the design of an economical antenna-gain and radiation-pattern measurement system

The Des ign of an Econom ica l Antenna-Gai n and Rad iat ion -Pattern Meas u rement System

Brandon C. Brown, Frederic G. Goora, and Chris D. Rouse

U n iversity of New Brunswick, Department of Electrical and .computer Engi neering Fredericto n , New Bru nswick, E3B 5A3 Canada

Tel . + 1 -506-453-456 1 ; Fax + 1 -506-453-3589 Emai l : {brandon . brown . f.goora . ch ris . rouse}@u n b . ca

http ://www.ee . u n b . ca/

Abstract

The design of a system capable of making antenna-gain and rad iation-pattern measurements at 2.4 GHz is presented . System performance based on component specifications is summarized and compared to measured data . Antenna measurements taken using the system were compared to those obta ined using commercial ly avai lable test equipment i n an anechoic test chamber. The accuracy of the system was found to be ±0.5 dB with in a dynamic range of 1 3 dB plus the gains of the antennas i n use. The system was shown to be capable of making h igh qual ity antenna rad iation-pattern measurements in an anechoic test chamber. For a total cost of less than $ 1 300 , the system presents an economica l alternative to more-sophisticated m icrowave measurement systems, and is wel l su ited for use in a learn ing environment.

Keywords: Antennas; antenna measurements; antenna rad iation patterns; antenna theory; electrical engineering education ; I EEE student design contest; I soTropic Thunder

1 . I ntroduction

Developing an understanding of antenna properties is essential for anyone hoping to pursue a career in wireless

systems. Perhaps the most important property is antenna gain, as it strongly impacts the range of a wireless link. Antenna gain is achieved by directing radio-frequency (RF) energy more favorably in some directions than others. Consequently, an antenna-gain specification is often accompanied by a radiation pattern. Due to the prohibitive costs associated with commercial antenna test equipment, it is impractical for large groups of students to gain hands-on experience in making antenna measurements. This motivates the development of a system that is capable of making such measurements with an accuracy of ±0.5 dB and that can be easily reproduced for less than $ 1 500 in cost.

This paper presents the design of a system that is capable of measuring the gain and radiation pattern of an antenna in accordance with these specifications. Commercial off-theshelf (COTS) components have been specified such that the system can be easily reproduced. The antenna radiation-pattern measurement is fully automated, and antenna gain is measured using the three-antenna method [ 1 ] . A graphical user interface (GUI) accessed on a laptop provides user control over the

system. The system was used to characterize a set of commercial off-the-shelf antennas, as well as an antenna that was custombuilt using a Pringles can. This form of antenna is colloquially referred to as a "cantenna," and is reported to exhibit upwards of 12 dBi of gain [2] .

2. System Overview

A block diagram of the antenna measurement system is shown in Figure 1 . The RF source consists of a dual-output frequency synthesizer. One of the outputs is connected directly to a transmitting (Tx) antenna. Filtering is not required, since the transmitting antenna is assumed to be narrowband and is designed for operation at 2.4 GHz. The other output is passed to the detector in the receiver stage via fixed RF attenuation.

The signal received by the antenna under test (AUT) is amplified by a low-noise amplifier (LNA) and is bandpass filtered (BPF). Signal detection is achieved using a gain detector. This device generates an analog voltage proportional to the gain in dB of the signals present at its two inputs.

The voltage output of the detector is digitized by a microcontroller for processing. The microcontroller provides a

1 88 IEEE Antennas and Propagation Magazine, Vol. 53 , No. 4, August 20 1 1

Tx AUT the frequency is swept from 2.4 GHz to 2 .45 GHz in ten discrete steps: see Section 3 .7 . 1 for details.

RF Source LNA B P F

Ga in

M icrocontro l ler

Attenuator

Frequency Control

Figure 1 . A block diagram of the system.

Figure 2. The system hardware as mounted on a 12 in x

20 in MDF board.

graphical-user interface that can be accessed via Ethernet connection to a laptop. In response to user commands, the microcontroller exercises control of the RF source and a stepper motor, which is used to rotate the AUT by means of a belt-drive assembly.

3. Hardware Descri ption 'I

A photograph depicting the hardware associated with the system is shown in Figure 2. The following subsections describe the RF and electrical details associated with each hardware component.

3 . 1 RF Source

The RF source consists of an Analog Devices ADF4360-0 evaluation board. The ADF4360-0 is an integrated frequency synthesizer and voltage-controlled oscillator, capable of generating crystal-referenced complementary microwave signals over a frequency range of 2.4 GHz to 2 .725 GHz [3] . Both the frequency and power levels o f the 5 0 Q outputs are adjusted by updating control registers serially from the microcontroller. Upon startup, the frequency is set to 2.4 GHz and the power level is set to -3 dBm. During a measurement,

3.2 Transmitti ng Antenna

One of the outputs of the ADF4360-0 i s fed to the transmitting antenna via 2 .8 m of RG-3 1 6 coaxial cable. The loss associated with this length of cable is approximately 4 dB. Consequently, the transmitted power is -7 dBm. The antenna is mounted 0.9 m above the table top on a fixed wooden stand to minimize perturbation of the electromagnetic fields . The transmitting antenna should exhibit an input VSWR of 2: 1 or less from 2.4 GHz to 2.45 GHz. It should also exhibit 5 dBi to 1 0 dBi of gain, in order to improve the dynamic range of the detector.

