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Wilson Flex-4 Antenna Adjustment

I've been running the Wilson 4' model on my TJ now for a couple of years and have very pleased with the RF performance and its survivability factor on the trail.  When I first spoke with Bob, he indicated he wanted to try the 4' model and I passed that info along to the Right Channel Radio folks.  You can view more of my comments regarding the Wilson Flex antenna. 

With the radio mounted and the antenna installed, it was time to check the antenna SWR.


The SWR meter supplied with the Cobra Off-Road Kit is very easy to use.  On the rear panel of the meter are two RF connectors.  The front panel is marked to indicate which connector gets the antenna cable (you can seen ANT in the above photo) and which connector gets the short 12" jumper attached to it and the remove interface box (as shown in the above photo.

The fundamentals of adjusting the antenna are pretty simple.  The goal is to get as low of a SWR as you can.  A ratio of 1 to 1 is perfect.  2:1 is not too bad and one can certainly live with that.  3:1 is the max and I would do some serious work to get mine lower than that if at all possible.  Anything higher than 3:1 means you have problems that require attention and you shouldn't use the radio.

Check your SWR at Channel 1 and then again at Channel 40.  If the SWR is high on Channel 1 and low on Channel 40, then antenna is too short and you need to lengthen the adjustable tip.

If the SWR is high on Channel 40 and low on Channel 1, then antenna is too long and you need to shorten the adjustable tip.
 

When adjusting the antenna tip, you should move it a very small amount, about 1/8", and then check the SWR again.  In most cases, a small amount of movement can change the SWR more than you might expect.  Moving the tip in large increments may cause you to miss the "low spot" in the SWR curve leaving you with the appearance of something being wrong with your antenna.

To check the SWR, place the FWD/REF switch in the FWD (forward) position.  Turn the channel control on the radio to the desired channel and push the push to talk (PTT) switch on the microphone.  With the radio now in transmit mode, do not speak into the microphone.  Keep the PTT switch pushed and adjust the CAL control on the front of the SWR meter such that the needle is aligned with the SET mark on the meter.  With the PTT switch still pushed, change the switch to the REF (reflected) position and read your SWR on the top scale of the meter.  A 1:1 SWR looks just as is shown in the above photo.....the red meter needle is resting on the "1" (far left end of the upper scale).  A 3:1 SWR places the meter needle on the "3" mark at approximately the half scale position.

Every time you change the channel on the radio and check your SWR, be sure to place the switch in the FWD position and adjust the CAL control to the SET mark.  Then move the switch to the REF position and read the SWR on the upper scale.

The SWR meter also indicates Percent of REFlected power.  To read this, use the lower section of the upper scale.  25% reflected power is right at a 3:1 SWR. 

Just a side note.....SWR meters are designed to work for a specific frequency range.  This meter is accurate for those frequencies used by a CB radio.  Use of the meter for other frequencies is not recommended as the accuracy is not guaranteed (or specified).

We adjusted the Flex-4 antenna on Bob's TJ the best we could which ended up being about 2.5:1.  That is not very good and as I cautioned Bob at the beginning of the installation, the mounting location placed too much of the antenna too close to the body of the TJ.  I've seen this problem many times in the past on other vehicles and the only cure for it is the relocation of the antenna (as was also discussed).  While poor SWR readings can be the fault of the antenna or coax, in this case it was not.

 

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