Cobra Electronics 148.GTL.ST operating instructions Tuning the Antenna for Optimum SWR

148.GTL.ST.MANUAL 10/1/98 10:56 AM Page 9

Section III Installation

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 148 GTL ST has ANL and NB controls, in some installa- tions 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 2-way radio technician for help in locat- ing 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.

Section III Installation (Continued)

TUNING 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 antenna length is directly related to the channel frequency, it must be tuned to resonate optimally all 40 channels of the transceiver. Channel 1 requires a longer antenna than Channel 40 because it is a lower 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 adjustable screws (set screws).

1.Start with the antenna extended and tighten the set screw lightly enough so that the antenna can be lightly tapped with your finger for easy adjustment.

2.Set your COBRA 148 GTL ST to Channel 21. 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 and then to Channel 40 and compare SWR readings. They should be almost equal.

B.Antennas which 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 once 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 a pliers.

NOTE

THE PROPER SETTING IS ACHIEVED WHEN THE SWR IS

1.5OR BELOW, AND WHEN IT HAS THE SAME READING FOR CHANNELS 1 AND 40.

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.

C. Check your coaxial cable routing (it may be pinched when routed into the car).

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