Andy, Without knowing the frequencies of the victim repeater, it's difficult to postulate a cause. However, I suspect that the APRS radio you hear is transmitting spurious signals in addition to the fundamental. A spectrum analyzer will reveal that as a cause, if you can get close to the offending radio. If you can determine the callsign of the APRS radio's operator, you might be able to locate it. If only one particular APRS radio is causing the interference, it is likely to be defective.
Radios that "go spurious" are not always Amateur-grade radios. My GMRS repeater was once beset with spurs being transmitted by a Motorola M100 radio used as a SCADA base station by a nearby petroleum company. In addition to its fundamental, it was transmitting a healthy signal just 75 kHz away. Replacing the radio cured the problem. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Andy Sent: Thursday, December 21, 2006 4:04 PM To: [email protected] Subject: [Repeater-Builder] A Different (not quite a) Mystery Signal Hi all, A local repeater around here has a similar problem to the one that has been discussed recently. In our case, an APRS packet can be heard over the repeater, only when the repeater is active. Sometimes it's very weak and is only heard during the squelch tail, other times it's strong enough to overpower repeater users. The strange part is that when I tune to 144.39, the interfering signal on the repeater coincides perfectly with packets coming across that frequency. This makes me think that it's not just someone with a radio on the wrong frequency. I haven't been able to record this happening yet, but hopefully I will soon...I've got a VOX recording program running now. Hopefully we'll be able to track down the APRS signal source, but how is it ending up in the repeater? Is this an intermod thing? Many thanks in advance, Andy KB9JOZ
