At 11:38 AM 08/07/10, you wrote: >Hi, I've got a new pager issue that has come up on a tower where we >have a VHF repeater. It's about 400 watts erp and 15 feet horizontal >distance and 4.3 and 4.7 MHz away (it switches). > >I'm thinking of using one bp/br can that will have the notch wide >enough to cover both channels and one pass can on our RX freq. I can >also add more pass cans or something like the DCI window filter. > >The real question is where to these can go and in what order. We >have a standard Q-202 duplexer that worked fine before the pager was >put in. I'm thinking that the pager cans will go on the RX side of >the duplexer, but does it matter if the pass goes on the duplexer >side or the RX side of the bp/br can? I'm kinda thinking that it >doesn't matter, but want to do it right in case it does. > >Dwayne Kincaid >WD8OYG
Is this a situation where the paging transmitter is a new install, or a situation where the paging company consolidated two transmitters, each a single frequency, into one that switches, or a new ham system install? If it's a new paging transmitter install, then 400 watts erp and 15 feet horizontal distance, is too damn close. Your antenna is broadside to its antenna and you have a nice coupling situation. It reminds me of a repeater that was located at an FM broadcast site, and we saw 35 (or so) watts of 90.7 MHz coming DOWN the ham feedline. The ham duplexer was nice and warm... If it's a consolidation I'll bet that the paging transmitter has no pass cavity, or any other filtering - if it ever had any it was probably removed in the consolidation. Digital paging transmitters use square wave modulation and are DIRTY when they have no filtering and local stories have it that the FCC has cited several consolidated transmitter systems. Paging companies are notorious for running overpower. One local situation had a license for 90w and they were found to be running 350w. They were cited, changed transmitters, and a year and a half later the interference returned. A no-notice visit from the FCC found then running 500w. Don;t order that DCI box yet - the first thing I'd do is reduce the antenna-to-antenna coupling. If the master site agreement has a no-interference clause perhaps you can get them to relocate an antenna, or go back to the two transmitters with filtering installed. Look at <http://www.repeater-builder.com/antenna/separation.html> Depending on your antenna gain I'd be very surprised if you are getting even 15db of isolation. Once you have reduced the coupling as best as you can, I'd then re-evaluate the situation by jacking a spectrum analyzer into your antenna and getting some hard numbers, a screen shot (even a digital photo of the CRT) posted here, etc. Then we can help a lot more. Mike WA6ILQ