3.3 Antenna U nder Test (AUT)

The AUT is mounted 0.9 m above the table top on the antenna-positioning system. In order to ensure that all measurements are made in the far field, the AUT is placed at least 1 .25 m from the transmitting antenna. At this distance, the freespace path loss is approximately 42 dB. Neglecting antenna gains, the received power is -49 dBm. The AUT should exhibit an input VSWR of 2 : 1 or less from 2.4 GHz to 2.45 GHz.

3.4 Low-Noise Am pl ifier (LNA)

The output of the AUT is fed to the low-noise amplifier via 2 m of RG-3 1 6 coaxial cable, which results in a loss of approximately 3 dB. The Mini-Circuits ZX60-272LN+ lownoise amplifier operates from 2.3 GHz to 2.7 GHz, and provides a gain of 1 4 dB. Consequently, the output power of the amplifier is approximately -38 dBm with 0 dBi antennas.

3.5 Band pass F i lter (BPF)

The input to the bandpass filter is connected directly to the output of the low-noise amplifier. The Mini-Circuits VBF-2435+ bandpass filter operates with a center frequency of 2.435 GHz and a bandwidth of 1 90 MHz. The insertion loss at 2.4 GHz is approximately 2 dB, resulting in an output power level of approximately -40 dBm with 0 dBi antennas. The output of the bandpass filter is fed to one input of the gain detector through a short length of coaxial cable.

3.6 Atten u ator

The other output of the ADF4360-0 is connected to an attenuation stage through a short cable exhibiting 0 .5 dB of loss. The attenuation stage consists of a 6 dB attenuator (MiniCircuits VAT-6+), followed by a 20 dB attenuator (Mini-Circuits VAT-20+) . As a result, a -29.5 dBm signal is fed to the other input of the gain detector.

IEEE Antennas and Propagation Magazine, Vol. 53 , No. 4, August 20 1 1 1 89

3.7 Detector

The detector consists of the Analog Devices AD8302 evaluation board. The AD8302 is an RFIIF gain and phase detector, which is capable of operating up to 2 .7 GHz and offers a nominal gain sensitivity of 30 mY/dB [4] . Note that the gain is measured between inputs INPA and INPB. The power at INPB acts as the reference level for the gain calculation. The device responds to signals between 0 dBm and -60 dBm. Consequently, the optimum reference power level at INPB is -30 dBm, which coincides with the -29 .5 dEm delivered from the attenuation stage. The linearity error at 2.2 GHz is specified as ±0.5 dB for a dynamic range of 5 1 dB [4] . Consequently, the power level appearing at INPA should range between -4 dBm and -55 dBm. For antennas with 0 dBi of gain, the power level at INPA is -40 dBm, setting the minimum dynamic range at 1 5 dB. Choosing a relatively high-gain transmitting antenna results in a flexible system, capable of measuring many different types of AUTs with accuracy.

The output voltage of the AD8302 is fed to a 1 0-bit analog-to-digital converter (ADC) on the microcontroller. Since the analog-to-digital converter uses a 3 .3 V reference voltage, the expected equation relating the gain between INPA and INPB and the analog-to-digital converter result is shown in Equation ( 1 ), where D is the value reported by the analog-todigital converter between 0 and 1 023, and G is the gain measured between INPA and INPB in dB :

G = 0. 1 075D - 3 0 . ( 1 )

3.7 . 1 Phase Sensitivity

Gain measurements made by the AD8302 are highly phase sensitive. Since the ADF4360-0 produces phase-coherent and frequency-locked signals, the detector output fluctuates sinusoidally about the true gain measurement as a function of the electrical-path-Iength difference between signals fed to INPA and INPB. The frequency of the ADF4360-0 is therefore swept from 2.4 GHz to 2 .45 GHz in ten discrete steps during a measurement, effectively sweeping the electricalpath-length difference. Averaging the set of results suppresses the phase sensitivity of the detector, and yields a proper gain measurement.

3.8 M icrocontrol ler

The microcontroller consists of the Making Things Make Controller Kit (MCK). The MCK features a l O-bit analog-todigital converter with a 3 .3 V reference, which is used to digitize the output voltage from the AD8302. There are eight highcurrent digital outputs : four outputs are configured to drive the stepper motor in a bipolar configuration, three outputs are used to communicate serially with the ADF4360-0, and one remains as a spare. The MCK also features both mini-USB and Ethernet

interfaces . An Ethernet cable is connected from the Ethernet port on the MCK to the laptop to enable graphical-user-interface access .

3.9 Stepper Motor

The Portescap 42L048D 1 U stepper motor is used to drive the antenna-positioning system. The motor is powered by 5 V, and features an angular resolution of 7 .5° . An external gear ratio of 7 .5 increases torque and results in an angular resolution of 1 0. Although the motor is unipolar, it is driven by the microcontroller in a bipolar configuration to reduce current requirements.

3.1 0 Power Considerations

The Power One MPB 1 25-4350G switching power supply provides power to the system. The supply is rated for 125 W, and offers a variety of dc voltage outputs: 3 . 3 V, 5 V, and 1 2 V. The 5 V line powers the low-noise amplifier, microcontroller, and gain detector, while the RF source is powered by the 1 2 V line. The other lines are available for future expansion.

