Instruction manual
DDF1 35
Final Calibration
A more accurate calibration can be achieved while the car is in motion. Posi-
tion a volunteer with a transmitter in a safe spot on the side of a long, straight
and vacant roadway about 1/4 to 1/2 mile away. Have them transmit on low
power (up to 5W) while traveling towards them. The RDF operator should
calibrate the RDF display to indicate 0 degrees as straight ahead. The dis-
play should change to 180 degrees indicating the signal is coming from di-
rectly behind the car as the vehicle passes the transmitter. The moving cali-
bration producer functions to average out false reflected signals caused by
multipath propagation. You may notice that the Doppler tone changes as the
car moves about. The Doppler tone will sound like a pure, undistorted 500
Hz sine wave in the absence of reflected or multipath interference. Direction
indications are most reliable under these conditions. When reflected or multi-
path signals are present the Doppler tone will sound raspy and distorted.
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 lengths 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.
You can determine the correct antenna length for other frequencies by inter-
polation of values from the table above. The Doppler DF’er will work on virtu-
ally any VHF/UHF frequency!
Frequency Antenna length in
inches
146 MHz 18
223 MHz 13 1/2
446 MHz 7 7/8