Hi everyone, A while ago I was troubleshooting a bad feedback or "growl" problem that was impacting a UHF repeater, of which the short term workaround was to not encode TX PL (PL or DPL would keep it locked until the signal dropped enough or timed out).
In doing some more research, I found a 1250kHz AM station within a mile or two that changes pattern between day and night. The interference mentioned above would appear around drive times (like 5pm) so that had me chasing other sources. Still, it was puzzling that a 5Mhz signal could be causing the feedback (it didn't appear when doing normal receiver testing with a service monitor). The recent give away was that I could hear talking underneath my test signal (like a sports show). So, if we take the 1250Khz signal or 1.25Mhz x 4 = 5Mhz. I realize that the 4th harmonic of a 5KW broadcast station isn't very powerful, but being in its nearfield might be enough to cause a mix with the UHF transmit output. Does this make sense? This phenomenon can be duplicated with both a 450 and 440 repeater system - both with standard 5Mhz offsets. I don't think any sort of filtering would work since the mix happens "in the air". Only by having split PL's can the lockup be prevented, and equipment was both MSF5000 and Micor systems, through correctly tuned duplexers. Thanks, Tony
