Hello, I helped in solving this same sort of problem several years ago with a amateur repeater experiencing RF getting into the repeater audio who's antenna was mounted on a shunt fed 5 KW AM Non-Directional vertical (Tower) antenna. In this particular installation the tower base was directly connected to the tower ground and radial counterpoise using a slant wire shunt fed arraignment. The 1/2" Andrew line was also bonded to the same ground connection as was the line periodically bonded to the tower on the way up to the VHF antenna. Still, enough RF circulated and caused a great deal of RF to find its way into the repeater audio path.
We installed a shorted 1/4 wave (VHF) stub via a Tee connector between the antenna port of the duplexer and the 1/2" line coming down the tower from the VHF antenna. The shorted stub served to short out or to shunt the AM RF while looking like a high impedance at the VHF frequencies used by the ham repeater. This completely solved the problem having eliminated the common mode current that was circulating from the AM radiator through the repeater and and back to the station ground system. You may also need to decouple the commercial AC mains feeding the repeater power supply as this as also been know to cause similar problems. Someone suggested disconnecting the antenna and replacing it with a dummy load to see if the problem disappeared. This is a good test as it will tell you if the RFI is getting into the system from some other point or points. This also breaks apart the most suspect path for common mode currents to circulate in your system. The iso couplers can be just a simple brut force choke that would need enough reactance at the AM frequency to adequately decouple the VHF line. Others are actually a parallel resonant circuit that requires a parallel capacitor in order to resonate the tune circuit at the AM frequency to again achieve proper decoupling. Try the 1/4 wave shorted stub first as this alone may do the trick. Good luck, Bill, WA8WG ----- Original Message ----- From: "robd53154" <[email protected]> To: <[email protected]> Sent: Tuesday, June 09, 2009 7:52 AM Subject: [Repeater-Builder] AM NOISE... > Hi we have a Ham reptr on a tower of AM radio station and its going > throguh an iso coupler, how ever the receiver is getting a lot of AM noise > in it the receive is in the 448 range, is there a filter we need to put on > the recvr side or is the iso coupler possbile bad, when you shut the AM > off the repeater works just fine! > > RD > > > > > ------------------------------------ > > > > Yahoo! Groups Links > > >