The power supply requires a minimum load of 5 W in order to achieve proper load regulation. While a simple power resistor would be sufficient, a small light bulb was chosen for both aesthetic and practical reasons: anechoic test chambers tend to be poorly lit, and the light bulb proved helpful when making measurements.

3 . 1 0 . 1 Poweri n g the M icrocontrol ler

Both the stepper motor and the microcontroller run from the same power source. The stepper motor is intended to be driven with 5 V; however, the first stage of regulation on the MCK specifies an input voltage of 6 V to 24 V to operate properly. In order to power both devices from the regulated 5 V output of the power supply, the first regulation stage is bypassed by directly connecting the power-supply lines to the power pins of the mini-USB interface on the MCK (refer to Section 6 .5 for details) .

4. Software Descri ption

The MCK is open-source and has the ability to run

freeRTOS, which is an open-source real-time operating system. The following subsections summarize software development.

4 . 1 Development Enviro n ment

The "mcbuilder" integrated development environment (IDE) is freely available and used for compiling and uploading the project to the MCK. The MCK uses the C programming


language. All of the code required to compile the freeRTOS is hidden by the integrated development environment to simplify software development.

4.2 G raphical User Interface (G U I)

Since the operating system includes an Internet protocol (IP) stack, the MCK has the ability to send data over any IPbased network. With the functionality of a simple Web server provided by Making Things, a Web page was created and is stored on the microcontroller. The MCK serves the Web page to a laptop or Web-enabled device upon request.

Although the controller is programmed in C, the Web page was written using the JavaScript, CSS, and HTML programming languages. A Web site was chosen over a dedicated application, as Web sites have better portability across many operating systems. By using Asynchronous JavaScript and XML (AJAX), data is able to be loaded in the background without causing a page reload, which results in a user interface with the feel of an application. In addition, the use of the HTML5 specification allows the radiation pattern to be generated algorithmically using JavaScript.

Upon request by the Web-enabled device, the Web server on the microcontroller responds by transmitting a character array containing the Web page. Due to the limited resources of the microcontroller, the number of characters used to implement the Web site was minimized. The user can begin taking measurements once the Web page is displayed. When a button is pressed on the Web site, a special function makes a background request for other Web pages. The microcontroller will take action based on which background Web page was requested. When the controller closes the connection after transmitting the requested data, the Web client detects that the transfer is complete. Actions such as displaying data, performing calculations, or drawing the radiation pattern are then completed by the Web client. An example of the user interface after a full set of measurements is shown in Figure 3 .

4 . 3 Fu n ctional Descri ption

Upon startup, the MCK programs the synthesizer to produce a 2.4 GHz tone using a customized serial communication protocol. The Web server routes corresponding to the various user-input commands are then defined. Each route defines an action that the MCK is required to take. Once initialization is complete, the microcontroller enters its normal mode of operation, and waits for a page request. When the user accesses the graphical user interface, the MCK will respond by sending a large static character array that holds all the required information. When a measurement is requested by the user, the Web server determines which function handler to call. The following subsections discuss the Web-server route-function handlers.

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Figure 3. A screenshot of the user interface, showing a complete set of measurements.

4.3 . 1 Websi teHandler ( . . . ) When called, this function simply returns a static charac

ter array containing the Web site. The MCK no longer takes any action; the graphical user interface is initialized and rendered by the Web-enabled device.

4.3.2 da taQuery ( . . . ) When a gain measurement is requested, the MCK initi

ates a sweep of the synthesizer 's output frequency. At each discrete frequency point, 32 analog-to-digital converter readings are averaged. The resulting ten measurement results are then averaged to produce the phase-insensitive result: refer to Section 3 .7 . 1 for further details. The measurement result is converted to a gain value in accordance with Equation (2) (given in Section 8 . 1 ), and is returned to the graphical user interface. The graphical user interface is responsible for any further calculations, such as compensating for free-space path loss and computing antenna gains.

4.3.3 reCalibrate ( . . . ) Pressing the calibrate button on the graphical user inter

face will trigger this function. The MCK assumes that a 20 dB attenuator is connected between the antenna feed cables and adjusts the offset term in Equation (2) accordingly.

IEEE Antennas and Propagation Magazine, Vol. 53 , No. 4, August 20 1 1 1 9 1

4.3.4 radiationTes t ( . . . ) This function is similar to dat aQue ry ( . . . ) , with the

addition of triggering the stepper motor between each successive measurement. Data are returned to the graphical user interface as they are taken, which eliminates the need for a large storage array in the MCK and ensures continuous data transfer. Once the MCK has completed the tests, the socket connection to the user is closed, which indicates to the graphical user interface that the measurement has finished. The graphical user interface then displays the radiation pattern, while the MCK rotates the antenna-positioning system in reverse to unwind the antenna feed cable.

4.3.5 moveHandler ( . . . ) A Web route has been defined that accepts an argument

that allows the user to specify how many degrees (and in which direction) the antenna positioning system is to be rotated: see Section 7.6 for operation instructions.

5. Antenna Positioning System

The primary considerations associated with the design of the positioner were ease of manufacturing; assembly and integration with a motor and belt-drive assembly; minimal impact on the quality of RF measurements; and low cost. Based on these criteria, a positioning system of primarily wooden construction was selected.

