This is one of the toughest intermod products to solve. 2 transmitters 600 kHz apart generate spurs every 600 kHz on both sides of each transmitter. When we were co-ordinating repeaters, it's one parameter we always looked at, and tried to keep nearby repeaters off of that 600 KHZ separation boundary. Are Both repeaters having problems or just yours? These signals don't even have to be generated in your equipment, I've seen them generated by stuff externally and if you look at a spectrum analyzer when both transmitters are on the air, you'll see the Christmas tree like display showing the spikes every 600 kHz decreasing with amplitude as they get farther away. Most suggestions made may help and all I can say is good luck, the only real way out of this problem may be a frequency change for one or the other machine.] Gary - K7NEY ----- Original Message ----- From: wa5luy To: [email protected] Sent: Sunday, February 15, 2009 6:15 AM Subject: [Repeater-Builder] Re: Help with intermod between repeaters.
Thanks to all for your replies. Joe M wrote "You need to reject the opposite TX on each repeater. Your BP filters were set wrong. You rejected the RX on the TX side - something the duplexer should be doing already. You need to install them in the repeater and reject the other TX. IOW, install a filter in the .16 repeater and notch the .36 TX. A good duplexer should do this as well, but many don't." Joe that's what I thought I did. Maybe I did not make myself clear. I put a BPBR cavity rejecting 146.76 and passing 147.36 between the repeater transmitter and the duplexer at the 147.36 repeater. This is where I was surprised that the mixing got worse. If I put the cavity in the wrong place let me know. Eric Lemmon WB6FLY wrote "The first question that enters my mind is, were both Micor stations originally built as repeaters, with the extra filters and shielding plates,or are one or both base stations that have been converted to repeaters?" I built our repeater. It's the 146.76 machine. It was originally a pager TX. All shielding and the lo pass TX filter is in place. I have looked at it with a spectrum analyzer and see no other signal than 146.76. I have no idea as to what the other repeater was made from. I will take a second look at their TX. The next time I go down there I plan to take a 50 watt radio and connect it to their duplexer to try to eliminate or prove their PA has a problem. "The second question is, are either or both repeaters equipped with ferrite isolators?" The 146.76 has no isolator. The 147.36 has a brand new, I believe Sinclair, isolator that was factory built for this frequency. The isolator has no affect on the problem although I don't think it's installed properly. I did not notice when I was there but I think it's mounted on a steel plate. Also there in no cavity between it and the duplexer. The mixing is there with or without the isolator in line. They paid big bucks thinking this would fix the problem. By the way they also replaced their antenna and feed line which may have made the mixing worse. John wrote "I hate to tell you, a definite way to eliminate the problem, is a frequency change so that the output of the two transmitters are no longer not 600 khz apart." Funny that`s the first thing I told them. I am familiar with two pagers 600 khz apart and the havoc that can be raised. I plan to go back down there when I have time and let the group know what I find. Again thanks to all.
