Frequency selection considerations

NEETS MODULE 10-Wave Propagation, Transmission Lines,

and Antennas

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2-44

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Q32. What are the two general types of variations in the ionosphere?

Q33. What is the main difference between these two types of variations?

Q34. What are the four main classes of regular variation which affect the extent of

ionization in the ionosphere?

Q35. What are the three more common types of irregular variations in the ionosphere?

2.12.3 Frequency Selection Considerations

Up to this point, we have covered various factors that control the propagation of radio

waves through the ionosphere, such as the structure of the ionosphere, the incidence

angle of radio waves, operating frequencies, etc. There is a very good reason for studying

radio wave propagation. You must have a thorough knowledge of radio wave propagation

to exercise good judgment when you select transmitting and receiving antennas and

operating frequencies. Selection of a suitable operating frequency (within the bounds of

frequency allocations and availability) is of prime importance in maintaining reliable

communications.

For successful communications between any two specified locations at any given time of

the day, there is a maximum frequency, a lowest frequency, and an optimum frequency

that can be used.

2.12.3.1 Maximum Usable Frequency

As we discussed earlier, the higher the frequency of a radio wave, the lower the rate of

refraction by an ionized layer. Therefore, for a given angle of incidence and time of day,

there is a maximum frequency that can be used for communications between two given

locations. This frequency is known as the MAXIMUM USABLE FREQUENCY (muf).