The positioner requires a turntable such that the base remains static while the platform above it is capable of rotation. This is accomplished through the use of a lazy Susan bearing (LS). A threaded-rod shaft is fixed to the upper platform, and extends through the middle of the lazy Susan and the lower platform. A large timing pulley is fixed to the end of the shaft, allowing the turntable to be driven by a stepper motor via a beltdrive assembly (BDA). A photograph of the completed beltdrive assembly installed on the underside of the lower platform is shown in Figure 4.

The belt-drive assembly uses a single timing belt to couple the threaded-rod shaft to the small timing pulley on the motor shaft. Due to challenges associated with attaching the drive shaft to the upper platform at its exact center of rotation, a tension assembly was required. The belt-drive tension assembly (BDTA) ensures that sufficient tension is applied to the timing belt, which mitigates slip in the belt-drive assembly. The entire assembly is mounted underneath the . lower platform: refer to Section 6 for assembly instructions.

Due to variations in antenna geometries, a universal mounting solution was not practical. A slot was cut into the tops of the antenna stands to permit insertion of PCB-type antennas. Other antennas may be mounted through the use of clamps and/or nonmetallic adhesive tape. Note that the antennas must

Figure 4. A photograph of the underside of the lower platform.

not be so large or heavy that the antenna-positioning system is overloaded. Due to the dynamic nature of the positioner, the user must not permit the antenna feed cable to become snagged by the rotating platform.

6 . Assembly

For practical reasons, all structural dimensions in this section are given in imperial units.

6 . 1 Ante n na-Position i n g System

The antenna-positioning system is comprised of a static lower assembly and a rotating upper-platform assembly. A lazy Susan is used as the rotary joint between the two platforms.

6 . 1 . 1 U pper Platform

A 1 ft x 1 ft x 1 in piece of wood forms the base of the upper platform. Note that the use of a drill press for all required drilling is recommended to ensure that holes are square to the assembly. Drill the holes as indicated in Figure 5 . Note that the middle 3/8 in hole is at the center of the base.

Cut a 114 in threaded rod to a length of approximately 3 . 5 in. Screw a nut to one end of the rod. Place a washer on the center hole on the base and feed the threaded rod through it. Install a washer and two nuts on the bottom of the base. Tighten the first nut against the base; then tighten the second nut against it. This ensures that the first nut will not loosen during use.

Cut a piece of 2 in x 3 in wood to approximately 3 1 .5 in in length. Using the dimensions shown in Figure 5, drill 5/8 in holes, as opposed to those indicated. DriIl to a depth of approximately 3/4 in. In each of the two outer holes, screw in a 1 14 in plain insert nut. The purpose of the center hole is


DIA 3/S"

DIA 3/8" I--,---+f--DIA

1+-----3I4"---J. 314" Figure 5. The locations of the drilled holes for the upper platform.

1'+----6"---+1

1+-----j-6 1 12"----.I 6"

9 1 12"

l '

6 1 12"

2 1/2"

1+-------- 1·--------.1

Figure 6. The bottom platform, showing mitered edges, stepper-motor notch, and eyelet screw location.

support structure on which the positioner will rest. Notch two of these pieces such that when they are installed, they do not interfere with the stepper motor. Affix the mitered edges to the bottom platform using two 1 .75 in wood screws per edge. Refer to Figure 4 and Figure 6 for guidance.

Use a hacksaw to increase the gap on a 1 14 in eyelet screw (near the end of the eyelet). Install the eyelet on the inside ofthe mitered edge, as shown in Figure 6.

Install four 1 12 in rubber feet on the bottom of the mitered edges, as shown in Figure 4 .

The lower platform is now assembled.

6 . 1 .3 Positioner Assembly

Center the lazy Susan on the base of the lower platform and mark the mounting holes . Drill these mounting holes using a 1 /8 in drill bit.

Center the lazy Susan on the bottom of the upper platform and secure it using four #4 x 1 12 in wood screws.

Install wood screws through each of the pre-drilled mounting holes on the lower platform and secure it to the lazy

Susan. The upper and lower platforms are now connected through the lazy Susan.

Using a clamp, glue, or a set screw, install the small timing pulley onto the shaft of the stepper motor. Install two nuts, a washer, and the large timing pulley onto the threaded rod. Position the two bottom nuts and washer such that the large timing pulley is at the same height as the motor-shaft timing pulley. Tighten the two nuts in place when the correct position is obtained. Place a washer above the timing pulley and tighten the assembly with another nut.

Cut a piece of 1 18 in-thick aluminum into a rectangle of approximately 2- 1 12 in by 3/4 in. Drill a 1 12 in hole centered 1 12 in from each end of the aluminum rectangle. Drill a 1 14 in hole in the center of the rectangle. Fold the rectangle into a channel such that the edge with the 1 14 in hole is centered and is approximately 1 12 in long. Refer to Figure 7 for a graphical representation of the channel, which is used for the belt-drive tension assembly.

Connect a 1 in spring to a 1 in machine-screw eyelet. Thread a nut onto the eyelet and insert the threaded portion into the end of the assembly shown in Figure 7. Secure the eyelet to the channel with another nut, and tighten the assembly. Install a timing belt onto a medium-sized timing pulley. Place a washer onto the 1 14 in x 1 .5 in bolt and insert it through one end of the channel, through the medium timing pulley, and through the opposite end of the channel. Install a washer and secure loosely into place with a nut. Wrap the belt around the small and large timing pulleys. Connect the opposite end of the spring to the eyelet screw as previously installed (refer to Figure 6). The result should resemble Figure 4.

