Tuning the Antenna for Optimum SWR

Section III Installation (Continued)

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 148GTL 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 anten- na you will notice the SWR reading will reach a low point and then start ris- ing 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).

Section III Installation (Continued)

D.Try a different location on your car (keeping in mind the radiation pattern you wish).

E.Is the antenna perfectly vertical?

F.Try a different location in your neighborhood. Stay away from large metal objects when adjusting (metal telephone or light posts, fences, etc.).

NOTE

The COBRA 148GTL will operate into an SWR of 2 to 1 indefinitely and sustain an SWR of 20:1 for a maximum of 5 minutes at rated operating conditions.

EXTERNAL SPEAKER

The external speaker jack (EXT. SPK.) on the rear panel is used for remote receiver monitoring. The external speaker should have 8 ohms impedance and be able to handle at least 4 watts. When the external speaker is plugged in, the internal speaker is disconnected.

PUBLIC ADDRESS

To use the transceiver as a public address system connect an external 8 ohm speaker (4 watts minimum) to the PA SPK. jack located on the rear panel. Direct speaker away from the microphone to prevent acoustic feedback. Physical separa- tion or isolation of the microphone and speaker is important when operating the PA at high output levels.