5.8 ghz backscatter sensor measurement across high voltage...
TRANSCRIPT
5.8 GHz Backscatter Sensor
Measurement Across High
Voltage Insulation Gaps
Document ID: PG-TR-092004-CRV Date: April 21, 2009
Professor Gregory D. Durgin, Christopher Valenta, Patrick Graf, Matthew Trotter, Gregory Koo,
Brad Schafer School of Electrical and Computer Engineering, Georgia Institute of Technology
777 Atlantic Ave. Atlanta, GA 30332-0250 E-mail: [email protected]
Voice: (404)894-8169 Fax: (404)894-5935 http://www.propagation.gatech.edu/
No portion of this document may be copied or reproduced without written (e-mail) consent of the Georgia Institute of Technology.
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Executive Summary
This report presents the test procedure and results for demonstrating the effectiveness of 5.8 GHz
backscatter radio sensors operating in a high-voltage commercial power line environment. These
microwave backscatter radio sensors allow constant, real-time monitoring of power line
conditions such as temperature, position, line-current, or other environmental information. Such
technology may play a key role in the development of a “smart power grid,” where sensors and
computer intelligence are used to enhance the efficiency and reliability of the country’s vast
power-related infrastructure [Ami01, Ami02].
Steady-state corona, transient arc, and steady-state arc tests were conducted to emulate the high-
voltage environments of the commercial power grid. The measured results demonstrate that the
microwave-bands are largely resistant to the plasma emissions and impulsive radio frequency
(RF) noise conditions of the high-voltage environment. When points of potential failure on the
existing system were measured, it was determined that modifications in the RF sensor tag’s
shielding, antenna design, or modulation scheme would easily remedy the problem.
Based on our new-found physical understanding of this unique RF environment, we are
confident that an ultra-reliable 5.8 GHz link for transferring sensor data can be designed to
accommodate the rigors of transferring sensor data across a large insulation gap (at least 10m).
The concluding chapter of this report presents a Stage 2 plan for designing and developing a
prototype 5.8 GHz backscatter system for power-environment sensing.
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Table of Contents
Executive Summary ......................................................................................................................... i Table of Contents............................................................................................................................ ii List of Figures ................................................................................................................................ iii 1 Introduction..................................................................................................................................1
1.1 Overview............................................................................................................................... 1 1.2 Measurement System Overview ........................................................................................... 4
1.2.1 RFID System - Backscatter Radio Overview ................................................................ 4 1.2.2 Spread Spectrum Modulation ........................................................................................ 7 1.2.3 Receiver Design ........................................................................................................... 17 1.2.4 Tag Design ................................................................................................................... 18 1.2.5 Channel Coefficient ..................................................................................................... 21
2 Measurement Plan.......................................................................................................................23 2.1 Steady-state Corona Testing ............................................................................................... 23 2.2 Transient – Flashover in Air ............................................................................................... 25 2.3 Steady-state Free Burning Arc............................................................................................ 26
3 Data Collection ...........................................................................................................................29 3.1 Testbed Calibration............................................................................................................. 29
3.1.1 Transmitter Power Calibration..................................................................................... 29 3.1.2 Receiver Chain Calibration.......................................................................................... 30
3.2 Steady-state Corona Testing ............................................................................................... 30 3.2.1 Tag in Presence of Corona........................................................................................... 31 3.2.2 Tag Adjacent to Corona ............................................................................................... 34 3.2.3 Corona Ring Test ......................................................................................................... 35
3.3 Steady-state Free-Burning Arc ........................................................................................... 36 3.4 Transient - Flashover Test .................................................................................................. 38
4 Data Analysis ..............................................................................................................................45 4.1 Calibration Results.............................................................................................................. 45
4.1.1 Transmitter Power........................................................................................................ 45 4.1.2 Receiver Chain............................................................................................................. 46
4.2 Steady-state Corona ............................................................................................................ 49 4.2.1 Tag in the Presence of Corona ..................................................................................... 49 4.2.2 Tag Adjacent to Corona ............................................................................................... 51 4.2.3 Corona Ring ................................................................................................................. 53
4.3 Steady-state Free-Burning Arc ........................................................................................... 54 4.4 Transient - Flashover in Air................................................................................................ 54
5 Conclusions and Future Research...............................................................................................63 6 Appendix.....................................................................................................................................65 7 Bibliography ...............................................................................................................................67
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List of Figures
Figure 1: Backscatter radio system composed of a reader and an RF tag. ..................................... 6 Figure 2: RF Tag block diagram demonstrating the principle of load modulation at the terminals
of an antenna [Gri09]. ............................................................................................................. 7 Figure 4: 5.79 GHz backscatter radio receiver [Gri09]. ............................................................... 18 Figure 5: Tag block diagram [Gri09]............................................................................................ 19 Figure 6: Tag antenna pattern at 5.79 GHz [Gri09]...................................................................... 20 Figure 7: Photograph of the RF tag with microcontroller............................................................. 20 Figure 8: Wire corona source connected to a high voltage set. .................................................... 24 Figure 9: Corona ring with tag mounting tape.............................................................................. 25 Figure 10: Metal contacts for transient arc. .................................................................................. 26 Figure 11: Block diagram for the 5.79 GHz backscatter transciever used for measurement. ...... 31 Figure 12: Diagram of the corona measurement setup. ................................................................ 32 Figure 13: RF tag position near the corona-generating wire. ....................................................... 33 Figure 14: Tag adjacent to corona source. .................................................................................... 35 Figure 15: Tag interior to corona ring........................................................................................... 36 Figure 16: RF tag near arc-welder used to generate a steady-state arc......................................... 37 Figure 17: Arc-welder setup and measurements........................................................................... 38 Figure 18: Transient arc measurement setup and measurements.................................................. 40 Figure 19: Spark gap setup and measurements............................................................................. 41 Figure 20: RF tag electronics with a makeshift aluminum foil shield.......................................... 42 Figure 21: Composite insulate sulfur hexaflouride pressure vessel.............................................. 43 Figure 22: Receiver chain calibration for corona test. .................................................................. 47 Figure 23: Receiver chain calibration for lightning test. .............................................................. 49 Figure 24: 5.79 GHz backscatter radio link in presence of corona............................................... 50 Figure 25: Raw received data reflected from tag.......................................................................... 51 Figure 26: 5.79 GHz backscatter radio link adjacent to corona.................................................... 52 Figure 27: Lightning observed by 5.79 GHz patch antenna. ........................................................ 56 Figure 28: Lightning observed by AM antenna............................................................................ 56 Figure 29: Lightning observed at receiver output with tag deactivated........................................ 57 Figure 30: Lightning observed at receiver output with tag activated. .......................................... 58 Figure 31: Lightning observed at receiver output with tag activated and shielded. ..................... 59 Figure 32: Lightning (through SF6) observed at receiver output with tag activated and shielded.
............................................................................................................................................... 61
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1 Introduction
1.1 Overview
This report presents the measured results of a feasibility study for the use of 5.8 GHz backscatter
sensors in the typical high-voltage environments encountered in the commercial power grid
infrastructure. In this setup, a transceiver unit (referred to as the “reader” hereafter) excites a
remote, low-power sensor (referred to as the “RF tag” hereafter) that returns information to the
reader via modulated backscatter. The measured results for this type of RF system demonstrate
the link resilience in the harsh, charged-particle and impulsive-noise power systems
environment. Additionally, the results thoroughly characterize the physical layer of this unique
radioscape. In fact, the exhaustive physical-layer characterization in this study will allow the
design of an ultra-reliable wireless link for future sensor systems that employ this technology.
