Advertisement
also work well. As a safety measure, secure each PopTop Mag-Mount with
20 pound fishing line when operating the vehicle at highway speeds.
Attach all four whip antennas to the mag-mount antenna bases placed about
the center of the car top. Connect the RF output of the antenna switcher to a
FM receiver transceiver tuned to a strong NOAA weather broadcast signal.
Caution: Again, make sure you disable transmit mode if you are using a
transceiver! It is best to use a smaller, handheld walkie talkie with your
DDF1. The unit connects easily to the antenna on your handheld and be-
cause they have less output power, there is less chance of doing damage by
accidentally transmitting into your DDF1.
Adjust the receiver's audio to a comfortable level in the external speaker.
Apply 12 V to the RDF unit and "spin" the antenna by closing switch S2. As
soon as S2 is closed, you should hear a 500 Hz tone imposed on the re-
ceiver audio. Rotate audio level adjust control R50 so that low signal level
LED D4 and audio overload LED D3 are extinguished. Never trust bearing
indications if D3 and/or D4 are illuminated. The direction indicating display
should be relatively constant with a single LED lit, or one or two adjacent
LEDs alternately illuminating. Adjust calibrate control R36 so that the direc-
tion indicating LED is consistent with the general direction of the NOAA
transmission with respect to your location and the position of the car. Have
the driver slowly circle while you observe the display. You should see the di-
rection indicating LED move in the opposite direction as the car is turning in
a circle. The position changes relative to the changing direction of the car,
however, the direction indicated from the center of the circle the car is driving
should remain fixed. If the display turns in the same direction as the car, flip
phase invert switch S3 to the opposite position to correct the 180 degrees
phasing offset. This completes the rough calibration procedure.
Final Calibration
A more accurate calibration can be achieved while the car is in motion. Posi-
tion a volunteer with an HT 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.
DDF1 • 31
Hide quick links:
Advertisement
