I think you may be on the right track with a ¼ wave open stub between the duplexer and the receiver. try and use a good low loss piece of coax for the stub...like ¼ or half inch superflex the lower loss the higher Q the notch will be. start a bit long and shorten it a bit at a time to tune. The reason that I Know this kind of thing will work for some types of problems is that I had the exact reverse happen to me. I had a 900 Mhz Paging transmitter getting into my 2 meter repeater a few years ago. I cured it with a BNC TEE and a very short piece of RG58 I got close to 27 or so db of notch at the Paging transmitter frequency and it cured the problem...This thing was getting into the front end of a GE MASTRII Receiver...It was one of thoes crappy third party 900 transmitters built FOR MOTOROLA by someone else. This fix is real cheap can't hurt to try.
AC0Y --- In [email protected], "David Epley" <[EMAIL PROTECTED]> wrote: > > What is your opinion on a ¼ wave open stub installed in the receiver side > cut for 104.9? > > > > _____ > > From: [email protected] > [mailto:[EMAIL PROTECTED] On Behalf Of Al Wolfe > Sent: Thursday, November 22, 2007 5:50 PM > To: [email protected] > Subject: [Repeater-Builder] Re: Receiver overload > > > > David, > I suspect that the issue is 104.9 mhz energy coming down the outside of > the feedline and into your receiver. As others have mentioned the maxtrac > has a lot of plastic. A shielded box with no unbypassed wires going in and > out may help. Ferrites on the feedline or a coil in the feedline may help as > > well as some more grounds on the feedline. There are also isolation > transformers that could be used at 900 mhz. but they are rare. Mini Circuits > > may have them. I once had to mount a UHF Micor receiver in a shielded box as > > it picked up cell phones otherwise. > > I have operated amateur repeaters in broadcast facilities since the > 1970's. It's not unusual to measure +30 or even +40 dbm coming down a > feedline from an antenna mounted not far from an FM broadcasting antenna. I > have been bitten with RF burns from such feedlines more than one. I have > enjoyed much success getting rid of these problems with just a 1/4 wave > shorted stub at the repeater frequencies. However, this apparently is not > your situation. With all the things you've tried to no avail, any 104.9 mhz. > > energy on the inside of the feedline doesn't sound like the culprit. That's > why I think it is RF on the shield. > > I have been in a great many broadcasting facilities. Many are very well > done with much attention to details. You could eat off the floor and feel > good about it. But many are a real pit with little attention to detail - > just get it on the air. The grounding in these installations is next to > non-existant. Having not seen your neigbor's setup on 104.9, I can't > evaluate it. I also don't know the amount of grounding and bypassing on your > > 900 mhz. setup. But, based on my experience, I would suspect a feedline hot > with RFon its outside. It might be interesting to visit your neighbor and > see how he is receiving his 940 mhz. studio-transmitter link, which is > apparently unaffected by his 104.9 mhz. transmitter. > > Al, > K9SI, BC Engineer/consultant, RETIRED! > > >David Epley wrote: > >> > >> I have a repeater receiver overload problem I am trying to cure. The > >> repeater is a 900mhz 927.7125/902.7125. There is an FM broadcast > >> station 100 yards away 104.9mhz. The repeater works fine at another > >> site. My transmitter is a Motorola Purc 5000 running 75 watts the > >> receiver is a converted maxtrac 800mhz radio. Duplexers are Telwave > >> BpBr 4 cavity. I have 10 to 12 db degradation when plugged into 3 > >> different antennas on the tower. When I use a 900mhz dish antenna > >> pointed away from broadcast tower I only have 3 db degradation. I have > >> tried 3 different maxtrac receivers, added 2 more BpBr cavities in the > >> receiver side and used 3 pole filters in the receivers with no > >> improvement. Today I looked at the signal level getting to the > >> receiver at 104.9. To my surprise I was getting -8 dbm at the > >> receiver. I believe this level is overloading the front end of my > >> repeater. I was wondering if a stub cut for the broadcast frequency > >> would work. Any thoughts would be greatly appreciated. > >> > >> > >> > >> > >> > >> David Epley, N9CZV > >> > >> Winchester, Indiana >