The high-voltage environment demonstrated several key behaviors of the physical layer RF
channel, many of which differ from conventional wireless systems. These attributes include
Thermal Noise – The measurements reveal that 5.8 GHz experiences virtually no
additional thermal noise in high-voltage scenarios, even when significant corona plasma
is present. Steady-state electrical arcs also did not seem to contribute measurable noise.
Impulsive Noise – The flashover tests demonstrated that, at most, 1 or 2 data bits in our
measurement setup on a radio link could be lost during an electrical discharge event.
This loss is readily overcome by using proper bit interleaving and forward error
correction coding at the tag.
Plasma Shielding – At very high line voltages (100 kV and higher), the RF tag
experienced plasma shielding when connected directly to the high-voltage wire.
Although this condition is extreme, the problem can be remedied by integrating a corona
shield into new RF tag/antenna designs.
Circuit Shielding – The flashover tests reveal that, for some cases, the prototype RF tag
(with its unshielded, bare electronics) experienced several “reset” conditions. Enhanced
electrical shielding should be included on future designs to avoid this condition.
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Overall, the results indicate interesting physical behavior that can be accommodated in future
sensor designs.
The 5.8 GHz backscatter sensor could be a powerful enabler for “smart grid” technologies that
enhance the reliability and security of our nation’s power infrastructure [Ami01, Ami02]. Since
the development of electric power and the application of electric power for the benefit of society
it has been necessary to measure various properties of the power system to maintain safety and
performance. Early developments in system measurement included the high voltage current
transformer for measuring current and the high voltage potential transformer for measuring
voltage. These technologies provide a majority of the information collected from the power grid
and have changed little in the past 50 years. The upgrade of power infrastructure to a “smart
grid” will require significant technical innovation to enhance these legacy sensing technologies.
With the evolution of better information systems and the desire for a more efficient power grid,
the need for increased measurement from the power grid has grown. Information on properties
such as voltage, current, temperature, vibration, gas density, mechanical position, velocity, etc
allow system engineers to provide greater reliability, create less waste, and improve safety.
The IT infrastructure has grown to handle increased data acquisition, but the limiting factor has
continued to be the difficulty of taking measurements at high voltage while maintaining the
insulation between high voltage and ground. Wireless devices have been employed to take
measurements and transmit data to ground based receivers. These devices have increased the
availability of information while maintaining the high voltage insulation, but they have suffered
from reliability issues. Since the wireless devices at line potential have no physical connection to
the ground, they draw power from some other source to operate. One solution has been to draw
energy from the currents flowing thru the high voltage lines. This solution is effective when
current is flowing thru the HV lines, but when lines are disconnected for maintenance or due to
an outage, the ability to take measurements ceases. To solve this problem some units have
employed batteries, which charge while the currents are flowing and then keep the units powered
when the currents are off. Batteries diminish the problems associated with unpowered lines, but
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lines can be disconnected for months at a time; even more importantly, batteries have a limited
life span and replacing units at high voltage is a very costly process. Other alternatives such as
solar cells have been employed to power devices, but again lifespan and reliability under all
weather conditions becomes a serious limitation.
What could considerably help the power grid is a device that allows for the communication of
data across the high voltage gap operates under all weather conditions, is not dependent on
current flowing through the line to power the unit, has a 20 to 30 year lifespan, and is
inexpensive. A device of this type would dramatically increase the stability and security of the
power system, reduce waste, and ultimately save money for utilities and their customers.
1.2 Measurement System Overview
1.2.1 RFID System - Backscatter Radio Overview RFID systems generally function in the following way: a reader, which consists of a transmitter
and receiver, interrogates a tag by sending an electromagnetic wave. This wave may energize
the tag and allow it to respond to the reader. The tag, which has little to no on-board power,
consists of some type of coupling element, such as an antenna, and a data-carrying device that
stores the tag identification information. When the tag is within the range of the reader, the tag is
powered up and can then transmit its data back to the reader [Fin03].
RFID tags source of power can vary between completely passive, semi-passive, and active
sources. Passive tags have no on-board battery and depend completely on the reader for energy
to power the on-board electronics and communication. Due of their simplicity, they have the
lowest cost of all tags and also the longest lifetime. They have no battery, transmitter, amplifier,
or oscillator of any kind because of the low power levels, which they must operate [Dob08].
Without any battery, their range is limited to the power they can rectify from the incoming RF
signal, usually a very small distance (up to 30m).
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Semi-passive tags have an on board battery that is used for powering the electronics on board,
but not for communication. Since the incoming wave is not rectified for power use, these tags
have a greater range of up to 100m. While these tags are more reliable than passive tags, they are
larger and more costly. Special care must be taken to ensure the battery has a sufficient lifetime
appropriate for the tag’s application.
Active tags use a batter to power all on-board systems, including an active method of
communication with the reader. While active tags have the longest range, they are the largest and
most costly due to the presence of a battery and other communication hardware. Since they
function as full-fledged radios, they must also be approved by government regulators who
oversee electromagnetic radiation. With the use of a full radio transmitter, these tags are capable
of communication at distances of approximately 1km. Many applications cannot warrant the
costs associated with an active tag, so passive or semi-passive tags are implemented for the
project to be feasible.
Passive and semi-passive RFID tags generally operate using the principle of modulated
backscatter radio. While a traditional radio system relies on each station providing power to
transmit data, backscatter radio stations rely on a single reader for power. This system is
advantageous for RFID applications because it allows for tags to be created with minimal parts,
size, and cost. During operation, a reader will send out some signal that the tags will detect. The
tags then modulate this signal by varying the reflection coefficient of their antennas. The reader
can then detect and demodulate the returned signal and extract the tag’s information. This
process is illustrated in Figure 1.
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Figure 1: Backscatter radio system composed of a reader and an RF tag.
A transmitter unit with antenna gain GTX sends a carrier wave to an RF tag that receives the
signal with antenna gain Gtag . This signal is then modulated by the tag according to ˜ Γ (t) and
then reflected back to the reader via the same antenna with gain Gtag . The tag’s data is embedded
in this reflection coefficient ˜ Γ (t). This signal then arrives back at the receiver where it is
received by a receive antenna with gain GRX and then processed by the reader. Figure 2 describes
the tag’s backscatter operation in more detail.
Figure 2: RF Tag block diagram demonstrating the principle of load modulation at the terminals of an antenna [Gri09].
As seen in Figure 1, the incoming signal from a reader is received by a tag. It is then modulated
according to a reflection coefficient and reflected back to the reader. Figure 2 shows the RFID
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tag’s basic components. The antenna receives the signal where it is then transmitted along a
transmission line. This transmission line is connected to a switch, usually a switching MOSFET,
which has two loads, ZLA and ZL
B . These different loads will change the amount of the incident
wave reflected back along the transmission line and back through the antenna to the reader.
While these loads can be any different values, it is most useful for them to be opposites so as to
distinguish the two opposing states as much as possible [Gri09]. Therefore, a short circuit and
open circuit are usually utilized. The modulating block is connected to the MOSFET to control
which load is connected to the transmission line. This modulation represents the data that the tag
sends back to the reader.
1.2.2 Spread Spectrum Modulation
Overview
Direct Sequence Spread Spectrum (DSSS) modulation is a multiple access technique created to
share RF bandwidth between multiple transmitters. The DSSS scheme at the most fundamental
level spreads out a narrowband RF signal in the frequency domain. The main advantages of
DSSS versus narrowband transmission schemes are resistance to small-scale or frequency-
selective fading, rejection of in-band interference, and the capacity for uncoordinated multiple-
access by simple sensors operating simultaneously in the same band. Additionally, the operation
of DSSS has useful signal-cloaking capabilities, providing a basic form of encryption and often
dropping the power spectral density of a signal beneath a detectable noise floor of common
measurement devices [Dix94].
