Repeater build problems

Question

I'm trying to build a UHF repeater using two Motorola radios. I have a cable connecting the two radios that was built for doing such a project. The radios have been programmed for the appropriate inputs/outputs for connection in the repeater mode. The homemade repeater operates flawlessly at my home feeding into a dummy load. However, when I take the radios to the site where my antenna is, the repeater seems to break into some type of oscillation (when accessed). The receiver will receive a transmission from a good four miles away via a handie-talkie....but whenever it starts receiving the signal from the handie-talkie, the repeater will go into transmit for about a half-second, then receive, transmit, then receive, over and over again. I have run tests to see if the receiver is getting a good signal from the handie-talkie and it has crystal clear audio from the repeater receiver (when the repeater transmitter is off). I have checked the reflected power from the antenna.....very low. I have checked the duplexer to see if it is working properly......seems to be working properly. I am running the repeater using CTCSS tones in/out. The repeater also works great (with no issue) if I am at the antenna site using a handie-talkie. I can't understand why (if I'm getting clear audio from 4 miles away) the repeater breaks into this sort of oscillation when trying to access it from 4 miles away, but works perfectly if I transmit on handie-talkie at the repeater site. Any suggestions would be sincerely appreciated. I've worked on this (what I thought would be a simple project) until I'm blue in the face. Thanks, Rusty

Answer 1

This sounds like desense cycling:

• Repeater receiver detects signal.
• Controller keys transmitter.
• Signal from transmitter gets into receiver and blocks incoming signal.
• Controller decides input signal gone, unkeys transmitter.
• Transmitter off, receiver detects signal.
• Cycle repeats.

Your duplexer might not be working as well as it should. Or possibly, you don't have enough isolation between your receiver and transmitter radios. You aren't using RG-58 are you?

It is difficult to completely eliminate desense. But you should be able to get it down to a negligible level.

[Edit to add:] Here is a way to get an estimate of desense. Requires a signal source with adjustable output level (e.g. signal generator).

• Have repeater set up normally with its antenna.
• Have signal source connected to another antenna setup some distance from repeater (not too close or too far. Need to be picked up by antenna, but be able to put in strong enough signal).
• Adjust signal level up and down to get an idea of what it takes to bring up repeater.
• Set signal level somewhat below minimum to bring up the repeater.
• Increase level in small (1 dB) steps until repeater just barely comes up. At this point the desense cycling will start.
• Keep increasing level until cycling stops and repeater stays up.
• Difference between these levels is a measure of desense.

Ideally, you'll get it down to zero. You can probably live with up to about 3 dB desense. If it is much more, you have a problem to work on.