A lot of strange things can happen. We had a situation where a school bus control station that was situated a couple blocks from an FM broadcast tower caused the control station to spur and get into our control station almost 20 miles away. When the offending control station was put on the bench (away from the FM broadcast station) it worked fine.
Have also had radio station Marti transmitters get into stuff as well. Was told by someone that Marti's were known to spur easily. Chuck WB2EDV ----- Original Message ----- From: "tracomm" <trac...@yahoo.com> To: <Repeater-Builder@yahoogroups.com> Sent: Monday, February 22, 2010 8:41 AM Subject: [Repeater-Builder] Re: Can the 4th harmonic of 1250 AM keep UHF repeaters locked up? > 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, 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 >