In conventional amplitude modulation, a data signal is used to vary the magnitude of a
transmitted carrier wave. The transmitted waveform can be observed on a spectrum analyzer as
a time-varying power level centered at an AM carrier frequency. The downside to this form of
communication is that transmission errors can rise rapidly if a nearby device emits spurious noise
or interference in or near the center frequency of the AM signal. If the interference is large
enough in magnitude, the AM signal will be overwhelmed and data loss will occur. An analogy
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for narrowband interference would be the case where two individuals are talking across a
crowded room are unable to hear one another due to surrounding noise.
Narrowband transmissions can also suffer from small-scale signal fading in multi-path
environments. Multi-path environments are usually highly reflective and result in constructive
and destructive signal interference. This event is analogous to playing a constant bass tone in a
room and observing a drop in loudness when standing in a null or an increase in loudness when
standing in a peak. These peaks and nulls occur due to relative phase differences between
reflected acoustic waves that cause them to add or cancel out at given points in space. By the
same mechanisms, RF waves undergo this effect when in a multi-path environment.
In order to overcome these two problems in narrowband communication systems, spread
spectrum modulation takes the narrowband signal and spreads it into a wideband signal. In
doing so, the transmitted signal power is also spread out across a broad frequency range. The
result is that only a portion of transmitted power is attenuated when certain frequencies
experience frequency selective fading. By the spreading the narrowband signal, it also becomes
less susceptible to narrowband interferers, since only small portions of the new wideband signal
suffer interference.
Digital Processing Characteristics
Direct sequence spread spectrum modulation is comprised of two main steps: 1) creating and
transmitting a wideband signal and 2) receiving and recovering baseband data. The following
sections explain the chronological process required from beginning to end for a DSSS
modulation setup.
Signal Spreading
In order to create a spread spectrum signal, binary data to be transmitted must be combined with
a high rate chipping sequence. As an example, consider a sensor monitoring voltage on a power
line. It outputs data at a constant rate of one kilobit per second. The sensor output stream is used
to directly modulate a carrier wave centered at 5.8 GHz. To modulate the carrier wave, an
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implementation of binary phase shift keying (BPSK) is implemented. When a bit output from
the voltage sensor is 0, the phase of the carrier wave being transmitted is modified by 180
degrees. When the binary output stream is 1, the phase of the carrier wave is left unchanged.
This process is shown in Figure 3.
Since the carrier wave is modulated with the low frequency 1kHz signal from the voltage sensor,
the transmitted waveform is inherently narrowband in the frequency domain. In order to spread
out the transmitted signal, a much higher rate signal must be used to modulate the carrier wave.
One method of achieving a high rate signal is to combine the low rate data stream with a high
rate chipping sequence. As an example, assume the chipping sequence runs at one megabit per
second (Mbps). To combine the data stream of the voltage sensor with the chipping sequence,
either a modulo-two addition can be performed digitally, or an XOR gate can be fed by the two
streams. The resulting output sequence will have a rate equal to the one Mbps chipping
sequence and be used to modulate the phase of the carrier wave.
For a DSSS implementation using phase shift keying, the relations between the signal bandwidth
and bit rate are as follows:
-3dB BW = 0.88*Bit Rate
Null-to-Null BW = 2*Bit Rate
Figure 3: Spread spectrum modulation example.
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With the one MHz chipping frequency from the voltage sensor example, we obtain a -3dB
bandwidth of 880 kHz and a null-to-null bandwidth of 2 MHz. If instead the 1kHz sensor output
were used for carrier modulation, the -3dB bandwidth would be 880Hz and the null-to-null
bandwidth 2kHz. As demonstrated, the low rate modulation frequency yields an extremely
narrowband signal, whereas the higher modulation frequency provides a much more broadband
one. The bandwidth of the transmitted signal is therefore proportional to the frequency of the
chipping sequence.
Another way to view the concept of signal spreading is to visualize how a square wave’s period
in the time domain relates to its bandwidth in the frequency domain. Begin by making the
association that the dual of a square wave in the time domain is a sinc function in the frequency
domain. As the period of a square wave decreases in the time domain, the bandwidth of the sinc
function increases in the frequency domain, and vice versa. The same results occur when the bit
period of a chipping sequence is decreased (i.e. the chipping rate is increased).
Signal De-spreading
After the wideband signal is created, it propagates through space before being acquired by a
receiving antenna. Once a reader captures the signal, it must be de-spread in order to regain the
baseband signal. This process of de-spreading is achieved by performing a correlation between
the received waveform and a chipping sequence identical to that of the transmitter.
Signal Correlation
Although the correlation can be performed through analog or digital circuitry, for ease of
understanding, it will be explained in the digital domain. The receiver begins by creating a
binary chipping sequence identical to that of the transmitting node. Next, the receiver will take
the received waveform and convert areas of relative phase shift to either 0’s or 1’s to create the
received sequence. In an ideal environment, the transmitted waveform would experience no
phase shifting from propagation through space and would arrive exactly as it was transmitted. If
this were the case, the conversion from the received waveform back into its binary representation
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would lead to an identical copy of the chipping sequence. However, this does not occur in real
world systems. A propagation induced phase shift of the transmitted signal will cause it to be out
of sync with the reference sequence at the receiver. Therefore, the initial correlation between the
reference sequence and the received sequence will lead to a non maximal value. In order to
ensure that the received sequence matches the reference, a cyclic correlation must be performed.
For a purely conceptual example, let the reference sequence and chipping sequence be the 5 bits
[1 0 0 1 1]. To perform a discrete correlation, the reference sequence is held stationary while the
received sequence is shifted across it. After each shift, aligned bits are multiplied and then
summed. Consider the case where the received sequence is out of sync with the reference and
delayed by one bit, [0 0 1 1 1]. Also replace the binary 0's with -1's. The reference signal
becomes [1 -1 -1 1 1], and a phase shifted received signal becines [-1 -1 1 1 1]. A correlation at
the receiver would execute as follows. Note the received sequence is zero padded.
Received Sequence: -1 -1 1 1 1 0 0 0 0 0 0 0 0 0 0
Cyclic Reference Sequence: x 1 -1 -1 1 1 1 -1 -1 1 1 1 -1 -1 1 1
Result: -1 1 -1 1 1 0 0 0 0 0 0 0 0 0 0
Correlation value = total bit sum = (-1) + 1 + (-1) + 1 + 1 = 1
Next iteration:
Shift received sequence and repeat
Received Sequence: 0 -1 -1 1 1 1 0 0 0 0 0 0 0 0 0
Cyclic Reference Sequence: x 1 -1 -1 1 1 1 -1 -1 1 1 1 -1 -1 1 1
Result: 0 1 1 1 1 1 0 0 0 0 0 0 0 0 0
Correlation value = total bit sum = 1 + 1 + 1 + 1 + 1 = 5
Since the received waveform was phase delayed by one bit sequence, only one shift in the
correlation was needed before the reference sequence and received sequence synchronized.
When a correlation is performed between two sequences that are identical but out of sync, this
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process is known as an autocorrelation. For discrete autocorrelation, when the sequences are
synchronized, the peak autocorrelation value will be equal to the bit length of the sequence. As
shown in the previous example, the chipping sequence of length five produced a maximum
correlation value of five.
Baseband Data
Once a peak autocorrelation is detected by the receiver, it acknowledges that the received data is
synchronized with the reference chipping sequence. Next, the receiver will XOR or modulo-2
add the received data with the reference data. The process of XORing the data sequence with the
chipping sequence on both the transmitting and the received ends is shown below.
