PROPAGATION OF ELECTROMAGNETIC WAVES
Ground wave
In Ground Wave Propagation wave glides over the surface of the earth.
A wave induces current in the ground over which it passes and it is attenuated as a result of absorption of energy by the earth. The attenuation of surface waves increases very rapidly with increase in frequency.
The maximum range of coverage depends on the transmitted power and frequency (less than a few MHz).
To radiate signals with high efficiency, the antennas should have a size comparable to the wavelength λ of the signal (at least ~ λ /4).
Sky waves
In the frequency range from a few MHz up to 30 to 40 MHz, long distance communication can be achieved by ionospheric reflection of radio waves back towards the earth.
This mode of propagation is called sky wave propagation and is used by short wave broadcast services.
Ionosphere extends from a height of ~ 65 Km to about 400 Km above the earth’s surface.
The ionospheric layer acts as a reflector for a certain range of frequencies (3 to 30 MHz).
Electromagnetic waves of frequencies higher than 30 MHz penetrate the ionosphere and escape.
Space wave
At frequencies above 40 MHz, communication is essentially limited to line-of-sight paths.
A space wave travels in a straight line from transmitting antenna to the receiving antenna.
Space waves are used for line-of-sight (LOS) communication as well as satellite communication.
Heights of Antennas
The maximum line-of-sight distance dM between the two antennas having heights hT and hR above the earth is given by
MODULATION AND ITS NECESSITY
Size of the antenna or aerial
For transmitting a signal, we need an antenna or an aerial. This antenna should have a size comparable to the wavelength of the signal (at least λ/4 in dimension)
That we need nearly 15 km length antenna to propagate 20 kHz EM wave. Frequency increases length decreases.
Effective power radiated by an antenna
A theoretical study of radiation from a linear antenna (length l) shows that the power radiated is proportional to (l/λ)2.
For a good transmission, we need high powers and hence this also points out to the need of using high frequency transmission.
Mixing up of signals from different transmitters
Allotting a band of frequencies to each message signal for its transmission prevents the possibility of mixing up of signals from different transmitting stations.
Modulation
Translating the original low frequency baseband message or information signal into high frequency wave before transmission is called Modulation.
The high frequency signal is known as the carrier wave. Types of modulation:
(i) Amplitude modulation (AM), (ii) Frequency modulation (FM) and (iii) Phase modulation (PM)
AMPLITUDE MODULATION
In amplitude modulation the amplitude of the carrier is varied in accordance with the information signal.
Let c(t) = Ac sin ωct represent carrier wave and
m(t) = Am sin ωmt represent the message or the modulating signal
Modulated signal cm(t )
Note that the modulated signal now contains the message signal.
Here μ = Am/Ac is the modulation index. Using the trigonometric relation, we get
Side bands
Here ωc – ωm and ωc + ωm are respectively called the lower side and upper side frequencies.
The modulated signal now consists of the carrier wave of frequency ωc plus two sinusoidal waves each with a frequency slightly different from, known as side bands.
PRODUCTION OF AMPLITUDE MODULATED WAVE
DETECTION OF AMPLITUDE MODULATED WAVE
Detection is the process of recovering the modulating signal from the modulated carrier wave.
There are two processes
2. Envelope detection
This envelope of the rectified signal is the message signal. RC circuit can retrieve the envelop.