Antennas For UWB Communications

Introduction An antenna is an electrical conductor or

system of conductors Transmission - radiates electromagnetic energy

into space Reception - collects electromagnetic energy

from space In two-way communication, the same

antenna can be used for transmission and reception

Antenna Definition

An antenna is a circuit element that provides a transition form a guided wave on a transmission line to a free space wave and it provides for the collection of electromagnetic energy.

Antenna Definition-cont’d

In transmit systems the RF signal is generated, amplified, modulated and applied to the antenna

In receive systems the antenna collects electromagnetic waves that are “cutting” through the antenna and induce alternating currents that are used by the receiver

Reciprocity

An antenna ability to transfer energy form the atmosphere to its receiver with the same efficiency with which it transfers energy from the transmitter into the atmosphere

Antenna characteristics are essentially the same regardless of whether an antenna is sending or receiving electromagnetic energy

Polarization Polarization is the direction of the electric

field and is the same as the physical attitude of the antenna A vertical antenna will transmit a vertically

polarized wave The receive and transmit antennas need to

possess the same polarization

Types of Antennas Isotropic antenna (idealized)

Radiates power equally in all directions Dipole antennas

Half-wave dipole antenna (or Hertz antenna) Quarter-wave vertical antenna (or Marconi

antenna) Parabolic Reflective Antenna

beamwidth

antenna

• A

Power 3dB down from maximum point A

Max power

2 dipole

Directional Antenna

Radiated energy is focused in a specific direction

Beamwidth

Beamwidth is the angular separation of the half-power points of the radiated pattern

Half-wave Dipole (Hertz) Antenna

An antenna having a physical length that is one-half wavelength of the applied frequency is called a Hertz antenna or a half-wave dipole antenna. Hertz antennas are not found at frequencies below 2MHz because of the physical size needed of the antenna to represent a half-wave

Vertical (Marconi) Antenna

Vertical Antennas are used for frequencies under 2 MHz. It uses a conducting path to ground that acts as ¼ wavelength portion the antenna above the ground. The above ground structure represents a /4 wavelength

Vertical (Marconi) Antenna – cont’d

Poor grounding conditions of the earth/soil surrounding the antenna can result in serious signal attenuation. This problem is alleviated by installing a counterpoise

Counterpoise

Counterpoise is a grounding grid established where the earth grounding cannot satisfy electrical requirements for circuit completion. It is designed to be non-resonant at the operating frequency

Counterpoise-cont’d

supports

antenna

radius = ¼

Antenna Array

Antenna array is a group of antennas or antenna elements arranged to provide the desired directional characteristics. Generally any combination of elements can form an array. However, equal elements in a regular geometry are usually used.

Yagi-Uda Antenna

The Yagi-Uda antenna is a simple form of a directional antenna based off of a reflector placed /4 from the dipole antenna’s placement. Complex analysis to define the radiated patterns are experimental rather than theoretical calculations

Yagi-Uda Antenna-cont’d

reflector/2

dipole antenna

/4

antenna

2 dipole radiated signal without reflector

2 dipole radiated signal with reflector

Radiated Directed Signal

The Antenna Formula

c 186,000 misec

• c is the speed of lightl is the wavelength of the signall use 3 x 108 when dealing in meters for the speed of light

frequency of the signal

The Antenna Formula - applied

If a half-wave dipole antenna needed to be constructed for a 60 Hz signal, how large would it need to be?

c 186,000 misec

60= 3100 mi

2 = 1550 miles!

