lecture 2.ppt

1. Antenna and Propagation
By
Waheed ur Rehman
wahrehman@gmail.com

2. Antenna
• Electrical conductor for conduction
electromagnetic energy
• Electric energy < -- > electromagnetic
energy
• Antenna characteristics are essentially the
same whether an antenna is sending or
receiving electromagnetic energy.

3. Radiation pattern
• A common way to characterize the
performance of an antenna
• Graphical representation of the radiation
properties of an antenna
• The simplest pattern is produced by an
idealized antenna known as isotropic
antenna (sphere with an antenna in
center)

4. Radiation pattern

5. Isotropic Antenna
• Isotropic Antenna is a point in space that
radiate power in all directions equally
• Its not a practical antenna just used as a
reference.

6. Antenna Types
• Isotropic radiator
• Monopole
• Loop
• dipole
• Rhombic
• Dielectric Rod
• Yagi
• Horn
• Parabolic dish
• Patch
• Dielectric lens
• arrays

7. Dipole Antenna
• Lamda / 2 half wave dipole
• Lamda / 4 Quarter-wave dipole

8. Parabolic Antenna
• Used for terrestrial and satellite
communication
• Signals reflected back to focus

9. Antenna Types

10. Antenna Types

11. Antenna types

12. Antenna Gain
• Measure of directivity of an antenna
• Defined as power output in a particular direction,
compared to that produced in any direction by a
perfect radiator (isotropic)
• Its only possible on the expense of radiation in
other directions
• If an antenna has a gain of 3dB that means that
antenna improves upon isotropic antenna in that
direction by 3dB.

13. Antenna Gain
• Antenna gain is not related to more output
power but with directionality
• Effective area of an antenna is related to
that of the physical size of the antenna
and its shape
G = 4 х pi х Ae/lamda ^2
Ae is different from antenna to antenna

14. Propagation Modes
• A radiated signal from antenna travels
along one of three routes
– Ground wave
– Sky wave
– LOS
• We will only be concerned with LOS

15. Ground Wave Propagation
• More or less follow the earth curvature
• Propagate considerable distance well over the visual
horizon
• Frequency upto 2MHz
• One factor is that electromagnetic wave induces a
current in the earth’s surface (causes a bend towards the
earth)
• Another factor is diffraction
• Electromagnetic waves in this frequency range are
scattered in such a way that they don’t penetrate the
upper atmosphere.
• AM radio

16. Ground Wave Propagation

17. Ground Wave Propagation

18. Sky Wave Propagation
• Signal from the earth-based antenna is
reflected from the ionized layer of the
upper atmosphere back down to the earth
• Seems like reflection but actually
refraction.
• 2 – 30 MHz
• BBC , VoA

19. Sky Wave Propagation

20. Sky Wave Propagation

21. LOS Propagation
• Above 30MHz
• Not reflected by ionosphere (satellite
comm)
• For ground-based LOS communication
both Tx and Rx antennas must be within
effective LOS of each others
• Optical Vs Radio LOS

22. LOS Propagation
• Optical LOS
– d = 3.57 √h
– d= distance b/w antenna and horizon
– h= height of antenna in meters
• Effective / Radio LOS
– d=3.57√Kh
– K = adjustment factor to account for refraction,
typically K=4/3
• Max distance between two antennas
– 3.57 (√Kh1 + √Kh2)
– h1, h2 are height of antennas

23. LOS Transmission
• Signal received is not similar to signal
transmitted
• Significant impairments are
– Attenuation and attenuation distortion
– Free space loss
– Noise
– Atmospheric Absorption
– Multipath
– Refraction

24. Attenuation
• Strength of the signal falls with the
distance
• Expressed in decibels dB
• For unguided medium attenuation is a
complex function of distance and makeup
of the atmosphere

25. Attenuation
• Attenuation involves these factors
– Receive signal must be sufficiently strong to
be detected and interpreted
– Signal level must be sufficiently higher than
noise
– Attenuation is greater at higher frequencies,
causing distortion.
Amplifiers
Repeaters
can be used
Amplifiers that
amplify higher
frequency more
Than lower
frequency

26. Free Space Loss
• Signal disperses with distance
• Signal spreads larger over distances
• This type of attenuation is called free
space loss
• In ideal free space propagation
– Pr = Pt Gt Gr (lamda/4 х pi х d) 2
• For microwave systems
• Ls = 32.45 +20log d(km) + 20 log f (MHz)

27. Noise
• Unwanted signal created from the source
other than the transmitter
• Four categories
– Thermal noise
– Intermodulation noise
– Cross talk
– Impulsive noise

28. Noise
Thermal Noise
• Due to thermal agitation of electrons
• Always present and cannot be eliminated
• Uniformly distributed across the frequency spectrum
hence referred to as white noise
• Independent of frequency
• Thermal noise in watts present in a bandwidth of B Hertz
can be expressed as
N = kTB where k = boltzmann’s constt
1.38 х 10-23 J/K
T is Temp, in Kelvin
• Or in decibels-watt
– N = 10 log k + 10log T + 10 log B

29. Noise
Intermodulation Noise
• When signal with different frequencies
share the same medium, results in I.N.
• It produces signal at frequency that is the
sum, difference or multiple of two other
frequencies.
• E.g. f1 , f2 would result in f1+f2

30. Noise
Crosstalk
• Unwanted coupling between signal paths.
• The effect of one wire over the other in
twisted pair
• Can also occur when unwanted signals
are picked up by microwave antenna

31. Noise
Impulse Noise
• Irregular , continuous pulses
• Unpredictable therefore not possible to engineer
a transmission system to cope with it
• Generated from external electromagnetic
disturbance like lightning and faults and flaws in
the communication system
• A sharp spike of energy of 0.01 s duration can
destroy 560 bits of data being transmitted at
56kbps