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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’.

13.4.1.1 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

change in phase which occurs at the point of reflection of the ground-reflected

wave.

The net result is that, for instance, an aircraft flying towards a ground station may

suffer fading or temporary loss of VHF communications with that station. The
range at which this occurs depends on ground aerial height above the surface,

aircraft altitude, and frequency.

GEN radio 2025 - Midtjysk Flyveskole 100