The antenna positioner is now complete.

6"2 Transm itti ng Antenna Stand

Drill two holes through a 1 ft x 1 ft x 1 in piece of wood in accordance with the outer holes depicted in Figure 5 .

Figure 7 . The belt-drive tension-assembly timing-pulley channel.


Cut a 2 in x 3 in piece of wood to 35 in in length. Drill 5/8 in holes to a depth of 5/8 in into one end of the 2 in x 3 in piece of wood, using the outer holes shown in Figure 5 . Install 1 14 in plain insert nuts into these drilled holes. Install washers onto the 1 .25 in hex bolts, and insert them through the platform into the plain insert nuts installed into the 2 in x 3 in piece of wood. Secure both bolts and install four 1 12 in rubber feet onto the four comers of the bottom of the platform.

The transmitting antenna stand assembly is now complete.

6.3 Pringles Cantenna

The Pringles cantenna was built using instructions available on the Internet [2] . Note that the length of the antenna feed was adjusted until an input VSWR of less than 2 : 1 from 2.4 GHz to 2 .45 GHz was obtained.

6.4 System Mou nti ng

All individual system components were installed onto a 1 12 in thick piece of wood that was cut to 12 in x 20 in. As shown in Figure 2, the components were raised on hex standoffs and installed using machine screws. The placements shown in Figure 2 are not critical and may be modified as required. The terminal block for mounting the light bulb was installed using hot glue. A custom aluminum platform was fabricated to mount the low-noise amplifier.

6.5 E lectrical I nterconnections

A detailed diagram, showing all of the required power connections, is shown in Figure 8. The light-bulb and steppermotor connections were completed used 20 American Wire Gauge (AWG) wire. All other connections were completed using 24 AWG wire.

As shown in Figure 8, 5 V is applied to the MCK through the dc plug mounted on the board. As stated in Section 3 . 1 0. 1 , a custom connection between the input voltage pads to the miniUSB connector is required on the MCK. This is accomplished through the modification of a mini-USB cable, in accordance with the electrical connections shown in Figure 8 .

The MCK provides screw terminals that are used to connect the MCK to the stepper motor. Using these screw terminals, output 0 and 1 are connected to one of the coils of the stepper motor, and outputs 2 and 3 are connected to the other coil of the stepper motor. The center taps of each coil are not connected, and must be electrically isolated from all other connections.

The MCK is also used to program the ADF4360-0 over a custom serial interface. Note that 5 . 1 k n resistors are required on the communication lines. Refer to Figure9 for a detailed diagram outlining the electrical connections between the MCK and both the frequency source and the stepper motor.

Figure 8. A diagram of the interconnections for the power system.

R F Sou rce

Out 7 Out 6

�lo.4-+tl;vr'--'\NI.�--l Out 5 Out 4

L----l GND Vout

Coi l A Stepper

.-----l Out 3 �---I Out 2

�--l Out 1 Out O GND Vout

Motor Coi l B

Make M icrocontrol ier

Figure 9. The microcontroller-to-RF-source and steppermotor interface.

An SMA cable is connected to the GAIN output of the detector. The opposite end of the cable had to be cut such that the inner conductor is exposed and the outer shield is grouped into a pigtail connection. The shield and inner conductor are connected to the pins labeled GND and AINO on the MCK board, respectively.

The light bulb terminal block is connected to the 5 V power supply output, as shown in Figure 8. Stranded wire was soldered to the terminal block and covered with heat-shrink tubing. The terminal block was hot glued to a convenient location on the 1 2 in x 20 in wood. A light bulb was installed into the screw terminals such that the bulb could be easily replaced in the event of a filament failure.

From the ADF4360-0, RFOUT is connected to the transmitting antenna using a 2 .8 m length of RG-3 1 6 coaxial cable. RFOUT is connected directly to the VAT-6+. The VAT-6+ is connected to the VAT-20+ using a 0.3 m length of coaxial cable. The VAT-20+ is connected directly to INPB on the AD8302.

1 94 IEEE Antennas and Propagation Magazine. Vol. 53 , No. 4, August 20 I I

The AUT is connected to the input of the ZX60-272LN+ using a 2 m length of coaxial cable. The VBF2435+ is connected directly to the output of the ZX60-272LN+. The VBF2435+ is connected to INPA on the AD8302 using a 0.3 m length of coaxial cable.

7. Operation

7.1 Basic Setu p

An Ethernet cable is connected from the MCK to a computer with a wired Ethernet connection. The computer network settings are modified as follows: the IP address is set to a static address of 1 92 . 1 68.0 .2 1 0, with a subnet mask of255 .255 .255 .0 . The default gateway does not need to be specified.

A Web browser is opened, and navigated to URL http:// 192. 1 68 .0 .200. The browser will display the graphical user interface, as per Figure 3 . It should be noted that the Web browser needs to have the latest HTML5 specification implemented. As of Version 8, Microsoft Internet Explorer is incapable of rendering the full graphical user interface; a browser such as Mozilla Fire/ox is preferable.

7.2 Cal i bration

To calibrate the system, a 20 dB attenuator is connected between the antenna's feed cables. The "Calibrate" button is clicked. A message is displayed, indicating that calibration was successfully completed. The system can be recalibrated by repeating this procedure; however, note that the only way to clear the calibration is to power-cycle the MCK.

