Practical_Antenna_Handbook_0071639586

524 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s
Doppler RDF Antennas
Figure 23.13 shows the basic concept of an RDF antenna based on the Doppler effect,
which was discovered in the nineteenth century. A practical example of the Doppler
effect is seen when an ambulance with wailing siren first approaches a stationary observer,
then passes beyond. The pitch of the siren heard by the motionless observer is
initially at its highest (and higher than the pitch heard at the source by the driver of the
ambulance). It continuously drops as the source approaches and then passes by. Although
useful, the acoustic analogy is not perfect, since the Doppler mechanism for EM
waves in a vacuum is a little bit different than that for the subsonic acoustic waves of
the ambulance.
In a radio system, when distance between a receiving antenna and a signal source
is changing, a Doppler shift in the received signal frequency is detected; the amount of
shift is proportional to the differential radial velocity between the two. (No Doppler
shift occurs as a result of any change in motion at right angles to the line connecting the
source and the receiver.) Thus, when a hobbyist listens to the telemetry signals from a
passing satellite or the international Morse code (CW) of a radio amateur making contacts
through an AMSAT repeater satellite (visit www.amsat.org), the received frequency
continuously slides lower as the satellite comes into radio “view”, passes
nearby, and finally disappears until its next orbit. At the point of the satellite’s closest
approach to the receiving station (whether directly overhead or off to the side), the received
frequency equals the actual transmitted frequency because neither antenna has
any radial velocity relative to the other for that brief moment. Before then, the received
frequency is higher; afterward, it is lower.
Axis of
Rotation
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No
Shift
Antenna
No
Shift
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Signal
Source
Figure 23.13 Doppler antenna.