Binary data to be transmitted
Chipping sequence XOR
Carrier modulation sequence
Reference sequence XOR
Reference sequence XORed
with received sequence
Performance of DSSS
Multiple Access
In the field of spread spectrum modulation, the most influential aspect of designing a DSSS
system is the selection of the chipping sequence. By providing each transmitter in an RF
environment with a unique chipping sequence, multiple nodes are able to transmit
simultaneously at a common carrier frequency. For example, if three nodes transmit
simultaneously, a receiver will acquire an RF signal containing the three combined signals. The
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receiver will begin by selecting one of three known chipping sequences and performing a
correlation against the received signal. All three sequences are embedded in the received signal
and combine to produce a noise like total signal. However, by correlating the composite signal
against one of the three chipping sequences, the signal transmitted from one node can be de-
spread while the other two are left as residual noise. This fact occurs because the other two
sequences are different from the reference sequence, and their cross-correlation values will be
lower than that of the matched sequence. When an aligned, identical sequence is XORed with
the reference sequence, the sequences result in a large correlation. For the case where a
misaligned code is correlated with the reference sequence, the differences between the two cause
cancellation to occur and the unwanted node's transmitted signal is suppressed.
Processing Gain
The figure of merit for how well a DSSS system recovers the narrowband data signal from the
broadband received signal is known as process gain. The physical meaning of process gain is
associated with the signal-to-noise ratio of the wideband signal received versus the signal-to-
noise ratio of the de-spread narrowband signal. The physical meaning can also be related to the
computational process gain given as follows:
Process Gain = 10 log (Rate of chipping sequence / Rate of data )
The data rate of 1kHz and chipping rate of 1MHz yield a processing gain of 30dB.
Interference Rejection
While the process gain represents the ability of a receiver to de-spread a signal, it is not the sole
quality factor for a DSSS system. For instance, if chipping sequences are chosen arbitrarily, the
correlation between the reference chipping sequence and a slightly different chipping sequence
may trigger a peak correlation value threshold that would cause false synchronization. The result
would be that a signal transmitted from a different node than expected would be de-spread an
lead to an erroneous baseband data sequence. Chipping sequences of other transmitters can then
be considered a digital interference. By carefully choosing sets of chipping sequences however,
such interference can be minimized to increase overall system performance.
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Gold Codes / Chipping Sequence Length
The most effective method for determining the probability of whether or not one chipping
sequences might be erroneously mistaken for another during the de-spreading process is through
performing cross-correlations between all chipping sequences used by transmitting nodes. For
example, for a given set of chipping sequences, if two sequences produce a high cross-
correlation value, it means that the two codes are very similar in terms of bit patterns. It also
implies that during the de-spreading process, the reader is far more likely to mistake one
chipping sequence for the other.
A method for selecting a set of sequences with an upper bound for all possible cross-correlations
was developed by Robert Gold in 1968. By knowing the upper correlation bound, a receiver's
peak correlation threshold can be set more efficiently since there is a guarantee that all sequences
within the set will produce a peak cross-correlation equal or less than the bound.
The receiver's ability to distinguish a correctly matched sequence during the correlation process
can also be improved through the use of longer chipping sequences. Due to the fact that
synchronized sequences provide discrete correlation peaks equal to their bit length, the longer the
sequence, the larger the peak. Creating a longer length sequence utilizing Gold's method then
allows for a correlation gain to be established.
For example, Gold's method can be used to produce a set of 1023 bit long chipping sequences
that exhibit a bound of 63 for all possible cross-correlations between pairs of sequences in the
set. If a receiver is correlating one reference sequence from the set against many other nodes
transmitting with different sequences from the set, then the maximum cross-correlation value the
receiver will see is 63. Upon synchronization between the reference sequence and the correct
received sequence though, the peak autocorrelation value becomes 1023. The difference
between 1023 and 63 corresponds to a 24.2 dB difference. Thus increasing the chipping
sequence length provides increased robustness against de-spreading an incorrect node's signal.
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1.2.3 Receiver Design The receiver used in this measurement campaign was built and prototyped by the Propagation
Group. This highly sensitive receiver operates in the entire 5725-5850 MHz ISM frequency band
and then down-converts the in-phase and quadrature channels of this signal to DC where it can
be sampled and processed by an analog-to-digital converter.
The direct-conversion receiver utilized consists of two, four-layer FR4 printed circuit boards, an
RF front-end and a baseband amplification board, encased in an aluminum housing as shown in
Figure 4. These boards are connected externally via a pair of SMA cables. This receiver requires
an RF local oscillator input at 5.79 GHz and a DC power supply capable of providing +12 and -8
volts simultaneously. The receiver also has the capability of providing an automatic gain control.
This gain is controlled by a PC, which uses an algorithm to adjust the DC offset of the receiver.
Finally, the receiver accepts an RF input and has a baseband in-phase and quadrature (BBI and
BBQ) output [Gri09].
Figure 4: 5.79 GHz backscatter radio receiver [Gri09].
1.2.4 Tag Design The tags used for this measurement campaign were prototyped by the Propagation Group. These
two layer FR4 printed circuit board tags are capable of being reprogrammed to transmit any
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variety of spread spectrum code and modified to accept any number of external sensor inputs.
These signals can then be backscattered back to the receiver for processing.
The tags each work by the principle illustrated in Figure 5. A 5.79 GHz half-wave slot antenna
with broadside gain of 3.8 dBi (antenna pattern in Figure 6) receives the transmitted 5.79 GHz
carrier wave sent by the test setup. The antenna’s load is then switched between an open and
short circuit by a microwave gallium arsenide (GaAs), pseudo-morphic, high electron mobility
transistor (PHEMT) switch (M/A-Com MASW-007207 V2). Since this tag does not rectify any
power from the received signal, open and short circuits were chosen to maximum the backscatter
power. DC blocking capacitors were placed at the antenna port and the short-circuited port per
the datasheet recommendations. The transistor switch’s two inputs (input and complement) were
controlled via a Microchip PIC microcontroller (PIC18F2520). The microcontroller includes
several internal oscillators up to 20 MHz or uses an external oscillator of 40 MHz. This
microcontroller can be programmed in the C-language via a 6-pin header to USB cable. A 3-volt
coin cell battery powers the tag’s electronics, but does not contribute to the tag’s data
transmission. Figure 7 shows an image of this tag. For this testing, a 31-bit long pseudo-random
(PN) sequence with a chip period of 2μs was programmed into the microcontroller.
Figure 5: Tag block diagram [Gri09].
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Figure 6: Tag antenna pattern at 5.79 GHz [Gri09].
Slot Antenna
SwitchingFETMicrocontroller
ProgrammingInput
Figure 7: Photograph of the RF tag with microcontroller.
1.2.5 Channel Coefficient The performance of the system, the available power, and the relative noise floor levels were all
determined from the channel coefficient of the link. The channel coefficient is a complex value
that describes how the transmitted signal’s phase and amplitude are affected by the operating
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April 23, 2009
environment. Effectively, the coefficient is the multiplicative combination of the forward and
backscatter channel responses (hf and hb). For these measurements, the channel coefficient was
not normalized and, therefore, still has units of volts (a normalized channel coefficient would
technically be unitless). Because the channel coefficient represents the voltage of the
communications channel, the square of its magnitude is proportional to the received power,
which dictates the reliability of communication link. Thus, quality of system performance can be
determined through analysis of the channel coefficients.
