Practical_Antenna_Handbook_0071639586

528 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
Switched-Pattern RDF Antennas
If an advancing wavefront arrives at two identical antennas from a direction perpendicular
to the line connecting the antennas, the phase of the wavefront will be the same at both
antennas. If signals from the two antennas are “chopped” at an audio rate with a square
wave and alternately fed to a simple receiver, the resulting phase modulation will appear
as an audio tone at the receiver output. Rotation of the antenna array until the tone is
nulled out, which occurs when there is no phase difference between the signals at the two
antennas, provides an accurate bidirectional “fix” on the source of the wavefront.
Double-Ducky Direction Finder (DDDF)
Figure 23.16 shows in simplified form a system originally presented to amateur radio
operators by David T. Geiser, W5IXM, for use with “rubber ducky” VHF antennas; he
called it the double-ducky direction finder (DDDF). Because of the sharpness of the null,
Geiser chose to use these short, loaded, and rubber-damped antennas rather than full
l/4 or 5/8l whips because vibration and wind can cause enough relative motion between
elements of an array built with full-length whips to make it difficult to get a clean
null in normal outdoor use.
Spacing of the two antennas is not critical; 0.25l to 1l apart over a good ground
plane (such as the roof of a car or truck) should work well. As with all vertical monopoles,
if no ground plane exists, a sheet metal ground plane should be provided.
In Fig. 23.16 the antennas are fed from a common transmission line to the receiver. In
order to keep them electrically the same distance apart, identical half-wavelength sections
of transmission line are used to connect the antennas to the shared section of line.
Switching is accomplished by using a bipolar square wave and PIN diodes. The
bipolar square wave (see inset to Fig. 23.16) has equal positive and negative peak voltages
on opposite halves of the cycle. The PIN diodes (D 1 and D 2 ) are connected in opposition
such that diode D 1 conducts on negative excursions of the square wave and D 2
conducts on positive excursions. In any given half-cycle of the audio frequency square
wave, one antenna is connected to the receiver when its diode is conducting and the
other diode is back-biased, creating a high impedance between its antenna and the
shared line. Thus, the active antenna alternates rapidly between ANT1 and ANT2.
The combined signal from the two antennas is coupled to the receiver through a
small-value capacitor (C 1 ) that blocks the baseband audio frequency square wave from
appearing at the receiver input. This allows use of the shared transmission line segment
for both the RF signal and the switching signal. An RF choke (RFC 3 ) is used to keep RF
from the antenna from entering the square-wave generator.
The DDDF antenna phase-modulates the incoming signal at the frequency of the
square wave, and the resulting tone can be heard in the receiver output. When the signal’s
direction of arrival is perpendicular to the line between the two antennas, the
phase difference is zero and the audio tone disappears.
The pattern of the DDDF antenna is bidirectional, so the ambiguity problem found
with loop antennas exists here, as well. In this system the ambiguity can be resolved by
either of two methods:
• Place a reflector l/4 behind each antenna. This is an attractive solution, but it
tends to distort the antenna pattern a little bit.
• Rotate the antenna through 90 degrees (or walk an L-shaped path).