Cobra Electronics Cobra 148 GTL - Manual (Page 4)

Section III Installation (Continued)
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 transceiv-
er 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 COBRA 148GTL has ANL and NB controls, in some installations
ignition interference may be high enough to make good communications impossi-
ble. 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 2-way radio technician for help in locating and
correcting the source of severe noise.
ANTENNA
Since the maximum allowable power output of the transmitter is limited by the
FCC, the antenna is one 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, boats, 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 attrac-
tive, compact and adequate for applications where the maximum possible dis-
tance 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 radia-
tion pattern is nondirectional. The slight directional characteristic will be observed
only at extreme distances. 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 sepa-
rate 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 re-
garding an adequate grounding system and prevention of electrolysis between fit-
tings in the hull and water.
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Section III Installation
LOCATION
Plan the location of the transceiver and microphone bracket before starting the in-
stallation. Select a location that is convenient for operation and does not interfere
with the driver or passengers in the vehicle. In automobiles, the transceiver is usu-
ally mounted below the dash panel, with the microphone bracket beside it.
MOUNTING THE CONNECTION
The COBRA 148GTL is supplied with a universal mounting bracket. When mount-
ing the bracket and radio to your car, make sure it is mechanically strong. Also
provide a good electrical connection to the chassis of the vehicle. Proceed as fol-
lows to mount the transceiver:
1. After you have determined the most convenient location in your vehicle, hold
the COBRA 148GTL with mounting bracket in the exact location desired. If
nothing will interfere with mounting it in the desired position, remove the
mounting bolts. Before drilling the holes, make sure nothing will interfere with
the installation of the mounting bolts.
2. Connect the antenna cable plug to the standard receptacle on the rear panel.
Most CB antennas are terminated with a type PL-259 plug and mate with the
receptacle.
3. Connect the red DC power input wire (with the fuse) to +13.8V DC. This wire
extends from the rear panel. In automobile installation, +13.8V DC is usually
obtained from the accessory contact on the ignition switch. This prevents the
set being left on accidentally when the driver leaves the car and also permits
operating the unit without the engine running. Locate the accessory contact on
most ignition switches by tracing the power wire from the AM broadcast receiv-
er in the car.
4. Connect the black lead to -13.8V DC. This is usually the chassis of the car. Any
convenient location with good electrical contact (remove paint) may be used.
5. Mount the microphone bracket on either side of the transceiver, using the two
screws supplied. When mounting in an automobile, place the bracket under
the dash so the microphone is readily accessible.
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