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April 23, 2009
2 Measurement Plan
The purpose of Stage 1 is to demonstrate the performance of the backscatter communication
technology in electromagnetic environments typically found in high voltage applications.
For the tests listed below, The Propagation Group used a battery-powered remote device capable
of communicating information across a 40-inch air gap and the corresponding transceiver. The
data to be transmitted is stored in the microcontroller of the remote device and transmitted in a
looped sequence. Use of the analog input of the remote device is not required since the test is
designed to measure the impact of the high voltage environment on data transmission and not on
the data collection process.
2.1 Steady-state Corona Testing Corona testing was performed at Southern States inside The McGarity Building using the
Southern States high voltage production test set. The test was performed in the evening to avoid
interference with production. To generate corona, a piece of wire is attached to a high voltage set
as seen in Figure 8. Measured data is taken over 50,000 V increments.
Figure 8: Wire corona source connected to a high voltage set.
With the transceiver and remote device at ground potential, a safe distance from the high voltage
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April 23, 2009
system, the following test sequences are performed.
1) Receiver on, transmitter off, tag off, line off – This test measures RF thermal and system
noise as well as the facility’s background RF interference. The results from this
measurement provide a useful baseline for interpreting other measurements.
2) Receiver on, transmitter on, tag off, line off – This test measures self-interference levels
in the backscatter radio system, again providing a baseline for interpreting further
measurements.
3) Receiver on, transmitter on, tag on, line off – This measurement represents RF tag and
receiver operation under ideal system conditions.
4) Receiver on, transmitter off, tag off, line on (50kV, 100kV, 150kV, 200kV and 250kV) –
This series of tests measures the 5.8 GHz RF emissions purely due to the steady-state
corona discharge
5) Receiver on, transmitter on, tag on, line on (50kV, 100kV, 150kV, 200kV and 250kV) –
This series of tests measures tag operation with the corona plasma present to varying
degrees.
A test was also made to determine the potential significance of corona in closer proximity to the
remote device. A large donut-shaped corona ring will be applied around the remote device to
shield the device from the strong electric field gradient as seen in Figure 9. A sharp point will be
located near the remote device, 3 inches outside of the corona shield as a source of corona. The
test sequence will be repeated.
Figure 9: Corona ring with tag mounting tape.
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April 23, 2009
2.2 Transient – Flashover in Air For the transient test, a high voltage impulse/Marx generator is used to create the flashover in air
at approximately 350kV. A test setup used to generate full-wave BIL testing should be
appropriate to create the desired flashover. Pictures will be taken of the test setup for
documentation purposes.
Figure 10: Metal contacts for transient arc.
The ambient electromagnetic noise of the environment is measured before the high voltage
equipment is energized for use as a baseline. The electromagnetic signal produced by the
flashover will be recorded. The flashover will occur over a time span measured in microseconds
so triggering will be required to record the desired information. The sensor unit and transceiver
will be separated by 40 inches and located a safe distance away from the high voltage equipment.
Data transfer will be initiated before the flashover and recorded during flashover to determine if
data was corrupted.
Measurements will be taken during ten flashovers in air at a fixed distance and ten additional
flashovers will be taken at varying distances and antenna angles.
2.3 Steady-state Free Burning Arc An arc will be drawn between two contacts with 200 amps of alternating current across an air
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gap. More current can be used if necessary to sustain the arc.
The ambient electromagnetic noise of the environment will be measured before the high voltage
equipment is energized for use as a baseline. The electromagnetic signal produced by the arc will
be recorded. The sensor unit and transceiver will be separated by 40 inches and located a safe
distance away from the arc generating equipment. Data transfer will be initiated before the arc is
created and recorded during arcing to determine if the data was corrupted.
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April 23, 2009
3 Data Collection
3.1 Testbed Calibration Before performing measurements in any environment, it is important to perform standard
calibration measurements in order to establish a baseline by which to judge the measurement
data, as well as determine proper system functionality. For the measurements performed in
support of this phase of testing, two separate calibration procedures were performed: transmitter
power calibration and receiver chain calibration.
3.1.1 Transmitter Power Calibration
As was described earlier, a signal generator is used to supply both the carrier waveform of the
system and the local oscillator (LO) signal used by the receiver to down-convert the modulated
backscatter signal. Thus, the output of the signal generator flows through both a signal splitter
and a power amplifier before it reaches the transmitting antenna. In order to calibrate the
transmitter portion of the system described above, the following steps were executed:
1. The transmitter antenna from the amplifier was removed and the amplifier output was
connected to a power meter through a 30dB attenuator to ensure the meter is not
overloaded.
2. The powers supplied by the signal generator to the power amplifier were varied between -
20 and 10 dBm. The output powers of the amplifier were recorded for the various input
powers.
3. It was verified that the resulting outputs correspond to expected results and a signal
generator power that provides maximum amplification, while remaining in the linear
regime of power amplification was chosen.
3.1.2 Receiver Chain Calibration
As was stated in the system description earlier, the receiver antenna was connected directly to the
5.8GHz receiver, which down-converts the received signal to baseband. The output of this
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April 23, 2009
receiver was subsequently connected to a computer, which was used for data sampling and
analysis. In order to calibrate the system, the following steps were performed:
1. A step attenuator, which could be used to provide 0dB to 70dB of attenuation, was
inserted between the receiver antenna and the receiver.
2. A powered tag was placed 1m from middle of the transmitter/receiver antenna pair
3. Attenuation was varied from 0dB to 70dB (in 10dB steps) and the received power was
measured for each step.
4. The result of this measurement was then used to determine whether the received power
was adequate and where the approximate noise floor power of the receiver was located.
3.2 Steady-state Corona Testing The purpose of the steady-state corona testing is to determine whether a 5.8GHz backscatter
radio system can operate in the presence of corona noise. Corona is a type of electrical
breakdown in air that occurs around high-voltage power lines. When the voltage is sufficiently
high, the air becomes ionized and conducts electricity, thus becoming a plasma. This plasma has
the potential to interfere with radio communication and operation due to its conductivity and RF
emissions.
The Corona tests took place on March 24, 2009 from 5:30pm – 1:00am in the McGarity Building
at Southern States’ facility in Hampton, Georgia. The McGarity Building has approximately 50-
foot metal ceilings and metals walls. Underneath the cement floor, a copper shield exists as well.
All metal pieces of the building frame are grounded, making the interior of the building similar
to a large, shielded room.
3.2.1 Tag in Presence of Corona The following procedure was followed in order to take data during the Corona test.
1. The test equipment was setup according to the schematic in Figure 11.
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April 23, 2009
Figure 11: Block diagram for the 5.79 GHz backscatter transciever used for measurement.
2. The test environment was setup and measured as seen in Figure 12.
Figure 12: Diagram of the corona measurement setup.
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April 23, 2009
3. The tag was placed on the top edge of the ceramic insulator on a foam holder as seen in
Figure 13.
Figure 13: RF tag position near the corona-generating wire.
4. The system was calibrated according to the procedures described in Section 3.1
5. At position 1, with the receiver on, transmitter off, tag off, and line off, the background
thermal noise and background interference were collected by running the RunTestbed.m
MATLAB file.
6. The following naming convention was used for data files:
RX1TX1T1L0P1N1
• RX – receiver (1/0 – on/off)
• TX – transmitter (1/0 – on/off)
• T – tag (1/0 – on/off)
• L – line voltage (0-250 kV, 0 denotes off)
• P – position (1-5)
• N- Test Number (1-?)
7. At position 1, with the receiver on, transmitter on, tag off, and line off, the self-
interference levels were collected by running the RunTestbed.m MATLAB file.
