How long is your feed line? Do you have any toriods on your feed line, where it comes in your shack?
> To: [email protected] > From: [email protected] > Date: Tue, 23 Feb 2010 16:01:45 -0600 > Subject: Re: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF > repeaters locked up? > > I was able to match up the audio coming through the repeater and the > local AM station. My latest theory is that their signal is so strong > that its blowing into the receiver's front end and multiplying/mixing > there (past the bandpass filters and all). They are heterodyne receivers > after all. > > I'm considering an ICE broadcast high-pass filter that cuts off at > 1.8Mhz (model 402). I have an email into them to see how well it might > work at 448 Mhz. > > Tony > > tracomm wrote: > > > > Had a similiar situation at our site, a station on 106.7 MHz, music on > > hang time on many repeaters, intermod runs gave no clue to reason, did > > all the usual, grounding, filters no resolve. > > Turned out to be the STL Marti system on 450.100 MHz, from an close > > studio site pointed right at our site, hitting our Rx multicoupler, > > mixing with our transmitters. Resolution was low passs & isolater on > > the STL system. > > Make certain which station the broadcast audio is coming from and give > > the station engineer a friendly call, may reveal some info to help > > your issue. > > > > Chris > > GMRS Inc. > > > > > > --- In Repeater-Builder@ yahoogroups. com > > <mailto:Repeater-Builder%40yahoogroups.com>, Tony KT9AC <kt...@...> wrote: > > Hi everyone, > > A while ago I was troubleshooting a bad feedback or "growl" problem > > that was impacting a UHF repeater, of which the short term workaround > > was to not encode TX PL (PL or DPL would keep it locked until the > > signal dropped enough or timed out). > > > > In doing some more research, I found a 1250kHz AM station within a > > mile or two that changes pattern between day and night. The > > interference mentioned above would appear around drive times (like > > 5pm) so that had me chasing other sources. Still, it was puzzling that > > a 5Mhz signal could be causing the feedback (it didn't appear when > > doing normal receiver testing with a service monitor). The recent give > > away was that I could hear talking underneath my test signal (like a > > sports show). > > > > So, if we take the 1250Khz signal or 1.25Mhz x 4 = 5Mhz. I realize > > that the 4th harmonic of a 5KW broadcast station isn't very powerful, > > but being in its nearfield might be enough to cause a mix with the UHF > > transmit output. > > > > Does this make sense? This phenomenon can be duplicated with both a > > 450 and 440 repeater system - both with standard 5Mhz offsets. I don't > > think any sort of filtering would work since the mix happens "in the > > air". > > Only by having split PL's can the lockup be prevented, and equipment > > was both MSF5000 and Micor systems, through correctly tuned duplexers. > > Thanks, > > Tony > > > > > > > ------------------------------------ > > > > Yahoo! Groups Links > > >
