At 2/23/2010 17:16, you wrote: >Mark et al, >Yes, this repeater is using the Motorola T1500 series bandpass cavities >(two each for rx and tx). I've tried running rx and tx both duplex and >seperate (borrowing a nearby antenna with permission). I can hear the >interference underneath my signal when I'm about 2 miles away and >monitoring my signal. When its strong enough, the PL encode of the >repeater keeps it locked up until the modulation from the AM station >overtakes the PL being looped (voice peak). Then the repeater drops >since I have a tone panel in between and not continuous PL outbound. > >I have tried changing the receive frequency about 75Khz lower and the >interference is not present (so a 4.925Mhz split), so that serves to >prove to me that this indeed a mix. > >I can try adding an attenuator the next time I'm out at the site. The >antenna is about 300 feet up and fed with 7/8" heliax, to a Polyphaser >and then superflex to the duplexer. I've also tried without the Poly, >but have the same result. I have some nice Mini-Circuit pads that should >work in the receive side after the duplexer, but think the receiver is >simply overloaded.
The cause of your interference problem is not RX "overload". It is as others have suggested: a mix occurring somewhere in the near field of the antenna. Pads may eventually mask the real source of the problem, once you've added enough to drop the signal below your RX's noise floor, but you'll end up with a deaf repeater. How far away were the separate TX & RX antennas when you tried that? I'd think if they were far enough apart that you would lose the mix. OTOH if a tower joint is the source of the mix (likely since a lot of length is required to couple in the AM BC station), it might be all over the tower. A similar problem was partially cured here by spraying some conductive paint into all the tower joints. Each time it was done the interference would disappear for a few months, then return. Bob NO6B
