At 06:52 AM 08/20/08, you wrote: >Be very careful when you put that pass cavity on the transmit side. >When I was in Virginia I had a repeater on 147.06 and late at night >I would get interference to it. Noticed the same to other repeaters >in the area. Narrowed it down to a repeater that a cavity was placed >on the transmitter side to help clear up some desense. Looking at >the spectrum analyzer you can see when the transmitter would become >active it would have a spur running from it assign frequency up the >band to the public service area. This happpened when the >transmitter was idle for a while. Removing the cavity cleared the problem.
Weird. Anybody ever put a spectrum analyzer on the bare transmitter output? Or pull the end off the cavity, do a visual inspection and figure out what caused the problem? 20+ years ago we had the same problem here in L.A.... I was involved in detecting it / discovering it, but not in affecting the cure. A certain low-usage 2M amateur repeater tube-based transmitter, if left idle for about 5 minutes, would sweep from about 146 to about 159 with full power and do the full sweep in about 1/2 second. If the transmitter was used and not left idle it didn't have the problem. A spectrum analyzer showed it was not a spread-spectrum broadband noise burst, but actually a full power narrow sweep. The system was split site with a half-mile RF link on 220.14 or so, and no duplexer. All the control stuff was at the receive site so the IDer would ID both the link and the main output. Repeaters with relay-based CORs would not trigger on that short a "blip", neither would ones with kerchunk filters. I only discovered it because I happened to have a rare combination of gear running in the same room at the same time - a scanner receiver (a brand new Bearcat 220 - does that date this story?) that happened to be parked on the repeater output, a receiver on 147.99, a PT400 listening to a 148 MHz Civil Air Patrol repeater, another handheld radio on a 151MHz channel, two more on two different 154 MHz channels, another receiver on a 155.55 PD channel, plus on the shelf over the bench was a Regency crystal scanner on the 155.16 local SAR channel, next to my 158.73 local PD monitor receiver. When the owner went 10-8 on the repeater every receiver (except the 220 and UHF ones) in the room went "sphht" (mentally replace that with a short squelch tail noise). HMMMM.... A immediate kerchunk didn't cause the receiver pool to fart, one 10 minutes later did. Over the next hour I narrowed it down to any time period over about 5 minutes. That 2M repeater was very used, so up until then nobody really put two and two together that when it was awakened from a deep sleep the entire low end of the high band world went "kerchunk". Once the repeater owner was aware of the situation (I had to bring him over to my garage workshop to prove it to him, after all HE wasn't seeing any problem, and HE was ABSOLUTELY SURE that HIS system was clean) he borrowed a spectrum analyzer and verified it on site. A tweak of the diddle stick into the plate tuning of the last multiplier had an affect on it, a new multipler tube fixed it. While he had the analyzer he retuned the entire transmitter for maximum narrow / clean power (which interestingly was not the point of maximum power). The owner commented that the bad tube made an appropriate "puck"-crunch sound as he stomped on it with his hiking boot - that particular tube was NOT going back into the tube caddy - not even for use in the electronic organ at his church. >Again be careful. Yep. You gotta be a good RF neighbor. A pass cavity on the system output from day 1 would have made all the difference in the world. >David Mike WA6ILQ
