VHF and above frequencies travel in straight lines. This is termed as Line of Sight (LOS) communication because the transmitting antenna must see the receiving antenna for the waves to reach. These waves do not bend along with the curvature of earth and if aimed at the ionosphere, they will pass through to space rather than being refracted. Such waves are called space waves. Any tall or massive object in between the transmitting and receiving antennae will obstruct the waves from reaching the receiving antenna. The area on the other side of the obstructing object (or beyond the horizon), looked from the transmitting side, will receive no signal and this is called the shadow
zone.
Fig 2.5 Space wave communication
Thus, the range of communication using space waves depends on the height of transmitter and receiver antennae. The formula which gives the range of communication using space waves is:
1.23( hr + ht ) NM
Where hr is the height (in feet) of the receiver antenna above sea level
ht is the height (in feet) of the transmitter antenna above sea level
If an aircraft is flying at 10,000 ft, approximate maximum range of communication using VHF would be 123 NM (229 Km) with the ground stations at sea level. Similarly, for an aircraft flying at 1000 ft altitude, the maximum communication range would be 39 NM (73 Km) with the ground station at sea level.
With the above examples, it can be seen that the height of the aircraft plays an important role in the range of space wave communications. Though the limited range of communications of VHF is a disadvantage, it also enables the usage of same frequencies at different places. For example, ATS units at Chennai, Delhi, Kolkata and Mumbai use the same frequency for their services (TWR – 118.1 MHz, APP – 127.9 MHz, SMC – 121.9 MHz)
2.7.3.1 Superrefraction (Ducting)
Under certain atmospheric conditions such as temperature inversion, complete bending down of the space waves takes place from the layer of atmosphere as low as just 30 metres from the ground. Thus, space waves are refracted back to the earth and reflected back by the surface, continuously, and they propagate around the curvature of the earth for a long distance which sometimes exceeds 1000 km. This phenomenon is called superrefraction. It is also called ducting because the earth surface and the refracting layer of the atmosphere act like a duct for propagation of waves.
Fig 2.6 Ducting phenomenon; space wave travels beyond horizon 2.8 Basic Radio Principles
Audio frequencies (such as a pilot’s speech) are converted into electrical energy by a microphone. If attempt is made to radiate this energy directly, it will pose certain problems: audio frequencies are low frequencies with large wavelengths, this would require enormous size of antenna to radiate the energy efficiently. For example human speech which is normally in the range of 3 KHz would require an antenna length of 1,00,000 metres (100 KMs). Even if half or quarter wavelength antenna is used it would still be 50 or 25 KMs long. Another problem is that the AF band is 20 Hz to 20KHz which is a bandwidth of 19.88 KHz. Very few channels can be accommodated and even then, all the transmissions would be hopelessly mixed up. Third problem is that very high power would be required to transmit such low frequencies to enable them to travel long distances. To overcome these problems a process called modulation is used for radio communications.
2.8.1 Modulation
Modulation is the process of superimposing audio frequency over radio frequency. Advantages are: RF has higher frequencies which require shorter antennae; RF spectrum has very large bandwidth which can accommodate a large number of channels; transmission power will be very less because of the higher frequencies. Modulation is done in the transmitter. Basically there are two types of modulation in analogue radio communications; (i) Amplitude modulation (ii) Angle modulation. Angle modulation has two types namely, frequency and phase modulations.
2.8.1.1 Amplitude Modulation (AM)
In this type of modulation, the amplitude of the RF is made to change as per the amplitude of the AF. Normal RT communication is amplitude modulated. Figure below illustrates the concept.
The RF carrier, which has a much higher frequency than the AF, gets shaped according to the shape of the AF which is the modulating wave. It can be seen that, if the peaks of the modulated result wave are joined by a line, it takes the shape of the modulating wave.
Fig. 2.7 Amplitude modulation 2.8.1.2 Frequency Modulation (AM)
In this type of modulation, the frequency of the carrier is varied according to the amplitude of the AF wave. When the amplitude of the modulating wave increases, the frequency of the carrier RF increases and when the amplitude of the modulating wave decreases, the frequency of the carrier RF decreases. Figure below illustrates the concept.
Fig. 2.8 Frequency modulation
Frequency modulation is used in commercial FM broadcast (88-108 MHz) and also satellite communications. SATCOM equipment fitted in aircraft use FM in conjunction with the VHF communication equipment.
2.8.1.3 Phase Modulation (PM)
This is a variant of Frequency Modulation in which the phase of the RF carrier is varied as per the amplitude of the amplitude of the modulating wave (AF). Phase and Frequency modulations fall under the broad category of Angle modulation.
Fig. 2.9 Phase modulation 2.8.1.4 Pulse Modulation
There is another class of modulation which is called pulse modulation in which the carrier is a series of pulses and not sinusoidal. This can be further categorised into Pulse Amplitude Modulation (PAM), Pulse Width Modulation (PWM), Pulse Position Modulation (PPM) and Pulse Code Modulation (PCM).
In pulse modulation, the RF carrier pulses are modulated depending upon the amplitude of the modulating wave.
2.8.2 Demodulation
Demodulation is the separation of AF from the carrier RF. This is the reverse process of modulation. Demodulation is done in the receiver.