7.3 Antenna Gai n Measurements

Antenna gain measurements are accomplished using the three-antenna method. Three suitable antennas are identified, and designated as numbers 1 , 2 and 3 . In accordance with the "Gain Measurement" block featured in the graphical user interface, antenna 1 is mounted on the transmitting antenna stand, and the appropriate coaxial cable is connected. Antenna 2 is mounted on the antenna-positioning system, and the appropriate coaxial cable is connected.

It must be ensured that the antennas are pointing directly at one another, and that they are polarization aligned. The distance between the two antennas is measured, and the value in meters is entered into the appropriate field. Note that for best results, it is recommended that the antennas be at least 1 .25 m apart. The "Capture" button is clicked. A number that reflects the sum of the two antenna's gains in dB should appear in the "Result" field.

This procedure is repeated for the two remaining antenna combinations. Once all three measurements have been com-

pleted, the individual antenna gains will be displayed in dBi in the "Gain Results" block.

7.4 Rad iation Pattern Measurements

An antenna with 5 dBi to 1 0 dBi of gain is mounted on the transmitting-antenna stand, and the appropriate coaxial cable is connected. The AUT is mounted on the antenna-positioning system, and the appropriate coaxial cable is connected. Polarization alignment between the two antennas must be ensured, along with an appropriate separation distance for far-field measurements. In the "Radiation Pattern" block, the "Measure Pattern" button is clicked. The antenna-positioning system should begin to rotate the AUT. It must be ensured that the coaxial cable feeding the AUT does not interfere with the operation of the antenna-positioning system. The radiation pattern will be displayed once a full rotation has been completed. The antenna-positioning system will execute a full reverse rotation to unwrap the coaxial feed cable.

The raw measurement data can be accessed by clicking the "Show raw data" link. This data can be selected and copied from the browser window for use in an external application. A copy of the radiation pattern may also be saved as an image in the Portable Network Graphics (PNG) format by clicking the "Save Graph" link.

7.5 Optional Network Mode

The firmware on the MCK sets the default IP address to 1 92 . 1 68 .0.200, but this can be dynamically reassigned should the user decide to connect the device to a network that has DHCP enabled. A limitation of the device is that there is no feedback to indicate the assigned IP address; it is left to the user to determine.

7.6 Add itional F u n ctional ity

To determine the calibration value used by the MCK, the user can navigate his or her Web client to the "/recal" subdirectory, which will trigger a recalibration and will display the value of the variable cal: this variable modifies the offset of Equation (2) .

The user can manually rotate the antenna-positioning system by navigating to the "/m?m=" directory and appending the number of degrees of rotation to the address of the graphical user interface. This argument can be negative. Note that a leading zero is required for rotations of less than 1 0° .

8. System Evaluation

The complete system, set up for measurements in the anechoic test chamber, is shown in Figure 10 . The following sections report the evaluation of the performance of the system.


Figure 10. The complete system set up for measurements in the anechoic test chamber.

O r---�---.----�---.----.---�---.----. 3

-1 0

-20 iii -0 � -30 0;

o

-40

-- linear F� o Measured Data

_ .. _ .... linearity Error

.. /1 /

�OOL---�1 ��--200��-2��··�-·�300�---��·--···-···-·��·��·-·--��

Figure 11. The system's linearity performance.

8 . 1 Gai n -Meas u rement Accu racy

The ability of the system hardware to accurately measure the gain between the antenna feed cables was tested using an S-band variable attenuator. The performance of the attenuator was characterized in 5 dB steps, from approximately - 1 0 dB to -60 dB using a calibrated Agilent performance network analyzer (PNA). The attenuator was then connected between the antenna feed cables and the analog-to-digital converter outputs were recorded as a function of attenuation. The relationship between gain and analog-to-digital converter output was determined using linear regression. This is shown in Equation (2), where G is the gain between the antenna feed cables in dB, and D is the analog-to-digital converter output:

G = 0. I 1 1 7D - 67 . (2)

In Section 3 .7 , the nominal slope relating the gain between INPA and INPB to the analog-to-digital converter result was determined to be 0. 1 075 . The experimental slope was 0. 1 1 1 7, representing a 4% relative error. The offset changed since the gain was no longer being measured between INPA and INPB.

The linearity performance of the system is depicted in Figure 1 1 . Specifically, the linearity error associated with Equation (2) is presented. Based on the accuracy specification of ±0.5 dB, the dynamic-range performance of the system could be determined. The absolute error remained below 0.5 dB for gain values between - 1 3 dB and -55 dB, resulting in a maximum dynamic range of 42 dB. For a free-space path loss of 42 dB and 0 dBi antennas, the minimum dynamic range was 1 3 dB. These results showed that the complete system had reduced dynamic-range performance relative to the AD8302. This was expected, as additional linearity error was introduced by each hardware component in the system, coupled with quantization errors introduced by the analog-to-digital converter and subsequent microcontroller calculations. As mentioned in Section 3 .2, it is recommended that the transmitting antenna exhibit 5 dBi to 1 0 dBi of gain in order to improve dynamicrange performance.