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April 23, 2009
8. At position 1, with the receiver on, transmitter on, tag on, and line off, the ideal system
operation values were collected by running the RunTestbed.m MATLAB file.
9. At position 1, with the receiver on, transmitter off, tag off, line on (50kV, 100kV, 150kV,
200kV, 250kV), the corona pure-RF levels were collected by running the RunTestbed.m
MATLAB file.
10. At position 1, with the receiver on, transmitter on, tag on, and line on (50kV, 100kV,
150kV, 200kV, 250kV), the tag operation with corona levels were collected by running
the RunTestbed.m MATLAB file.
11. Steps 5-10 were repeated for positions 2-5. These separate positions were achieved by
moving the ceramic insulator, on which the tag was on top of, in one-inch increments.
The results were then averaged to eliminate the effects of small scale fading.
Table 1 : Separation distances from RF tag to corona-generating wire.
Position Number Distance (Inches)
1 56
2 57
3 58
4 59
5 60
3.2.2 Tag Adjacent to Corona The following procedure was followed in order to take data during the Corona test.
1. The tag was moved from its position on the ceramic insulator to 4 inches under the
corona source. It was taped to the wooden broom supporting the wire as illustrated in
Figure 14.
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April 23, 2009
Figure 14: Tag adjacent to corona source.
2. At this new position, with the receiver on, transmitter on, tag on, and line off, the ideal
system operation values were collected by running the RunTestbed.m MATLAB file.
3. With the receiver on, transmitter on, tag on, and line on (50 kV, 100kV, 250kV), the tag
operation with corona levels were collected by running the RunTestbed.m MATLAB file.
3.2.3 Corona Ring Test The following procedure was followed in order to take data during the corona ring test.
1. The tag was placed inside a corona ring in an effort to shield the tag from the effects of
the corona. This setup is illustrated in Figure 15.
Figure 15: Tag interior to corona ring.
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April 23, 2009
2. With this new configuration, with the receiver on, transmitter on, tag on, and line off, the
ideal system operation values were collected by running the RunTestbed.m MATLAB
file.
3. With the receiver on, transmitter on, tag on, and line on (50 kV, 100kV), the tag
operation with corona levels were collected by running the RunTestbed.m MATLAB file.
3.3 Steady-state Free-Burning Arc The purpose of the steady-state free-burning arc test is to determine whether a 5.8GHz
backscatter radio system can operate in the presence of continuously burning arcs. In power
systems, steady-state arcs can occur when large, mechanical switches open and close. Air
between conductors becomes ionized and creates a path for current to flow. Until this path is
broken, the arc will sustain. This event corresponds to noise generated and can lead to
interference with communication systems. For this test, an arc-welder was used to generate a
steady-state arc as seen in Figure 16.
Figure 16: RF tag near arc-welder used to generate a steady-state arc.
The steady-state arc tests took place on March 24, 2009 from 5:30pm – 1:00am in the McGarity
Building at Southern States’ facility in Hampton, Georgia.
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April 23, 2009
The following procedure was followed in order to take data during the steady-state arc test.
1. The tag was placed on a foam holder and then onto a chair adjacent to the arc-welding
site. The environment was measured as illustrated in Figure 17.
Figure 17: Arc-welder setup and measurements.
2. With this new configuration, with the receiver on, transmitter on, tag on, and line off, the
ideal system operation values were collected by running the RunTestbed.m MATLAB
file.
3. With the receiver on, transmitter on, tag on, and arc on, the tag operation with steady-
state arc levels were collected by running the RunTestbed.m MATLAB file.
3.4 Transient - Flashover Test The purpose of the transient – flashover ‘Lightning’ test is to determine whether a 5.8GHz
backscatter radio system can operate in the presence of transient - flashover arcs. In power
systems, arcs can occur when large mechanical switches open and close or when lightning
occurs. These flashovers discharge large voltages in short periods of time. This event
corresponds to high frequency noise generated and can lead to interference with communication
systems. For this test, a Marx generator was used to generate a transient flashover between two
conducting rods.
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April 23, 2009
The transient - flashover tests took place on April 2, 2009 from 8:30am – 6:00pm at the JMX
Services, Inc. facility in Fayetteville, Georgia. The room where the test was conducted had 20
foot ceilings and grounded, metal walls and ceiling, save for 2 skylights. The floor was cement.
The following procedure was followed in order to take data during the transient – flashover test.
1. All electrical equipment used was connected through an isolation transformer in order to
protect it from the high voltage.
2. All antennas and equipment were connected to a common building ground to prevent
capacitive buildup.
3. The environment was measured as illustrated in Figures 18 and 19.
Figure 18: Transient arc measurement setup and measurements.
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April 23, 2009
Figure 19: Spark gap setup and measurements.
4. The backscatter radio system was setup the same way as in the corona tests, except the
analog-to-digital converter used an internal 40 MHz oscillator instead of an external 80
MHz oscillator. This modification allowed the system to be remotely triggered off of AM
noise.
5. The system was calibrated.
6. Using a spectrum analyzer, the frequency content of the free-burning arc was measured
with an AM antenna, a wide band antenna, and a 5.8 GHz patch antenna over a wide
range of voltages and sweep times.
7. Due to triggering problems, the receiver was sampled by a JMX analog-to-digital
converter that was triggered by direct coupling from the high voltage (instead of the PC
from the setup diagram).
8. With the receiver on, arc on, and tag off, background measurements were taken of the I
and Q channels separately by the JMX system (JMX system had limited inputs – 1
trigger, 1 data).
9. With the receiver on, arc on, and tag on, arc measurements were taken of the Q channel
only with the JMX system. Tag was reset during test by arc.
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April 23, 2009
10. Tag electronics were wrapped in aluminum foil and tape as a crude shielding mechanism
as seen in Figure 20.
Figure 20: RF tag electronics with a makeshift aluminum foil shield.
11. With the receiver on, arc on, and tag on, arc measurements of the shielded tag were taken
of the Q channel only with the JMX system.
12. The shielded tag was then moved behind the spark gap so that the signal had to travel
‘through’ the spark gap to be received.
13. At this new position, with the receiver on, arc on, and tag on, arc measurements of the
shielded tag were taken of the Q channel only with the JMX system.
14. The spark gap was then replaced with a composite insulate Sulfur Hexaflouride ( 6SF )
pressure vessel as illustrated in Figure 21. When a spark discharges through this gas, the
time period of the discharge is on the order of nanoseconds, instead of microseconds.
Thus, the noise would theoretically be more broadband and could have a greater effect on
system reliability.
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April 23, 2009
Figure 21: Composite insulate sulfur hexaflouride pressure vessel.
15. With the tag back at its original position and with the receiver on, arc on, and tag on, arc
measurements of the shielded tag were taken of the Q channel only with the JMX system.
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April 23, 2009
4 Data Analysis
4.1 Calibration Results
4.1.1 Transmitter Power Corona Test (McGarity Building)
As was stated in the calibration procedure section, the actual transmitted power was measured
for various signal generator (SigGen) output power levels and these measurements can be found
in Table 2. It was decided that the signal generator should be set to output 0dBm to provide
maximum transmitted (Tx) power, while avoiding excessive gain compression.
Table 2: Transmitter Power Calibration – Corona Test
SigGen Power Level (dBm) Tx Power (dBm) -20 -5.5 -15 -0.5 -10 4.5 -5 9.6 0 14.6 1 15.6 2 16.4 3 17.2 4 17.8 5 18.2 6 18.6 7 18.9 8 19.1 9 19.2 10 19.31
Transient Arc Test (JMX Services)
As was stated in the calibration procedure section, the actual transmitted power was measured
for various signal generator output power levels and these measurements can be found in Table
3. It was again decided that the signal generator should be set to output 0dBm to maintain
consistency with the previous test, while avoiding excessive gain compression. The numbers
seen in this calibration are slightly different than the previous calibration. However, this
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April 23, 2009
difference can be attributed to slight changes in the system, e.g. the use of different cables and
cable lengths. Also, fewer points were measured because the outputs were approximately
matching the values seen during the calibration before the corona tests.
