Hunting Intermod Issues

We spent the last two Thursdays on the hill hunting RF intermodulation mixes that 435 is part of.   On Aug 6th, 2015, we found the commercial VHF system that is transmitting the 3-way mix affecting a commercial system.  We did field strength measurements on that spur to find the shelter and then the antenna of the offending system.  We traced coax to the rack.  The circulator is suspect and the filters clearly need to be upgraded.  The commercial engineer expects to upgrade that system next week and that mix should be fixed.

Three other ameteur repeaters have reported hearing occasional RF mixes on their inputs which involve 435.  Usually these are brief 3-way mixes and tough to find.   Given the strength of the spur we found and the inadequate filter engineering on the offending commercial VHF system, there is a good chance that same system is throwing other spurs.

Two technicians have looked the 435 repeater on spectrum analyzers and concluded that we are *not* producing any of the reported spurs.  We will continue to cooperate with other repeater systems to find mixes and be good hilltop citizens.

All the 435 equipment and installation is top-notch commercial grade:

  • Motorola Quantar
  • Motorola dual circulator
  • Henry repeater amplifier
  • Bird TX bandpass filters
  • Bird RX bandpass and notch/pass filters
  • RX antenna at 100’ (Commscope 4-pole)
  • TX antenna at 30’ (Commscope 4-pole)
  • Professionally installed 7/8″ Heliax coax.

Roger Bly working on the 147.435 repeater rack:


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