7
Nothing
comes close to a Cobra
®
Installation and Connection
6
English
Installation and Connection
Installation and Connection
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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.
However, 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 your mobile radio has ANL and NB controls, in some 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. Consult your Cobra dealer or a
two-way radio technician for help in locating and correcting the source of severe noise.
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Antenna
In addition to transmitter power, the antenna is an important factor affecting transmission
distance. Only a properly matched antenna system will allow maximum power transfer
from the 50 ohm transmission line to the radiating element. In mobile installations
(cars, trucks, etc.), an antenna system that is non-directional should be used.
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 purposes, however, the radiation pattern
is non-directional. The slight directional characteristic will be observed only at extreme
distances. A standard antenna connector (type SO 239) 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.
Installation and Connection
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4
Tuning the Antenna for Optimum SWR
Because such a wide variety of base and mobile antennas are available, this section
will concern itself only with the usual types of mobile adjustable antennas.
Antenna length is directly related to signal frequency. Therefore, it must be
tuned to resonate optimally throughout the frequency range of the transceiver.
Lower frequencies require a longer antenna than higher frequencies.
Due to the various methods of adjusting antennas for proper SWR, we have
chosen what we think is the optimum method:
A. Antennas with adjustable screws (setscrews).
1.
Start with the antenna extended and tighten the setscrew lightly enough so
that the antenna can be lightly tapped with your finger for easy adjustment.
2.
Set your mobile radio to your desired operating frequency or the center of
the range of frequencies you plan to use. Press the PTT (Press-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 the middle frequency.
3.
Extend the antenna a short distance and again follow the procedure above.
4.
When the lowest point has been reached, switch to 28.0 MHz and then
to 29.7 MHz and compare SWR readings. They should be almost equal.
B. Antennas that must be cut to proper length.
1.
Follow the procedure as in A 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 off the antenna at one time.
Once it is cut, it can no longer be lengthened.
3.
The whip is easily cut by filing a notch all the way around,
then breaking the piece off with pliers.
NOTE
The proper setting is achieved when the SWR is 1.5 or below and when it has
the same reading for the low and high frequencies in the range you plan to use.
Your Mobile Radio
Your Mobile Radio
