Cobra Electronics 18 WX ST operating instructions CB Antenna, Ignition Noise Interference

Models: 18 WX ST

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18.WX.ST.MAN.ORIG. 9/19/97 2:54 PM Page 2

3.Connect the antenna cable plug to the standard recep- tacle on the unit. Most CB antennas are terminated with a type PL-259 plug which mates with the recepta- cle marked "ANT."

4.Connect the red lead of DC power cord to +13.8 VDC. In automobile installations, +13.8VDC is usually ob- tained from the accessory contact in the fuse box. This prevents the set being left on accidentally and also per- mits operating the unit without the vehicle's engine running.

Before installing the CB radio, visually check the ve- hicle battery connections to determine which battery terminal, positive or negative (positive is the larger of the two) is grounded to the engine block (or chassis).

5.Connect the black lead to the negative side of the auto- mobile. This is usually the chassis of the car. Any con- venient location with good electrical contact (remove paint) may be used.

6.Mount the microphone bracket on right side of the transceiver or near it using two screws supplied. When mounting in an automobile, place the bracket under the dash so the microphone is readily accessible.

CB Antenna

Since the maximum allowable power output to the trans- mitter is limited by the FCC, the antenna is one important factor affecting transmission distance. Only a properly matched antenna system will allow maximum power trans- fer from a 50-ohm transmission line to the radiating ele- ment. In mobile installations (cars, trucks, boats, etc.), an antenna system that is non-directional should be used.

A vertically polarized quarter-length whip antenna pro- vides the most reliable operation and the greatest range. The shorter loaded-type whip antennas are more attractive, compact and adequate for applications where the maxi- mum possible range is not required. Also the loaded whips do not present the problems of height imposed by the 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 vehi- cle. 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 con- nector (Type SO-239) is provided on the transceiver for easy connection to a standard PL-259 cable termination. Cobra load- ed-type antenna models AT-35, AT-60 and AT-70 are highly rec- ommended for most installations. Consult your Cobra dealer for further details (or see order form at the back of this book).

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, con- sult your dealer for information regarding an adequate grounding system and prevention of electrolysis between fit- tings in the hull and water.

3-Way Combination Antennas are available which allow op- eration of all three bands (AM-FM & CB), using a single antenna. However, use of this type of antenna usually results in less than normal transmit and receive range when compared to a standard-type "Single Band" antenna designed for CB only.

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 alternator 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 this COBRA radio has an automatic noise limiter, 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 varia- tions 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.

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Cobra Electronics 18 WX ST operating instructions CB Antenna, Ignition Noise Interference