Table 3: Transmitter Power Calibration – Lightning Test
SigGen Power Level (dBm) Tx Power (dBm) -20 -4.8 -15 0.2 -10 5.2 -5 10.2 0 15.1 1 15.9 2 16.6
4.1.2 Receiver Chain Corona Test (McGarity Building) As was stated in the receiver calibration procedure, the channel coefficient and its magnitude
squared were determined for various levels of attenuation and the results can be seen in Table 4
as well as Figure 22. In this calibration the noise floor appears to be reached between 30dB and
40dB of attenuation, which is approximately equivalent to a free-space distance of 5.62 meters to
10 meters.
Table 4: Receiver Chain Calibration – Corona Test
Attenuation (dB) |Channel Coefficient|2 (Volt2) 0 2.9354 x 10-7
10 3.0681 x 10-8 20 2.9798 x 10-9 30 4.0865 x 10-10 40 3.1476 x 10-11
50 3.5297 x 10-12
60 7.8856 x 10-12
70 1.4719 x 10-11
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April 23, 2009
Figure 22: Receiver chain calibration for corona test.
Transient Arc Test (JMX Services)
As was stated in the receiver calibration procedure, the channel coefficient and its magnitude
squared were determined for various levels of attenuation and the results can be seen in Table 5
as well as Figure 23. In this calibration the noise floor appears to be reached at around 20dB to
30dB of attenuation, which is approximately equivalent to a free-space distance of 3.16 meters to
5.62 meters. The noise floor occurs much earlier because this environment is far more reflective
(more metal objects) and is far more confined than the environment used for corona testing.
Table 5: Receiver Chain Calibration – Lightning Test
Attenuation (dB) |Channel Coefficient|2 (Volt2) 0 4.9318 x 10-8
10 5.8035 x 10-9 20 5.1284 x 10-10
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April 23, 2009
30 4.0686 x 10-11 40 4.3583 x 10-11
50 3.3370 x 10-11
60 3.9659 x 10-11
70 1.0749 x 10-12
Figure 23: Receiver chain calibration for lightning test.
4.2 Steady-state Corona
4.2.1 Tag in the Presence of Corona With the tag placed on a movable ceramic insulator in the presence of corona, several sets of data
were taken. For each voltage level, the tag was moved to 5 different locations (on the order of a
wavelength) in order to minimize the effects of small-scale fading. For each of these
measurements, each position was time-averaged 16 times to minimize the effects of transient
noise sources. This data was then plotted in Figure 24. Additionally, a time-domain plot of the
received waveform is plotted in Figure 25.
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April 23, 2009
Figure 24: 5.79 GHz backscatter radio link in presence of corona.
Figure 25: Raw received data reflected from tag.
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April 23, 2009
Figure 24 clearly shows that a tag in the presence of corona has no difficulty communicating
with a reader. The received signal strength from the RF tag is consistently 16 dB higher than the
RF background noise, both with and without the corona present. This result is independent of
the voltage (and, hence, corona plasma) level. A backscatter radio link without a tag in the
presence of corona deviates only slightly from the background noise in the environment,
indicating that the 5.8 GHz RF emissions of corona plasma are much lower than HF frequencies,
where corona effects can impair basic analog radio links. When the tag is turned on in the
presence of corona, the link only deviates slightly from the ideal system operation case.
4.2.2 Tag Adjacent to Corona After demonstrating that a tag can successfully work in the presence of corona, the unprotected
tag was then moved immediately adjacent to the corona source (4 inch separation). For this
testing, space averaging was not implemented, but time averaging was still used. Also, data was
only taken for 50kV, 100kV, and 250kV. The results are shown in Figure 26.
Figure 26: 5.79 GHz backscatter radio link adjacent to corona.
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April 23, 2009
This testing yielded more interesting test results. At 250kV, corona buildup concentrated around
the sharp edges of the tag, and the tag swayed back and forth approximately 5 degrees. While no
signal was readable at this voltage level, the tag signal was recovered when the voltage was
lowered. Based on this testing, a tag could be read when 4 inches from a corona source when the
voltage is under 150kV under ideal conditions. Likewise, this test demonstrated that a new type
of antenna suited for corona environments should be investigated.
The communication failure above 150kV can be attributed to the large electron density in the
proximity of the corona source. At high voltages, the air around the conductor becomes plasma,
an ionized cloud of gas. The electrons in this cloud create a conductive medium that beneath a
certain cutoff frequency, prohibits wave propagation [Roh91]. This occurrence is the same
phenomenon that happens when spacecraft re-enter the Earth’s atmosphere. Plasma builds up
around the spacecraft up re-entry and this effectively shields all radio communication below a
certain frequency for the remained of re-entry. This critical frequency ( fc ) is related to the
number of electrons per cubic meter ( NMax ) and is listed below as Equation 1.
fc = 80.5NMax (1)
The use of 5.8 GHz microwave frequencies is particularly effective at penetrating into plasmas,
although there are still upper-limit thresholds of charge density that can shield the RF tag from
communications. Future designs should be able to raise this limit by designing shielding and
metal housing shapes that minimize corona discharge in the area immediately around the tag
antenna.
4.2.3 Corona Ring When the tag was placed inside the corona ring, it was very difficult to receive any signal in the
setup. In an attempt to solve this problem, the tag was moved to the front side of the corona ring
and both transmit and receive antennas were positioned closer to the tag. However, none of
these solutions yielded better result. The inability of the tag to communicate with the reader is
most likely do to reflections off of the ring interfering with the transmitted and reflected signal.
Additionally, the presence of metal in such a close vicinity of the tag can de-tune the tag
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April 23, 2009
antenna’s characteristic impedance, which was optimized for free-space operation. When this
impedance is changed, the tag will no longer resonate efficiently at 5.8 GHz and the
communication link will degrade. However, these problems can be mitigated through an
intelligent antenna design suited for operation near corona rings and high voltage.
4.3 Steady-state Free-Burning Arc Data collection during arcs proves to be a difficult process due to the triggering necessary to
capture the data. Some of these arcs occur for very short time intervals on the order of
microseconds. Therefore, triggers must be very quick and analog-to-digital converters must have
very high sample rates (and large buffers) in order to obtain useful data.
The data collected showed no effect of the steady-state arc on the backscatter radio system. The
free burning arc power remained in the noise floor, and tag operating during the arc had no
observable difference from ideal operation. This data verifies our prediction that the steady-state
arc would not affect the communication because of the lack of high frequency noise and/or
plasma.
4.4 Transient - Flashover in Air Measurement results for the transient - flashover proved difficult to obtain, due to the extremely
small time scale of a lightning strike. Initially, it was believed that that a spectrum analyzer
could be used to witness the lightning strike. However, after several trials, it was determined
that the resolution/sweep time of the analyzer was not nearly high enough to observe the very
brief event of a lightning strike, which was on the order of microseconds. Also, since the
lightning strike occurs in such a short time frame, the traditional system setup could not be used.
Therefore, an alternative measurement approach was improvised. The JMX system has a second
input port on its data acquisition system, which can be used for data capture. This system is
beneficial to use since it is triggered on the lightning strike itself through direct coupling.
Therefore, it accurately synchronizes data collection with the discharge.
