1. ***Radio Frequency**
**** Waves****
Presented by:-
*Sameer Farooq (1442).
*Gowher Qadri (1426).
*Lt. Ishfaq Malik
(1439).

2. Radio Wave
Radio wave is an electromagnetic wave having wavelength between
1mm-30km & having frequency between 10KHz-300 GHz.
A radio wave moves outward from the radiator with its electric and
magnetic fields at right angles to the direction of the wave-font motion
and to each other. These waves moving through free space are
transverse electromagnetic (TEM) waves consisting of mutually
perpendicular electric and magnetic fields varying and travelling
together in synchronism.

5. Radio wave is now commonly
used in transmitting sound and
information of images.
The unit, hertz, is also used
in radio broadcast.

8. Propagation Modes
 Ground-wave propagation.
 Sky-wave propagation.
 Line-of-sight propagation.

10. Radio Waves in the
Atmosphere

11. Ground Wave Propagation
 Follows contour of the earth.
 Can propagate considerable distances.
 Frequencies up to 2 MHz (all frequencies will
have some ground wave/near field).
 Examples
 AM radio (generally).
 LF and MF.
 Low frequencies which can be effected by
daytime/night time.
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12. Ground Wave Propagation
(LF/MF)

13. Sky Wave Propagation
 Signal reflected from ionized layer of
atmosphere back down to earth (dependent
on sun’s radiation).
 Signal can travel a number of hops, back and
forth between ionosphere and earth’s surface;
both a short path and a long path (opposite
direction around earth) can occur.
 Reflection effect caused by refraction.
 Examples (3 – 30 MHz).
 Short-wave radio (sw).

14. Sky Wave Propagation (HF)
Normally only one hop

15. 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.
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16. Line-of-Sight Propagation
(VHF and above)

17. 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 K = 4/3
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hd  57.3

18. Line-of-Sight Equations
 Maximum distance between two
antennas for LOS propagation:
 h1 = height of antenna one in meters
 h2 = height of antenna two in meters
 Note that d is in kilometers (km)
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 2157.3 hh 

19. Radio Waves

21. LOS Wireless Transmission
Impairments
 Attenuation and attenuation distortion.
 Noise.
 Atmospheric absorption.
 Multipath.
 Refraction.
 Thermal noise.

22. Attenuation
 Strength of signal falls off with distance over
transmission medium (exponential).
 Attenuation factors for unguided media:
 Received signal must have sufficient strength so that
circuitry in the receiver can interpret the signal
(without overloading the front-end of the receiver)
– receiver sensitivity related to internally generated
noise
 Signal must maintain a level sufficiently higher than
noise to be received without error.
 Attenuation is greater at higher frequencies and any
attenuation results in signal distortion.

23. Other Impairments
 Atmospheric absorption – water vapor and
oxygen contribute to attenuation (microwave).
 Multipath – obstacles reflect signals so that
multiple copies with varying delays are
received (shadow fading – obstruction of
signal by objects in the straight-line path).
 Refraction – bending of radio waves as they
propagate through the atmosphere.

24. 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
the wavelength of the radio wave (signals
received without a direct line-of-sight)
 Scattering – occurs when incoming signal hits
an object whose size in the order of the
wavelength of the signal or less (difficult to
predict)
 If there isn’t a clear LOS, multipath can be
the primary means of signal reception

25. Generation of Radio Waves;
 Accelerating charges radiate EM energy.
 If charges oscillate back and forth, get time-varying
fields.
E
+
+
+
-
-
-
+
-
-

-
-
-




26. Generation of Radio Waves
If charges oscillate back and forth, get time-varying magnetic
fields too.
Note that the magnetic fields are perpendicular to the electric
field vectors.
B
+
+
+
-
-
-
+
-
-

-
-
-




27. Encoding Information on
Radio Waves
 Two common ways to carry analog
information with radio waves
 Amplitude Modulation (AM)
 Frequency Modulation (FM): “static free”

28. Amplitude Modulation
 Amplitude Modulation (AM) uses changes in
the signal strength to convey information.
pressure modulation (sound)
electromagnetic wave
modulation

29. AM Radio in Practice
 Uses frequency range from 530 kHz to 1700
kHz.
 each station uses 9 kHz.
 spacing is 10 kHz (a little breathing room)  117
channels.
 9 kHz of bandwidth means 4.5 kHz is highest audio
frequency that can be encoded.
 falls short of 20 kHz capability of human ear.
 Previous diagram is exaggerated:
 audio signal changes slowly with respect to radio
carrier.
 typical speech sound of 500 Hz varies 1000 times
slower than carrier.
 thus will see 1000 cycles of carrier to every one cycle
of audio.

30. Frequency Modulation
 Frequency Modulation (FM) uses changes in
the wave’s frequency to convey information.
pressure modulation (sound)
electromagnetic wave
modulation

31. FM Radio in Practice
 Spans 87.8 MHz to 108.0 MHz in 200 kHz intervals
 101 possible stations.
 example: 91X runs from 91.0–91.2 MHz (centered at
91.1).
 Nominally uses 150 kHz around center.
 75 kHz on each side.
 30 kHz for L + R (mono)  15 kHz audio capability.
 30 kHz offset for stereo difference signal (L - R).
 Again: figure exaggerated.
 75 kHz from band center, modulation is > 1000 times
slower than carrier, so many cycles go by before frequency
noticeably changes.