Cobra 148GTL-DX, Instruction Manual

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TUNING THE ANTENNA FOR OPTIMUM SWR
Since there is such a wide vaiiety of base and mobile antennas, this section
will strictly concern itself to the various types of mobile adjustable antennas.
Because the abtenna length is directly related to the channel frequency, if
must be tuned to resonate optimally all 120 channels of the transceiver. Channel
1 requires a longer antenna than Channel 120 because it is lower in 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 adjusîment 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-DX to Channel 21 @ Mid band. 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 continu-
ing to shorten the antenna you will notice the SWR reading will reach a low
print and then start rising again. This means that you have passed the opti­
mum 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 @ Low band and
then to Channel 40 @ Hi band and compare SWR readings. They sould he
almost equal.
B. Antennas which must be eut to proper length.
1. Follow the same procedure as above, but adjust the length by cutting in 1/8"
incréments until a good match is obtained.
2. Be very careful not to eut too much at one time, as one it is eut, it сап по
longer be lengthed.
3. The whip is easily eut b y filing a notch all the way around and 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 CHAN­
NELS LOW BAND 1 AND HI BAND 40.
If you are having difficulties in adjusting your antenna, check the following:
A. AU 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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