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April 23, 2009
Lightning Test Results Before testing tag functionality, it was of interest to see how the lightning waveform appeared on
the receiving patch antenna, which has localized frequency reception at 5.79 GHz. The time
domain plot of the received waveform can be seen in Figure 27, which was taken with an
oscilloscope using the patch antenna. As should be noted, the overall waveform has peak
magnitude of less than 1 V, which is significantly less than what would be seen on an antenna
that captures lower frequencies. Also, the noise has duration of less than 1 microsecond,
meaning that the effect on data transmission and bit loss would be minimal (providing chip time
was sufficiently low). An example of the response of this spark on an AM (low frequency)
antenna can be seen in Figure 28. The voltage levels are significantly higher than the patch
antenna at 5.8GHz. This occurrence is due to the fact that noise power of a lightning strike is
much more dominant at low frequencies than at high frequencies. Thus, this design, which is
built to operate at 5.8GHz, strongly mitigates the effects of lightning which might otherwise be
seen.
Figure 27: Lightning observed by 5.79 GHz patch antenna.
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April 23, 2009
Figure 28: Lightning observed by AM antenna.
After viewing the time domain response of the patch antenna directly, the antenna was connected
to receiver and the receiver output was viewed. As should be noted the waveform is still visible,
which was to be expected, but the duration is approximately the same as before.
Figure 29: Lightning observed at receiver output with tag deactivated.
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April 23, 2009
Finally, after analyzing the system without an active tag, the tag was activated and the effects of
a lightning strike on the tag were witnessed. As can be seen in Figure 30, the strike caused the
tag to reset, and it stopped broadcasting a sequence for approximately 65 milliseconds.
However, it is important to note that the tag reactivated itself and continued broadcasting. This
shows that, due to the early stage of prototype development and lack of proper shielding, the
lighting was able to reset the microprocessor, but the tag was undamaged.
Figure 30: Lightning observed at receiver output with tag activated.
Since the lack of shielding caused the tag to reset, rudimentary shielding was constructed using
aluminum foil, and the test was run once again. This time the tag functioned throughout the
strike, although the transient noise was still visibly present. The time domain response can be
seen in Figure 31 with the PN sequence visible before and immediately after the impulse.
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April 23, 2009
Figure 31: Lightning observed at receiver output with tag activated and shielded.
For the lightning test, another interesting parameter to address is the RMS delay spread of the
environment. The RMS delay spread describes the time-domain dispersion due to reflections in
the environment [Dur03]. In short, this parameter can be used to determine the maximum
interference time a lightning strike could have on the data signal. For this calculation, two
different sets of measured data were used: data from a lightning strike as measured at the output
of the receiving patch antenna and data from a lightning strike as measured at the output of the
5.8 GHz receiver. The delay spreads calculated from these two sets of data and were 1.1718 μs
(as seen by at the antenna output) and 3.8272 μs (as seen by the receiver output). The receiver
output has a slightly higher delay spread than the antenna because it performs low-pass filtering,
which provides smoothing and stretches out the delay. However, both delay spreads are
extremely short and would cause minimal chip distortion. Also, it should be noted that these
delay spreads are a worst-case representation because the test facility was essentially a Faraday
cage and reflections were thus higher than in an outdoor line of sight (LOS) environment. Thus,
it is not expected that lightning would significantly hamper data transmission.
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April 23, 2009
As an additional test, Southern States requested that the voltage should be discharged through
SF6 gas. SF6 gas is used by power companies because of its excellent ability to quickly suppress
arcs. A discharge through SF6 is on the order of nanoseconds, instead of on the order of
microseconds, as is seen in air. Since the spike is even more narrowband than before, the noise
will be more broadband, pushing more noise into the operating frequency band. The time-
domain response of the system under these conditions can be seen in Figure 32. Due to the
additional noise at higher frequencies, the microprocessor sometimes resets, due to the
rudimentary nature of the shielding. Superior shielding will have to be designed for the final
product to mitigate the effects of this voltage spikes. However, the tag still successfully
recovered.
Figure 32: Lightning (through SF6) observed at receiver output with tag activated and shielded.
At the very end of testing, the lightning test system was set to discharge directly through the RF
tag. Immediately after the discharge, the tag was found to be non-functional. Back at the
laboratory, the tag was tested, and it was confirmed that it was no longer functional. Although
the tag no longer functioned, this event has a low probability of occurrence. The tag has no
reference to earth ground. Lightning would only discharge through the tag when it happened to
be in lightning’s path to ground.
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5 Conclusions and Future Research
As a result of this Phase I study of the effectiveness of 5.8 GHz backscatter radio sensors
operating in a high voltage commercial power line environment, the Propagation Group makes
the following conclusions:
• A 5.8 GHz backscatter radio system can successfully operate in the presence of corona.
• A 5.8 GHz backscatter radio system can be affected by corona when directly adjacent to a
source of corona at very high voltages (greater than 150 kV from line to ground). It would
be possible to design a corona-resistant antenna to minimize this problem.
• A 5.8 GHz backscatter radio system is not affected by steady-state arcs.
• A 5.8 GHz backscatter radio system can potentially suffer bit errors during transient arcs.
Appropriate shielding and communication protocols can resolve this problem.
While there are points of further optimization, the basic backscatter system worked
extraordinarily well in the high voltage environment. Since backscatter tags are low-powered,
capable of operating on RF-harvested power, this form of communications opens up the
possibility of robust, battery less line state and fault detection under any circumstances.
The next step in research and development of a 5.8 GHz backscatter radio sensor system is to
design and implement a radio communications data protocol that is tailored to the unique
physical-layer demands of the high voltage environment. Based on our findings in this report,
we recommend the following protocol attributes be incorporated into the second phase of the
research project:
• Packetized Binary Modulation – The RF tags should be integrated with current-sensing
hardware from Southern States to demonstrate how sensor measurements may be
sampled, quantized, and sent on digital packets using conventional binary modulation
schemes. Results in these measurements demonstrate ample signal-to-noise ratio for
conventional binary modulation.
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• Forward Error Correction Coding – In addition to conventional binary modulation, the
final design should incorporate forward error correction coding on the information link.
If designed with both the thermal and impulsive noise levels measured during Phase I, it
is possible to make a digital transmission link completely error-proof using these codes.
• Multiple Access Protocol – Since one reader is expected to simultaneously read data from
multiple sensor tags, the final design must include a multiple access protocol. This will
likely be built upon the direct sequence spread spectrum waveform used in the Phase I
experiments, since the signal was quite robust to noise, interference, and jamming during
the measurement campaign. Thus, the final system should involve some form of code
division multiple access (CDMA).
The expected output of a second phase project would be a working prototype of multiple sensors
and a stand-alone reader unit to prove the sensing concepts and objectives.
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6 Appendix The DVD included with this report contains the following folders:
• Documents – Contains this report and the proposal (Microsoft Office .doc) • Images – Contains images from the corona and steady-state arc tests at Southern States,
from the transient arc tests at JMX Services, and from the ice tests at Georgia Tech (.jpg) • Data – Contains the data taken at Southern States and JMX Services (.dat, .txt, .pdf) • Code – Contains the code used to take and process data (Matlab script .m, .exe) • Video – Contains video taken at Southern States and JMX Services (.mpg)
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7 Bibliography
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[Fin03] K. Finkenzeller, RFID Handbook: Fundamentals and Applications in Contactless Smart
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[Gri09] J.D. Griffin, “High-Frequency Modulated-Backscatter Communication Using Multiple
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[Roh91] P. Rohan, Introduction to Electromagnetic Wave Propagation, Boston, MA: Artech
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