IGNITION NOISE INTERFERENCE
Use of a mobile receiver at low signal levels is normally limited by the
presence of electrical noise. The primary source of noise in automobile
installations is from the generator and ignition system in the vehicle. Under
most operating conditions, when signal level is adequate, the background
noise does not present a serious problem. Also, when extremely low level
signals are being received, the transceiver may be operated with vehicle
engine turned off. The unit requires very little current and therefore will not
significantly discharge the vehicle battery.
Even though the transceiver has ANL and NB controls, in same
installations ignition interference may be high enough to make good
communications impossible. The electrical noise may come from several
sources. Many possibilities exist and variations between vehicles require
different solutions to reduce the noise.
ANTENNA
A vertically polarized, quarter-wavelength whip antenna provides the
most reliable operation and greatest range. Shorter, loaded-type whip
antennas are more attractive, compact and adequate for applications where
the maximum possible distance is not required. Also, the loaded whips do not
present the problems of height imposed by a full quarter-wavelength whip.
Mobile whip antennas utilize the metal body of the vehicle as a ground
plane. When mounted at a corner of the vehicle they are slightly directional,
in the direction of the body of the vehicle. For all practical purpose, however,
the radiation pattern is nondirectional. The slight directional characteristic
will be observed only at extreme distance. A standard antenna connector
(type SO239) is provided on the transceiver for easy connection to a standard
PL 259 cable termination.
If the transceiver is not mounted on a metal surface, it is necessary to
run a separate ground wire from the unit to a good metal electrical ground in
the vehicle. When installed in a boat, the transceiver will not operate at
maximum efficiency without a ground plate, unless the vessel has a steel
hull.
Before installing the transceiver in a boat, consult your dealer for
information regarding an adequate grounding system and prevention of
electrolysis between fittings in the hull and water.
TUNNING THE ANTENNA FOR OPTIMUM SWR
Since there is such a wide variety of base and mobile antennas, this
section will strictly concern itself to the various types of mobile adjustable
antennas.
Because the antenna length is directly related to the channel frequency,
it must be tuned to resonate optimally on all channels of the transceiver.
Channel 1 requires a longer antenna than Channel 40 because it is lower in
frequency.
Due to the various methods of adjusting antennas for proper SWR we
have chosen what we think is the optimum method:
A. Antennas with adjustment screws (set screws)
1. Start with the antenna extended and tightens the set screw lightly enough
so that the antenna can be lightly tapped with your finger for easy
adjustment.
2. Set your transceiver to Channel 21. on Band C or Band D. Press the PTT
(push-to-talk) switch, and tap the antenna (making it shorter). The SWR
meter will show a lower reading each time the antenna is tapped. By
continuing to shorten the antenna you will notice the SWR reading will
reach a low point and then start rising again. This means that you have
passed the optimum point for Channel 21. Extend the antenna a short
distance and again follow the procedure above.
When the lowest point has been reached, switch to Channel 1 on Band A
and then to Channel 40 on Band F and compare SWR readings. They
should be almost equal.
B. Antennas that must be cut to proper length.
1. Follow the same procedure as above, but adjust the length by cutting in
1/8” increments until a good match is obtained.
2. Be very careful not to cut too much at one time, as one it is cut, it can no
longer be lengthed.
3. The whip is easily cut by filing a notch all the way around and breaking
the piece off with pliers.
If you are having difficulties in adjusting your antenna, check the following:
A. All doors must be closed when adjusting the antenna.
B. Make sure the antenna base is grounded.
THE PROPER SETTING IS ACHIEVED WHEN THE SWR IS
1.5 OR BELOW, AND WHEN IT HAS THE SAME READING
FOR CHANNELS BAND A Ch.1 AND BAND F Ch.40.
NOTE
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