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Those signal components may arrive from different directions causing false
bearing indications. The LED display tends to jump around randomly under
these conditions. Avoid taking bearing information when the Doppler tone
sounds raspy. You can minimize display jitter by slowing the response time
of the digital filter. This is accomplished by increasing damping control R19.
With a little time, you can master the art of Doppler RDFing.
The wide bandwidth antenna switcher can be used for DFing on other bands,
provided the proper antenna whip lengths and antenna spacing are used.
Arrange the antennas in a square pattern. Typical antenna spacing for com-
mon frequencies is shown in the table below. Whip lengths are measured
from the top surface of the mag-mount PC board base to the tip of the anten-
nas.
Frequency
146 MHz
223 MHz
446 MHz
You can
the cor-
tenna length for other frequencies by interpolation of values from the table
above. The Doppler DF'er will work on virtually any VHF/UHF frequency!
FINAL CASE UP
When all your tests are complete, you will want to case up your main PC
board. First, attach the overlay to the plastic case front. The best (only) way
to do this is to spray the surface with a light coating of window cleaner. Next,
peel the backing from the overlay and position it so that all the holes line up.
At that point, smooth down the overlay with your fingers and leave the case
top with the overlay properly positioned until the window cleaner evaporates.
If you try to place the sticker on a dry surface and do not place it correctly the
first time, you will be unable to remove the overlay without ruining it. Once
the overlay is dry, you can attach the board to the case using the screws pro-
vided. While you're waiting you can attach the speaker to the case bottom
using the screws included in your kit. The speaker fits over the holes drilled
into the plastic case bottom. Insert the two screws into the holes in the case
bottom and then place the speaker mounting holes through the screws. A nut
on each screw will hold the speaker tight to the case bottom. Plug the
Antenna length in
inches
18
13 1/2
7 7/8
DDF1 • 32
determine
rect an-
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