An open qtr wave at 104.5 will notch out 104.5, but make sure it is not off muliple qtr wave at your 925/905 freq or it could notch it out also. Just have to do the calculation.
Qtr wave with .66 velocity factor cable at 104.5 MHz is about 1.55 ft. I believe this is 9 qtr wavelengths at 940.5 MHz notching it out also. The skirts on this low Q filter might be a factor. 73, ron, n9ee/r >From: David Epley <[EMAIL PROTECTED]> >Date: 2007/11/22 Thu PM 05:18:42 CST >To: Repeater-Builder@yahoogroups.com >Subject: RE: [Repeater-Builder] Re: Receiver overload > > >What is your opinion on a ¼ wave open stubinstalled 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, 20075:50 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 > > > Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