8.2 Antenna Gain Measurements

Three commercial off-the-shelf antennas were acquired in order to evaluate the performance of the system prior to characterizing the cantenna. A 2.4 GHz monopole antenna was purchased, due to its relatively constant H-plane radiation pattern. A 2.4 GHz printed circuit board (PCB) Yagi antenna was purchased, due to its relatively high forward gain, and a PCB log-periodic dipole (LPD) antenna, which operated over 900 MHz to 2.6 GHz, was borrowed from UNB for testing purposes. The input VSWR of each of these antennas was confirmed to be less than 2: 1 between 2.4 GHz and 2.45 GHz, using the performance network analyzer.

The three-antenna method was carried out for the commercial off-the-shelf antennas inside an anechoic test chamber, using both the system and the performance network analyzer. The results are summarized in Table 1 . The system was confirmed to meet accuracy specifications, as the absolute error associated with the antenna gain measurements was less than 0.5 dB.

The three-antenna method was repeated using the Yagi, monopole, and cantenna. The antenna gains were measured to be 5 .6 dBi, 0.2 dBi, and 5 . 5 dBi, respectively. Despite a somewhat legendary status among RF hobbyists on the Internet, the Pringles cantenna fell short in its promise of providing upwards of 1 2 dBi of antenna gain. However, at a cost of less than $ 1 5 in parts, the cantenna offered 5 .5 dBi of gain, which rivals the gain offered by a commercial off-the-shelf PCB Yagi antenna sold at over double the price. Also, unlike the Yagi, the Pringles cantenna includes a delicious snack.

8.3 Antenna Rad iation-Pattern Measurements

H-plane radiation-pattern measurements were made for the Yagi and monopole antennas using the system inside an anechoic test chamber, and were compared to results obtained


Table 1. Antenna gain measurements.

Antenna PNA Resuits isoTropic Thunder Absolute Error

(dBi)

Yagi 5 .6 Monopole 0.3

LPD 4.7

using the UNB Antenna Positioning System with the performance network analyzer. Figures 1 2 and 1 3 show the results. Note that the angular resolution for each measurement was 1 °, and the data was normalized such that the pattern maximum was 0 dB.

While the UNB system had greater accuracy and dynamicrange performance, there was strong agreement between the pattern results . As the measurements were taken using different measurement hardware and with different feeding cable arrangements, some variation in the measured patterns was expected. Nevertheless, it was clear that the system was capable of making high-quality automated radiation-pattern measurements.

The H-plane radiation pattern of the Pringles cantenna was measured using the validated system, and is shown in Figure 14 . As expected, the cantenna pattern was qualitatively similar to that of a Yagi antenna, with a front-to-back ratio of approximately 1 1 dB. It should be noted that since the cantenna feed was unbalanced and lacked a proper ground connection, the radiation-pattern results were very sensitive to feeding cable orientation.

8.4 Budgetary Considerations

A bill of materials is included in the Appendix. The total cost to reproduce the system was $ 1 240.64 (CAD), which was in compliance with the maximum specified budget of $ 1 500.

R---J 90°

1 80°

I ·· .... ··· PNA Pattern 1 -- isoTropic Thunder Pattam

Figure 12. The radiation-pattern measurements for the Yagi antenna.

Results (dBi) (dB)

5 .4 0.2 0.2 0 . 1 4.6 0. 1

1 80° ········· PNA Pattern -- isoTropic Thunder Pattern

Figure 13. The radiation-pattern measurements for the monopole antenna .

180°

Figure 14. The radiation-pattern measurement for the cantenna.

9. Conclusion

The motivation for this project was to design and build a system capable of making antenna-gain and radiation-pattern measurements with an accuracy of ±0.5 dB for less than $ 1 500 in cost. Due to the fact that many of the specified components were commercial off-the-shelf, the system presented here can be easily reproduced, and met budgetary constraints at a cost of $ 1 240.64. It has been shown to achieve a gain measurement accuracy of ±0.5 dB over a dynamic range of 1 3 dB plus the combined gains of the two antennas in use. Antenna-gain and radiation-pattern measurements made inside an anechoic test chamber were validated through comparison with results obtained using a commercial performance network analyzer


and the UNB antenna-positioning system. The flexible nature of the graphical user interface allows for system access and control independent of operating system or hardware platform.

The system has been used to measure the gain and radiation pattern of a homemade Pringles cantenna. Despite claims from Internet RF hobbyists that the cantenna is capable of achieving a gain of 12 dBi, measurements made by the system indicate a gain of 5 . 5 dBi .

In summary, the system presented here is accurate, economical, robust, and easily reproducible. With minor enhancements, the system would be suitable for use in a learning environment such as an undergraduate laboratory.

1 0 . Acknowledgements

The authors would like to thank Dr. Bruce Colpitts, Dr. Brent Petersen, Ryan Jennings, Michael Wylie, and Lars Woodhouse for their support and guidance throughout the course of the project.

1 1 . Appendix

The bill of materials is summarized in Table 2. Note that all prices are in Canadian dollars.

Table 2. Bill of materials.

