A few questions to which the answers would be extremely helpful: * What are the frequencies in question? * Are they harmonically related? * How much vertical separation is there between the two antennas on the tower? * Are they directly one above the other? * What is the gain of the two antennas? * Which band is on the top? * Did the system *ever* work without desense, or has something changed?
If something has changed: * Are the antennas and feedlines visibly OK? * What sort of feedline is being used? 9913? * Any new neighbors on the hill? * Any recent lightning? A low-pass filter on the LB Tx may help. It could be the harmonics of the LB Tx clobbering the VHF Rx. Have you looked at the output of the LB Tx on a spectrum analyzer in the VHF area? Some (Motorola Micor) LB Tx's have a low-pass filter *after* the PA and before the antenna output. Does your Midland have something similar? This would definitely help suppress the harmonics. If your Midland doesn't seem to have a LPF after the PA and the spectrum analyzer shows lots of noise in the VHF area, I would get a hold of a Micor LB LPF and put it in series and see if that helps. They are designed to be 50 ohms in /out and are rated for 100W or so. Scott Scott Zimmerman Amateur Radio Call N3XCC 474 Barnett Road Boswell, PA 15531 tritium242 wrote: > Hello all. We have a cross band repeater (Midland 100W VHF low w/ 1W VHF high > band repeater built in). > > What we noticed is that in the High Band --> Low Band direction, the high > band side will receive the signal, then the low band Tx will key up to repeat > the signal. Normally, all works just fine. But at extended distances, the low > band TX keying up will deafen the high band side trying to copy the faint > signal (from an HT), resulting in a choppy, unintelligible signal at the max > distance. > > This is only with a faint/distant enough signal, like a HT at a distance. In > the other direction (low band --> high band), there is a HUGE increase in > maximum distance, even with just 1W from the high band Tx going out, since it > seems to be not enough power to desense the low band Rx. > > Long story short, will adding a high pass filter to the high band side to > shield the high band Rx from the powerful low band Tx do the trick? Where can > one find such a thing? It just has to pass from say, 100-120 Mhz, and block > everything below it, and tolerate 1W of Tx power is all. > > We'd like to avoid using a big, bulky cavity filter if possible, since we > don't need monster isolation figures, or high power handling capabilities. > Thanks in advance for any ideas! > > > > ------------------------------------ > > > > Yahoo! Groups Links > > > >
