At 11/12/2008 20:20, you wrote: >Ron, > >Contact EMR and request a copy of the bulletin, "Field Tuning of Isolators." >Be aware that typical circulators and isolators can be field-tuned perhaps >+/- 2% from the frequency for which they were made. If you want to use it >at a new frequency that is more than 3 or 4 MHz away, it probably will need >to be remanufactured at about 1/3 to 1/2 the cost of a new unit.
This is incorrect. If it was, then virtually all surplus VHF isolators would be unusable at 2 meters. My collection of isolators I've acquired at swap meets over the years that have been retuned & work just fine at 2 meters is proof that they do in fact move further than "3 or 4 MHz away". > You should >use a vector network analyzer, or at least a spectrum analyzer with tracking >generator and a return-loss bridge, to properly tune a circulator or >isolator. Not necessary. I use a transmitter & power meter, & first apply power in the forward direction through the isolator & tune the adjustments near the input & output ports (1 & 2) for max. power. Then reverse the input & output connections & tune the adjustment near the reject load (3) for minimum power. Repeat the procedure a 2nd time. You should get anywhere from 20 to 30 dB of isolation, depending on how good the load on port 3 is. BTW, this is probably why you almost never see an isolator with SO239 connectors, even at VHF HB. Bob NO6B
