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Desense Issue

Jul 16th, 19:03

BaoFeng2020

Joined: Apr 10th, 08:24
Total Topics: 0
Total Posts: 0
Hi, I am a rookie very little radio experience. So of course, I bought a used repeater :) The issue I am having is the repeater works fine within a 1/8 of mile, but beyond that I only hear the repeater kurchunk (nothing is transmitted). I was told this is called desense. The seller of the repeater requested that I send the duplexer back to make sure it was tuned correctly. The seller tested the duplexer and I was told the duplexer was tuned correctly. When the duplexer was sent back, I reinstalled the duplexer and made sure all the connections were correct.

At this point, the repeater will work on occasion (repeater worked well with a very clean signal little over 4 miles out) and then later when I tried I just heard the repeater kurchunk (nothing is transmitted). Then, when I get about 1/8 mile from the house I can hit the repeater again.

There is no real pattern when the repeater works and when it doesn't. I leave the repeater on (don't turn it off at night) and the repeater doesn't appear to be overheating. The antenna is only 40 feet high as the intent was to initially have the repeater just cover the township. Also, the repeater frequencies used are not active frequencies in this area.

Do duplexers go bad and if so is what I am describing a symptom? If not, any ideas on what is wrong?

Thanks again for you time!
Jul 16th, 20:28

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The Repeater Builder web site has a ton of information.
http://www.repeater-builder.com/antenna/ant-sys-index.html

Zak Lau W1VT
ARRL Senior Lab Engineer
Jul 17th, 11:08

WB1GCM

Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The de-sense effect may be happening not in your repeater, but in the handheld that you are using to test the repeater. If you are listening to the output on you handheld while you are close to your repeater, your handheld will de-sense from the transmitter of the repeater. A 25 Watt repeater that is only 600 kHz away from the frequency you're listening on can de-sense an inexpensive radio. De-sensing of inexpensive handhelds caused by stations close to the tuned frequency is quite common. Try another radio of good known quality, first.

Also, if the tuning of the repeater's duplexer cavities is off just a little bit (caused by mechanical vibrations, dropping it a few inches, or by someone trying to tune it by ear), it will de-sense the receiver of the repeater. Tuning the cavities for maximum rejection should be done while viewing a spectrum analyzer.

Bob Allison
WB1GCM
ARRL Lab Assistant Manager

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