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Old 05-19-2017, 10:53 AM
jim202 jim202 is offline
Join Date: Mar 2002
Location: New Orleans region
Posts: 2,517

The above information is good.

A simple change of the coax should not effect the SWR of the antenna. If it does, then maybe there was a high loss in the old cable that was hiding the reflected power from being seen at the radio. A simple check can be made by putting a watt meter at the far end of the coax with a dummy load. Measure the TX power at the far end. Then pull the watt meter and put the dummy load on the coax. Move the watt meter to the radio end of the cable and measure the TX output power again. The difference is the cable loss. In 50 feet of cable, I would not expect to see much if any difference.

I would do testing on a few things at this point.

If you have an ohm meter, disconnect the LMR400 from both the antenna and the radio. Check to make sure you don't have a short in the cable. Put one lead on the shell of the coax connector. Place the second lead on the center pin of the coax connector. You should not see anything but infinity. Then put a 50 Ohm termination on the far end of the coax cable. Do the test again and you should see about 50 Ohms. You can double check your reading by doing the same test right on the dummy load RF connector.

If you have a dummy load you know is good, put it on the end of the LMR400 and connect it back up to the radio. Do the normal transmit check at channel one and again at channel forty. You should see a very low SWR on both channels.

Connect the coax back up to the antenna. I forgot the readings you supplied already. But if the reading is lower at the high channel, and higher at the lower channel, this indicates that the antenna is probably too short. On the other hand, if the reading is higher at the lower channel and lower at the high channel, this indicates your antenna probably is not long enough.

The instructions indicate that there is a screw adjustment at the base to lengthen or shorten the entire antenna by rotating it. I couldn't figure out how to secure the screw turning to keep it from moving with the limited information provided. Maybe if I had the antenna in front of me it would be more obvious.

There is also a ground plane kit that is available for that antenna. I didn't do any research on just what the effects of the ground plane hit have. I will say that the length of the ground plane elements will effect the SWR if you do have that kit installed.

This information is not much to go on, but maybe it will give you some ideas.

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