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INTRODUCTION
Radio direction finding is a fascinating hobby that has been becoming more
and more popular in today's portable world. More recently, Doppler "df-ing"
has become the rage, with a display that gives you a direct bearing on the lo-
cation of the transmitter. Pretty neat trick considering you don't need multiple
separate receivers at different locations to triangulate on the mystery trans-
mitter.
DDF1 CIRCUIT DESCRIPTION
The classic example of the Doppler effect is that of a car approaching a sta-
tionary observer. The car's horn sounds higher in pitch (frequency) to an ob-
server as the car approaches. The change in frequency occurs because the
motion of the car shortens the wavelength. The horn sounds lower in pitch
(frequency) to the observer as the car speeds away. This occurs because the
car is speeding away from the observer effectively increasing the wave-
length. Fewer cycles per second, hence, lower-frequency sound. A similar
effect occurs when an antenna is moved toward or away from a transmitting
source. The signal received from an antenna moving toward the transmitting
source appears to be at a higher frequency than that of the actual transmis-
sion. The signal received from an antenna moving away from the source of
transmission appears to be lower in frequency than that of the actual trans-
mission. Imagine a receiving antenna moving in a circular pattern as pic-
tured in Figure 1A. Consider the antenna at position A, nearest the source of
transmission. The frequency of the received signal at point A equals that of
the transmitted signal because the antenna is not moving toward or away
from the source of transmission. The frequency of the received signal de-
creases as the antenna moves from point A to point B and from point B to
point C. Maximum frequency deviation occurs as the antenna passes
through point B. The frequency of the received signal at point C is the same
as that of the transmitted signal (no shift) because the antenna is not moving
toward or away from the source of transmission. As the antenna moves from
point C to point D and from point D back to point A, the frequency of the re-
ceived signal increases. Maximum frequency deviation occurs again as the
antenna passes through point D. The Doppler frequency shift as a function
of antenna rotation is illustrated in Figure 1B.
dF= ( rf
)/c
c
where:
dF =Peak change in frequency (Doppler shift in Hertz)
= Angular velocity of rotation in radians per second (2 x frequency of ro-
tation)
r = Radius of antenna rotation (meters)
f
= Frequency of transmitted signal (Hertz)
c
DDF1 • 4
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