Page 88 - Communication IFR_Neat

are separated by 8.33kHz, all 6 digits of the numerical designator should be used
to identify the transmitting channel.

Three digits after the decimal are used for all channels:

118.005 transmitted as ONE ONE EIGHT DECIMAL ZERO ZERO FIVE

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9.6 VHF PROPAGATION CHARACTERISTICS

Propagation Paths
The path of a radio wave from a transmitter to a receiver many miles away is not

necessarily direct. The following paragraphs describe the various paths a radio

signal can follow. In many cases, the signal may be reaching the receiver by more
than one path at the same time, and because of the different path lengths there will

be phase differences between the signals. Such phase differences affect the

resultant signal strength. For instance, if two waves from the same transmitter
travel by different paths and arrive 180° out of phase, they will cancel each other if

their amplitudes are the same. The resultant signal strength will be zero, so no

signal will be received. Changes in phase difference will cause changes in signal
strength so producing the effect known as ‘fading’.

Direct and Ground-reflected Waves.
A signal which travels in a straight line

between transmitter and receiver is called

the direct wave. In addition to this, there is
normally a signal arriving at the receiver

after reflection at the earth’s surface.

This is the ground-reflected wave.
These two waves are jointly known as the Space Wave. (In this and other diagrams,

the abbreviation Tx is used for transmitter and Rx for Receiver.)

Since the direct and reflected waves follow different paths they may arrive at the

receiver with large phase differences. The situation is further complicated by a
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