ANTENNA AND PROPAGATION

Electromagnetic WavesRadio wave has both electric (E) field and

magnetic (H) fieldE and H fields are transverse i.e. at right

angles to the direction of the wave propagationE and H are mutually perpendicular i.e. at

right angles to each otherE and H are in phaseVelocity of electromagnetic waves in free space

is equal to c (speed of light ≈ 3 × 108 m/s)In other media, the velocity can be less than c

Antenna Antenna = metallic conductor system

capable of radiating and capturing electromagnetic energy

In a free-space radio communications system,At transmitter

Antenna converts electrical energy travelling along a transmission line into electromagnetic waves that are emitted into space

At receiver Antenna converts electromagnetic waves in

space into electrical energy on a transmission line

Waveguide = special type of transmission line that consists of conducting metallic tube through which high-frequency electromagnetic energy is propagated

Radio waves = electrical energy that has escaped into free space in the form of transverse electromagnetic waves

Basic antenna operationSize of antenna is inversely proportional to

frequencyHigh-frequency waves require small antennaLow-frequency waves require large antenna

Every antenna has directional characteristics i.e. it radiates more energy in certain directions relative to other directions

Radiation PatternRadiation pattern = polar diagram or graph

representing field strengths or power densities at various angular positions relative to an antenna

Terms:Major lobe(s) = the primary beam(s)Minor lobe(s) = secondary beam(s)Front lobe = front of the antenna, where the major lobe isSide lobe(s) = lobe(s) adjacent to the front lobeBack lobe = lobe in a direction exactly opposite to the

front lobeLine of shoot = the line bisecting the major lobe (pointing

from the center of antenna to the direction of maximum radiation)

Major lobes propagates/receives the most energy

Minor lobes normally represent undesired radiation/reception

Radiation from an actual antenna is 3-dimensional. Therefore, radiation patterns are taken in both the horizontal and vertical planes

Near Field and Far FieldNear field

Radiation field that is close to the antennaPower in this field is continuously radiated and

returned back to the antennaFar field

Radiation field that is far from the antennaPower that reaches the far field continues to

radiate outward and is never returned to the antenna

Near Field and Far Field

Not all the power supplied to antenna is radiated. Some are lost as heat

Radiation resistance:

Rr = radiation resistance (Ω)

Prad = power radiated by antenna (W)

i = antenna current at the feedpoint (A)

2i

PR radr

Antenna efficiency:

η = antenna efficiency (%)Prad = radiated power (W)

Pin = input power (W)

100in

rad

P

P

In terms of resistance and current, antenna efficiency is:

η = antenna efficiency (%)Rr = radiation resistance (Ω)

Re = effective antenna resistance (Ω)

100

er

r

RR

R

Antenna GainDirective gain:

D = directive gain (no unit) P = power density at some point with a given antenna (W/m2) P ref = power density at the same point with a reference antenna (W/m2)

ref

D P

P

Power gain, Ap:

D = directive gain (no unit)η = antenna efficiency

DAp

Example 1Given a transmit antenna with radiation

resistance Rr = 72 Ω, effective antenna resistance Re = 8 Ω, directive gain D = 20, and input power Pin = 100 W. Calculate:Antenna efficiencyAntenna gainRadiated power in watts and dBm

Friis TransmissionFriis Transmission Equation is used for

calculating the power received by the receiver antenna from the transmitter antenna.

The transmitter and receiver are separated by a certain distance R

Both antennas are operating at a certain frequency f

Friis Equation

PLF = Polarization Loss FactorPT = transmitted power (W)PR = received power (W)GT = transmitter gainGR = receiver gainc = speed of light (ms-1)f = frequency of operation (Hz)R = separation distance between antennas (m)Note: PLF = 1 if both antennas are polarization-matched

22

4)(

Rf

cGGPPLFP RTT

R

Basic AntennaThe simplest type of antenna is the elementary

doubletElementary doublet has uniform current throughout

its length. Instantaneous value of the current:

i(t) = instantaneous current (A)I = peak amplitude of the RF current (A)f = frequency (Hz)t = instantaneous time (s)θ = phase angle (rad)

ftIti 2sin)(

Antenna Noise TemperatureAntenna noise temperature = a parameter

which describes how much noise an antenna produces in an environment

Types of antenna noise:Noise due to the loss resistance of the antenna

itselfNoise picked up from the surroundings

Power radiated by a noise source, when intercepted by an antenna, generates power PA at its terminals

Antenna temperature, TA = equivalent temperature associated with the received power PA

PA = received power (W)

K = Boltzmann’s constant (≈ 1.38 × 10-23 J/K)∆f = frequency band (Hz)TA = antenna temperature (K)

fkTP AA