Quantity Cost

Vendor Part ($) AD8302 Evaluation Board 1 2 1 2.99 Analog Devices

ADF4360-0 1 1 2 1 .44 Analog Devices

2 m SMA cable 1 26.70 Assemble / Digi-Key

2 .5 m SMA cable 1 30.33 Assemble / Digi-Key

0.3 m SMA cable 5 53 .05 Digi-Key

2 Position Terminal Block 1 0.32 Digi-Key

42L048D I U 1 24.00 Digi-Key

9 V Battery Snap Connector 1 0 .35 Digi-Key

DB9 Female 1 3 .94 Digi-Key

Ethernet Cable 1 3 .50 Digi-Key

Heat Shrink 1 6.00 Digi-Key

Molex Headers and Pins 1 5 .00 Digi-Key

Monopole Antenna 1 5 . 1 9 Digi-Key

Power Cord (5 .2mm barrel jack) 1 2 .37 Digi-Key

Power Cord (AC with Ground) 1 5 .00 Digi-Key

Power Supply 1 94.5 1 Digi-Key

Resistors 3 1 .00 Digi-Key

Ring Connector 1 0 .35 Digi-Key

SMA Barrels 3 1 4. 1 3 Digi-Key

Standoffs 20 5 .00 Digi-Key

USB A to mini B Cable 1 4 . 1 3 Digi-Key

Wire, 20 AWG 1 25 .70 Digi-Key

Wire, 24 AWG 2 34.80 Digi-Key

Cantenna Parts 1 1 5 .00 Grocery/Hardware

Store Belt Drive Tension Parts 1 8 .50 Hardware Store

1 I4x l - 1 I2" Hex Bolts 4 2 .00 Hardware Store

2"x2"x8 ' Wood Stud 1 1 .50 Hardware Store

2"x3"x8 ' Wood Stud 1 2 .50 Hardware Store

Dowel (3/8"O.D.x4' ) 1 3 .48 Hardware Store

Large Lazy Susan 1 6 . 1 9 Hardware Store

Light bulb 1 1 .50 Hardware Store


Table 2. Bill of materials (continued).

MDF ( 1 ' x 4') 1 5 .00 lIardvvare Store

Nuts (#8) 1 2 1 . 80 lIardvvare Store

Nuts (3/8") 6 1 .50 lIardvvare Store

Plain Insert Nut 4 2 .50 lIardvvare Store

I " D.D. Screvv-in Rubber Feet 1 2 1 2 .00 lIardvvare Store

Scrap Aluminum 1 5.00 lIardvvare Store

Screvvs (#4) 36 3 .00 lIardvvare Store

Screvvs (#8) 1 2 2 .79 lIardvvare Store

1 " Thick Wood 1 1 0 .00 lIardvvare Store

Threaded Rod (3/8") 1 12 .00 lIardvvare Store

Washers (#4) 26 1 . 82 lIardvvare Store

Washers (#8) 1 2 1 .08 lIardvvare Store

Washers (3/8") 6 2 .00 lIardvvare Store

Wire Clamps 14 5 .00 lIardvvare Store

Wood Screvvs (pack) 1 3 . 1 8 lIardvvare Store

Make Microcontroller 1 1 20.00 Making Things

VAT-20+ 2 29.60 Mini-Circuits

VAT-6+ 3 14 .80 Mini-Circuits

VBF2435+ 1 43 .06 Mini-Circuits

ZX60-272LN+ 1 49.95 Mini -Circuits

Yagi Antenna 1 32.95 Ramsey Electronics

Belt (A 6B 6M 1 93060) 1 4.47 Stock Drive Products

Timing Pulley 1 3 .27 Stock Drive Products

(A 6M 6M1 0DF06003) Timing Pulley

1 3 .37 Stock Drive Products (A 6M 6M25DF06008) Timing Pulley

1 7.29 Stock Drive Products (A 6M 6M75DF06008)

Tax :Ii

1 42_74 Total $ (CAD) 1240.64


1 2 . References

1 . C. A. Balanis, Antenna Theory: Analysis and Design, Third Edition, New York, John Wiley & Sons, Inc . , 2005 .

2. R. Flickenger, "Antenna on the Cheap (er, Chip)," July, 200 1 , July; available at http ://www.oreillynet.com/cs/weblog/ view/ wlg/448.

3 . Analog Devices, ADF4360-0 Datasheet Rev. A, Norwood, MA, 2004.

4. Analog Devices, AD8302 Datasheet Rev. A, Norwood, MA, 2002.

Introd ucing the Authors

Brandon C. Brown was born in Kitchener, Ontario, in 1 983 . He received a Bachelor of Applied Science (Computer Engineering) from Queen's University in 2006. After spending a short time working in industry, he enrolled at the University of New Brunswick (UNB) and received his Masters in 2007. Currently, he is enrolled at UNB and is working toward a PhD degree. His research interests include wireless systems, signal propagation, and various aspects of networking.

Frederic G. Goora was born in Sydney, Nova Scotia, in 1 977. He received a Bachelor of Science in Engineering (Electrical Engineering) and a Master of Science in Electrical Engineering from the University of New Brunswick (UNB) in 2000 and 2003, respectively. After more than six years of industrial experience, he returned to UNB and is currently pursuing a PhD degree in Electrical Engineering. His research interests include magnetic-resonance imaging and microwave systems. He is registered as a Professional Engineer in New Brunswick.

Chris D. Rouse was born in Halifax, Nova Scotia, in 1 986. He received a Bachelor of Science in Engineering (Electrical Engineering) from the University of New Brunswick (UNB) in 2009, and is currently pursuing a PhD degree in Electrical Engineering at UNB. His research interests include wireless systems, communications, and fiber optics. @

200 IEEE Antennas and Propagation Magazine, Vol. 53, No. 4, August 20 1 1

the design of an economical antenna-gain and radiation-pattern measurement system

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