What is your opinion on a ΒΌ wave open stub installed in the receiver side cut for 104.9?
_____ From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Al Wolfe Sent: Thursday, November 22, 2007 5:50 PM To: Repeater-Builder@yahoogroups.com 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
