Eric,
Yeah. we're always trying to do something for nothing! We're a low-budget operation and I'm pretty much left to my own devices to try things that the club wants to do. The good thing is, if this one just doesn't come to fruition, we haven't spend any money on it. The radio is an old Yaesu Memorizer. I'm sure that the front end (or lack thereof) is a major part of the problem. I've looked at the site noise on a spectrum analyzer and the repeater seems clean at the link frequency (with the spectrum analyzer hooked to the link antenna) so it's just poor rejection on the part of the Yaesu. With the antenna unhooked, there is no noise (additional . beyond normal static) on the Yaesu with the repeater transmitter on and with the Yaesu hooked to a signal generator, the repeater transmitter has no effect. so the offending signal must be coming through the antenna. The beam is (unfortunately) also a given. We can't climb the watertower but we're allowed to use an antenna already in place. so we're stuck with what we've got. I've been looking at that GE bulletin already. that's actually my next plan of attack. A plain quarter-wave stub has actually been fairly effective (much to my surprise) but I need to do better. I'll keep plugging away at it. it may turn out to be a total bust or perhaps a case of 'okay. this is as good as it gets'. I knew I was out of my mind when I volunteered to become the repeater manager! 73, Mike WM4B From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Eric Lemmon Sent: Saturday, September 27, 2008 11:57 AM To: [email protected] Subject: RE: [Repeater-Builder] Quarter Wave Stubs/Notching Interference Geez, Mike, you're not asking for much, are you? I assume you are using a half-duplex radio for the link, and it receives on 145.110 MHz and transmits to the distant repeater on 144.510 MHz. So, you need a high-Q notch at 146.850 MHz to protect the link receiver at 145.110 MHz, while not affecting the link transmitter at 144.510 MHz. Whew! It would help if you specified what make and model radios are involved, along with what duplexer and filters are in place. It's not easy to figure out potential fixes without knowing the quality and performance data of the radios. A Micor or Mitrek link radio, with the helical front-end filter, will perform much better than a broadband 2m mobile radio. My first choice would be a bandpass cavity on the TX side of your repeater, to attenuate broadband noise that might fall on the link receive frequency. Since that is not an option, perhaps a coaxial-cable notch filter in the same location might be a solution. Here's one reference: www.repeater-builder.com/ge/datafile-bulletin/df-10002-01.pdf An eight-element beam is major overkill for a link, IMHO, and may be part of the problem. Perhaps a corner reflector would be a good choice. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> [mailto:[email protected] <mailto:Repeater-Builder%40yahoogroups.com> ] On Behalf Of Mike Besemer (WM4B) Sent: Saturday, September 27, 2008 8:12 AM To: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> Subject: [Repeater-Builder] Quarter Wave Stubs/Notching Interference I'm working on collocating a 2-meter remote base with our repeater, and naturally every time the repeater transmits, it makes the remote-base radio totally deaf. I've been playing with quarter-wave stubs with some success (and have lots more ideas/experimenting in mind) but would like to hear from others who are running collocated equipment without the aid of cavities/duplexers. The local repeater is running 30 watts on 146.85 to a DB-224 at about 125 feet. The link is on 145.11 10 watts to an 8-element beam at about 70 feet. The whole purpose of the remote base is to link to another repeater 30 miles away. And before anybody asks, a link on another band isn't a possibility (primarily because of limitations with antenna systems), so it's 2-meter remote base or nothing. Thanks, Mike WM4B
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