Radiation & Induction Fields The mechanics launching radio

frequencies from an antenna are not full understood. The RF fields that are created around the antenna have specific properties that affect the signals transmission. The radiated field field is known as the (surprisingly!) radiation field

Radiation & Induction Fields-cont’d There are two induction fields or

areas where signals collapse and radiate from the antenna. They are known as the near field and far field. The distance that antenna inductance has on the transmitted signal is directly proportional to antenna height and the dimensions of the waveR 2D2

Radiation & Induction Fields-cont’d

R 2D2

Where: R = the distance from the antenna

D = dimension of the antenna

= wavelength of the transmitted signal

Radiation Resistance

Radiation Resistance is the portion of the antenna’s impedance that results in power radiated into space (i.e., the effective resistance that is related to the power radiated by the antenna. Radiation resistance varies with antenna length. Resistance increases as the increases

Effective Radiated Power (ERP)

ERP is the power input value and the gain of the antenna multiplied together dBi = isotropic radiator gain dBd = dipole antenna gain

Radiation Pattern Radiation pattern is an indication

of radiated field strength around the antenna. Power radiated from a /2 dipole occurs at right angles to the antenna with no power emitting from the ends of the antenna. Optimum signal strength occurs at right angles or 180° from opposite the antenna

Radiation Patterns Radiation pattern

Graphical representation of radiation properties of an antenna

Depicted as two-dimensional cross section Beam width (or half-power beam width)

Measure of directivity of antenna Reception pattern

Receiving antenna’s equivalent to radiation pattern

Radiation Pattern for Vertical Antennas

antenna

/4

/2

Antenna Gain Antenna gain

Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna)

Effective area Related to physical size and shape of antenna

Antenna Gain

Antenna gain is the measure in dB how much more power an antenna will radiate in a certain direction with respect to that which would be radiated by a reference antenna

Antenna Gain Relationship between antenna gain and effective

area

G = antenna gain Ae = effective area f = carrier frequency c = speed of light (» 3 ´ 108 m/s) = carrier wavelength

2

2

2

44

c

AfAG ee

Propagation Modes Ground-wave propagation Sky-wave propagation Line-of-sight propagation

Ground Wave Propagation

Ground Wave Propagation Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example

AM radio

Sky Wave Propagation

Sky Wave Propagation Signal reflected from ionized layer of atmosphere

back down to earth Signal can travel a number of hops, back and forth

between ionosphere and earth’s surface Reflection effect caused by refraction Examples

Amateur radio CB radio

Line-of-Sight Propagation

Line-of-Sight Propagation Transmitting and receiving antennas must be

within line of sight Satellite communication – signal above 30 MHz not

reflected by ionosphere Ground communication – antennas within effective line

of site due to refraction Refraction – bending of microwaves by the

atmosphere Velocity of electromagnetic wave is a function of the

density of the medium When wave changes medium, speed changes Wave bends at the boundary between mediums

Line-of-Sight Equations Optical line of sight

Effective, or radio, line of sight

d = distance between antenna and horizon (km) h = antenna height (m) K = adjustment factor to account for refraction,

rule of thumb K = 4/3

hd 57.3

hd 57.3

Line-of-Sight Equations Maximum distance between two antennas

for LOS propagation:

h1 = height of antenna one h2 = height of antenna two

2157.3 hh

LOS Wireless Transmission Impairments

Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction Thermal noise

Thermal Noise Thermal noise due to agitation of electrons Present in all electronic devices and

transmission media Cannot be eliminated Function of temperature Particularly significant for satellite

communication

Noise Terminology Intermodulation noise – occurs if signals with

different frequencies share the same medium Interference caused by a signal produced at a frequency

that is the sum or difference of original frequencies Crosstalk – unwanted coupling between signal

paths Impulse noise – irregular pulses or noise spikes

Short duration and of relatively high amplitude Caused by external electromagnetic disturbances, or

faults and flaws in the communications system

Other Impairments Atmospheric absorption – water vapor and

oxygen contribute to attenuation Multipath – obstacles reflect signals so that

multiple copies with varying delays are received

Refraction – bending of radio waves as they propagate through the atmosphere

Multipath Propagation

Multipath Propagation Reflection - occurs when signal encounters a

surface that is large relative to the wavelength of the signal

Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave

Scattering – occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less

The Effects of Multipath Propagation

Multiple copies of a signal may arrive at different phases If phases add destructively, the signal level

relative to noise declines, making detection more difficult

Intersymbol interference (ISI) One or more delayed copies of a pulse may

arrive at the same time as the primary pulse for a subsequent bit

Types of Fading Fast fading Slow fading Flat fading Selective fading Rayleigh fading Rician fading

