Hey Jeff: I think there are some out there that might question "agita". Haven't heard that term in a while (and don't want to have it).
Ciao, Tony Jeff DePolo wrote: >>We have a 222 MHz repeater comprised of a converted Micor >>mobile, Telewave TPRD-2254 BpBr duplexer, AM-6155 PA modified >>for class C operation at 250W, and a DB-264JJ antenna at 80 >>ft. fed by 1/2' Heliax on a commercial FM broadcast tower . >>With the duplexer terminated into a load, we have about 1 dB >>degradation in sensitivity when transmitting. However, with >>the antenna connected to the duplexer, we experience in >>excess 15 dB of desensitization. We have eliminated other >>narrowband transmitters and analog TV transmitters as >>contributing factors. We are left with a channel 12 digital >>TV transmitter at an adjacent site as a key contributor to >>the problem. Our hypothesis is that we have broadband IMD >>products from the mix of our transmitter and the DTV >>transmitter that are appearing in and near our receiver >>passband. Is it a "rusty bolt" problem or is there some other >>non-linear component somewhere on the site or in our system >>that is the mixing point - we don't know. > > > If you suspect IMD between the channel 12 DTV transmitter and your Tx > carrier, work on attenuating the DTV signal. Using cavity notch filters to > reject the entire 6 MHz of DTV isn't too practical, so instead, try adding > pass cavities on your Tx before the duplexer input. That will help > determine if the IM is originating in your PA. An isolator *may* help, but > with channel 12 being the better part of 20 MHz away (about 10%), it may not > afford full protection -- isolators don't have infinite bandwidth. > Likewise, finding a 250 watt 220 MHz isolator may not be easy. I think I > have some 220 isolators that came off a combiner (Sinclair), but doubt > they're good for 250 watts judging by their size. > > If you suspect a "rusty bolt" mix, use an alternate antenna for testing. If > nothing else, try a quarter-wave whip (suitable for operation at your 250 > watts TPO), even if it's just temporarily mounted on the tower (be sure it's > at a sufficient height to prevent desense due to close proximity to the > repeater itself). > > Another good possibility is IM in your receiver front end (or preamp, if > you're using one). Again, pass cavities are your friend here. Attenuate > the channel 12 signal as much as possible and see if it makes a difference. > Have you looked at what sigs are reaching your receiver input on a spectrum > analyzer? With 15 dB of desense, you should be able to see the culprit(s); > it's not like they're going to be buried in the noise if it's causing 15 dB > of desense. > > Even that 1 dB of desense would give me some agita. I'd verify that the > duplexers are properly tuned and the transmitter is clean before even > starting down any other paths related to the channel 12 issue. IIRC, the > Telewave cavities have adjustable coupling. If necessary, sacrifice a > little extra loss for additional rejection if necessary. > > I also assume you're using all known-good interconnect cables (no foil+braid > or other cables not suitable for duplex operation). > > Are you using a Polyphaser or other type of surge arrestor? If so, try > bypassing it. I've seen gas discharge tube type surge arrestors become > noisemakers after absorbing a strike. > > Has the VSWR changed at all on your antenna? If so, it could indicate water > in a connector or the harness which will cause all kinds of grief, including > wideband noise. > > Finally, does the desense change appreciably if you vary transmitter power > output (it probably will). Do you any have desense when running on just > exciter power? > > --- Jeff WN3A > > > > > > > > Yahoo! Groups Links > > > >
