Tim, I plugged the same values as before into CommShop, but this time with a 1 MHz split, and the result is about 85 dB isolation, which can be met with about 7.75 miles of horizontal separation.
The developer of CommShop was calculating distances that would result in NO desense, using radios available at that time. Many repeater owners simply put up with a small amount of desense, either because they have no means to measure it or they say that the repeater is working fine. I'll readily admit that many repeaters will "work fine" with some desense, and their owners get on with their lives. I daresay that a split-site 6m repeater using Micor or Mastr II radios will supremely outperform a similar system using Alinco or similar mobile radios that have broadband front ends. Doug Bade made a very good point that some desense may result from other stations that may be closer to your receive site than your transmitter; in this case, horizontal separation between your sites is meaningless. Obviously, a neighboring transmitter that is only 500 kHz away from your receive frequency will completely swamp your transmitter that is 1 MHz away. The definitive method for testing whether your transmitter is causing desense to your receiver is to radiate a weak signal to your receive antenna that results in a 12 dB SINAD reading on your service monitor with your transmitter off. Then, energize your transmitter. If the SINAD reading drops, you have desense. As others have pointed out, the use of one or more bandpass cavities on either or both ends may greatly reduce or eliminate desense. You may be pleasantly surprised. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Tim Ahrens Sent: Friday, January 22, 2010 6:57 AM To: [email protected] Subject: RE: [Repeater-Builder] 6 Meter Repeater spacing (no duplexers) Thanks to all for the great response! Well, since we are 1meg splits here -thanks Don-, that should help some also. (I had looked up a couple of repeaters around here & saw .5 split, so thought that's how it was). Anyhow, it's just in the thinking stages. Thanks again, Tim W5FN
