Bob, Depending upon the frequency band and the modulation being used, it's possible that the cellular transmitter is radiating a low-level spurious signal. All cellular telephone sites transmit one or more control channels continuously, and the associated mobile units home on this channel to receive calls and adjust their power levels. You didn't state whether this cellular site was 800 MHz, 1800 MHz, or 1900 MHz, so let's assume that it is an 800 MHz cell site. Note that an "800 MHz" cellular base station actually transmits in the 869-899 MHz band. Some brands of 800 MHz cellular base stations use a doubler just before the PA, which means that a low level signal in the 434-449 MHz range might be leaking out of the site, and could be continuously desensing your receiver even if it is not exactly on your input frequency. Even if the cell site is in a different band, it still uses doublers and triplers in various configurations to reach the channel frequencies.
To determine if this scenario is valid, shut off your repeater and hook a spectrum analyzer to your antenna lead. Look for any more or less continuous carriers on or close to your repeater input frequency or to its image frequency. It's also possible that a technician left off some shields or didn't tighten all the screws on a cover plate during routine maintenance. 73, Eric Lemmon WB6FLY "Bob M." wrote: > Hi. A local UHF repeater is having massive desense > problems. Everything is clean with a dummy load > attached to the duplexer, so we know it's a problem > with the connectors, coax, antenna, or jumper between > the two. This is a two-month-old system at a cellphone > site; nothing else there. The repeater is the only > thing on 444.5 MHz. > > I'd like to try a circulator before buying one, since > if it doesn't help the situation, it's a waste of > money to have it there. I would agree however that its > presence would be mandatory if this was a busy site, > but for now it's just his repeater and all the > cellphone equipment. > > He's running 200 watts out of a Henry amp, through a > TxRx 4-section duplexer. We see a 20dB rise in the > noise level at the Rx port when the transmitter comes > on, with the system connected to the coax/antenna. > Absolutely no change with a good dummy load attached > to the duplexer's output. We even have desense with > the 20 watts out of the exciter (power amp bypassed). > The antenna is a super stationmaster cut for the > 440-450 band. The coax is 7/8 and there's about 150 ft > of it. > > So if someone has a circulator and reject load that > would be rated for 200 watts to pass through it, > please contact me. If there's a way we could try it, > and buy it if it helps the situation, that would be > ideal. > > Thanks. > Bob M. > > > __________________________________ > Yahoo! FareChase: Search multiple travel sites in one click. > http://farechase.yahoo.com > > > Yahoo! Groups Links > > > > Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