Error Compensation Mechanisms Forward error correction Adaptive equalization Diversity techniques

Forward Error Correction Transmitter adds error-correcting code to data

block Code is a function of the data bits

Receiver calculates error-correcting code from incoming data bits If calculated code matches incoming code, no error

occurred If error-correcting codes don’t match, receiver attempts

to determine bits in error and correct

Adaptive Equalization Can be applied to transmissions that carry analog

or digital information Analog voice or video Digital data, digitized voice or video

Used to combat intersymbol interference Involves gathering dispersed symbol energy back

into its original time interval Techniques

Lumped analog circuits Sophisticated digital signal processing algorithms

Antenna Height

Antenna height above the ground is directly related to radiation resistance. Ground reflections causing out-of-phase signals to be radiated to receiving antennas will degrade the transmission. Physical length and electrical length of most antennas are approximately 95% of the physical length. Ideal antenna height is usually based on trial and error procedures

Smart Antennas

Smart Antennas smart antennas are base station

antennas with a pattern that is not fixed, but adapts to the current radio conditions

smart antennas have the possibility for a large increase in capacity: an increase of three times for TDMA systems and five times for CDMA systems has been reported.

Smart Antennas-cont’d Major drawbacks and cost factors

include increased transceiver complexity and more complex radio resource management

Smart Antennas-cont’d

The idea of smart antennas is to use

base station antenna patterns that

are not fixed, but adapt to the

current radio conditions. This can

be visualized as the antenna

directing a beam toward the

communication partner only

Smart Antennas-cont’d Smart antennas add a new way of

separating users, namely by space, through SDMA (space division multiple access)

By maximizing the antenna gain in the desired direction and simultaneously placing minimal radiation pattern in the directions of the interferers, the quality of the communication link can be significantly improved

Elements of a Smart Antenna

Smart antennas consists of a number of radiating elements, a combining/dividing network and a control unit

Phased Array Antenna

Phased Array antennas are a combination of antennas in which there is a control of the phase and power of the signal applied at each antenna resulting in a wide variety of possible radiation patterns

Types of Intelligent Antennas

Switched lobe (SL): This is also called switched beam. It is the simplest technique, and comprises only a basic switching function between separate directive antennas or predefined beams of an array. The setting that gives the best performance, usually in terms of received power, is chosen

Intelligent Antennas-cont’d

Dynamically phased array (PA): By including a direction of arrival (DoA) algorithm for the signal received from the user, continuous tracking can be achieved and it can be viewed as a generalization of the switched lobe concept

Intelligent Antennas-cont’d

Adaptive array (AA): In this case, a DoA algorithm for determining the direction toward interference sources (e.g., other users) is added. The radiation pattern can then be adjusted to null out the interferers. In addition, by using special algorithms and space diversity techniques, the radiation pattern can be adapted to receive multipath signals which can be combined. These techniques will maximize the signal to interference ratio (SIR)

SMDA

Space Division Multiple Access (SDMA) implies that more than one user can be allocated to the same physical communications channel simultaneously in the same cell, only separated by angle. In a TDMA system, two users will be allocated to the same time slot and carrier frequency at the same time and in the same cell

SMDA-cont’d

In systems providing full SDMA, there will be much more intracell handovers than in conventional TDMA or CDMA systems, and more monitoring by the network is necessary

Antenna Installation Considerations

Safety standard operating procedure

priority Grounding

lightning strikes static charges

Surge protection lightning searches for a second

path to ground

Antenna Installation Considerations-cont’d

Adaptive array antenna placement needs to be considered differently than current technologies serving the mobile environment. They need to be place so they have a greater angular approach to the receiving units. Existing tower placement with close proximity to roads and highways would need to be reconsidered.

Antenna Installation Considerations

Base, mast, and supporting structure needs clearance, serviceability (access), and complies with state, federal, and municipal guidelines