Basic Communication theory

2. Electromagnetic Waves
 Radio wave has both electric (E) field and magnetic
(H) field
 E and H fields are transverse i.e. at right angles to the
direction of the wave propagation
 E and H are mutually perpendicular i.e. at right
angles to each other
 E and H are in phase
 Velocity 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

3. 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

4.  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

5. Basic antenna operation
 Size of antenna is inversely proportional to frequency
 High-frequency waves require small antenna
 Low-frequency waves require large antenna
 Every antenna has directional characteristics i.e. it
radiates more energy in certain directions relative
to other directions

6. Radiation Pattern
 Radiation 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 is
 Side lobe(s) = lobe(s) adjacent to the front lobe
 Back lobe = lobe in a direction exactly opposite to the front
lobe
 Line of shoot = the line bisecting the major lobe (pointing
from the center of antenna to the direction of maximum
radiation)

7.  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

8. Near Field and Far Field
 Near field
 Radiation field that is close to the antenna
 Power in this field is continuously radiated and returned
back to the antenna
 Far field
 Radiation field that is far from the antenna
 Power that reaches the far field continues to radiate
outward and is never returned to the antenna

9. Near Field and Far Field

10.  Not all the power supplied to antenna is radiated.
Some are lost as heat
 Radiation resistance:
P
R rad
2 i
r 
Rr = radiation resistance (Ω)
Prad = power radiated by antenna (W)
i = antenna current at the feedpoint (A)

11.  Antenna efficiency:
rad
P
 100
η = antenna efficiency (%)
Prad = radiated power (W)
Pin = input power (W)
in
P


12.  In terms of resistance and current, antenna efficiency
is:
100
R
r
R 
R

r e

η = antenna efficiency (%)
Rr = radiation resistance (Ω)
Re = effective antenna resistance (Ω)

13. Antenna Gain
 Directive gain:
D  P
P
ref
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)

14.  Power gain, Ap:
A D p 
D = directive gain (no unit)
η = antenna efficiency

15. Example 1
 Given 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 efficiency
 Antenna gain
 Radiated power in watts and dBm

16. Friis Transmission
 Friis 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

17. Friis Equation
P G G c
P PLF T T R
R
 2
2
4
( )
Rf


 PLF = Polarization Loss Factor
 PT = transmitted power (W)
 PR = received power (W)
 GT = transmitter gain
 GR = receiver gain
 c = 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

18. Basic Antenna
 The simplest type of antenna is the elementary doublet
 Elementary doublet has uniform current throughout its
length. Instantaneous value of the current:
i(t)  I sin2ft  
i(t) = instantaneous current (A)
I = peak amplitude of the RF current (A)
f = frequency (Hz)
t = instantaneous time (s)
θ = phase angle (rad)

19. Antenna Noise Temperature
 Antenna 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 itself
 Noise picked up from the surroundings
 Power radiated by a noise source, when intercepted by
an antenna, generates power PA at its terminals

20.  Antenna temperature, TA = equivalent temperature
associated with the received power PA
P kT f A A 
PA = received power (W)
K = Boltzmann’s constant (≈ 1.38 × 10-23 J/K)
Δf = frequency band (Hz)
TA = antenna temperature (K)