Mark et al, Yes, this repeater is using the Motorola T1500 series bandpass cavities (two each for rx and tx). I've tried running rx and tx both duplex and seperate (borrowing a nearby antenna with permission). I can hear the interference underneath my signal when I'm about 2 miles away and monitoring my signal. When its strong enough, the PL encode of the repeater keeps it locked up until the modulation from the AM station overtakes the PL being looped (voice peak). Then the repeater drops since I have a tone panel in between and not continuous PL outbound.
I have tried changing the receive frequency about 75Khz lower and the interference is not present (so a 4.925Mhz split), so that serves to prove to me that this indeed a mix. I can try adding an attenuator the next time I'm out at the site. The antenna is about 300 feet up and fed with 7/8" heliax, to a Polyphaser and then superflex to the duplexer. I've also tried without the Poly, but have the same result. I have some nice Mini-Circuit pads that should work in the receive side after the duplexer, but think the receiver is simply overloaded. Tony Mark HARRISON wrote: > > Hi Tony, > > Are you using a duplexer on this repeater? > A lot of cavity filters act as a short circuit to DC and low > frequencies, so additional filtering is unlikely to help. > I can only think of one type of cavity that has a DC path between from > input to output (via an internal inductor) and not to ground. This > type could I guess pass low frequencies. It's simple to test - > disconnect the antenna and receiver leads and measure the DC > resistance with a meter between the centre and outer on the cavity > connectors. If it's 0 ohms then it's likely to be a very good high > pass filter for broadcast frequencies! > > Also, using a temporary attenuator you should be able to determine if > there is an intermod problem within the receiver, or parts of the > antenna filtering system on the receive side of a Duplexer. Inserting > an attenuator will reduce the interference (and desired signal) by the > same amount if everything on the receiver side of the attenuator is > functioning correctly. If instead the interference drops by 2-3 times > (in dB, and the desired signal drops only by the attenuator value) > then you've found your problem! > Placing a power attenuator in the duplexed antenna line is more > complicated because you are attenuating both the Tx and Rx signals. > You would expect the interfering signal to drop by more than double > the attenuation value, and you can't really tell if the problem is in > the antenna, antenna feeder, or something external. > > 73, > Mark VK3BYY > > -----Original Message----- > From: Repeater-Builder@ yahoogroups. com > <mailto:Repeater-Builder%40yahoogroups.com> [mailto:Repeater-Builder@ > yahoogroups. com <mailto:Repeater-Builder%40yahoogroups.com>] On > Behalf Of KT9AC > Sent: Wednesday, 24 February 2010 09:02 AM > To: Repeater-Builder@ yahoogroups. com > <mailto:Repeater-Builder%40yahoogroups.com> > 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 > > ------------------------------------ Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/Repeater-Builder/join (Yahoo! ID required) <*> To change settings via email: [email protected] [email protected] <*> To unsubscribe from this group, send an email to: [email protected] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
