Steve, You did a great job of hitting all of the key points, and I agree with the majority of your estimates.
One point that should be clarified is that when dual antennas are used, they must be separated vertically, one over the other, by no less than 30 feet to achieve about 55 dB of isolation. A similar isolation would require more than 450 feet of horizontal separation which is, of course, impractical due to line loss. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Steve Kometz Sent: Thursday, December 13, 2007 10:12 AM To: [email protected] Subject: Re: [Repeater-Builder] simple repeater As others have posted, I think you will be disappointed using that setup. Here are things to consider: There are fairly standard "graphs" available from several sources giving guidance as to need for duplex isolation, isolation curves for cavities, freq offset, antenna separation, etc. GE, Motorola, most of the duplexer manufacturers have these graphs posted, so go do some looking and you will find them. For example, at 600 kHz, in the two meter band the TX-RX Tech-Aid says you need 98 dB of isolation with a GOOD receiver and a 100 watt transmitter. If you are going to run less power, correct that to about 90 dB.(just estimating 8 dB less TX power) If you use a weird split, and make that 2 MHz offset instead of 600 kHz, the 100 watt figure is down to about 76 dB. (again correct for low tx you are around 68 dB). Again, going to a TX-RX Tech-Aid chart, you can get about 56 dB of isolation between two antennas perfectly aligned, 30 ft separation.. So, somewhere you need to get a LEAST 12 dB of isolation to even run 2 MHz. And thirty some for 600 kHz spacing. And one of the ways to "get" that isolation is to just suffer some receive desense. A few dB is not that big of deal in a portable set up. But if you start out with a system that suffers more that say 6 or 8 dB, you will be disappointed. Weak signals get clobbered, and the repeater just goes "kerchunk" a lot. As other posters said, those particular radios have very wide front end receivers. If you have a radio that "hears" a -125 dB signal, as compared to an older more selective radio that is more like -117 you will need that additional 8 dB somewhere. Not to mention the wide band noise the Tx would make somewhere off the Rx freq. A little disclaimer here: These figures from the various charts are not precise. (altho the charts are all pretty close to the same from one manufacturer to another) I am just quickly estimating, and not covering all variables (like cable and connector differences, losses, etc) With many radios to choose from, and performance differences frequency to frequency there are even more variables. I am not trying to talk you out of experimenting, just suggesting things that will help you be more successful (and less disappointed). Generally, you may have more success with Commercial radios ( I like the Maxtrac, Radius & GM300 lines, others I know use the Midlands and Kenwoods) If you don't really need frequency agility, use the separate antennas, and a single small cavity on each. There are several combinations that may work, but try not to start out with something that won't work good enuf to satisfy your need. Do that little bit of "math" and get a duplex "budget" figured out. Then shoot for about 10 dB better than you think you need. And last but not least, if it is easy enuf, TRY it.
