Absolutely you need some reserve. The same if you are designing a point to point path. You don't select equipment that will "just do the job". You always need a certain amount of reserve for changes of equipment etc. the idea is that some think the repeater is going to "work better" with more isolation in the duplexer just because it has more isolation. Once you meet the isolation requirement and some reserve built in to cover things that drift etc., then more is not going to help you.
73 Gary K4FMX _____ From: [email protected] [mailto:[email protected]] On Behalf Of Chuck Kelsey Sent: Sunday, August 23, 2009 8:45 AM To: [email protected] Subject: Re: [Repeater-Builder] isolation I agree with Kevin. You need a little headroom built in for conditions that could change. Equipment ages and changes it's operating characteristics. Temperature swings cause the same issues. Does one need to go overboard? Probably not. But if you happen to be right on the edge under perfect conditions, you may be unhappy when something moves a bit out of tolerance. Chuck WB2EDV ----- Original Message ----- From: Kevin Custer <mailto:[email protected]> To: [email protected] Sent: Sunday, August 23, 2009 9:30 AM Subject: Re: [Repeater-Builder] isolation Gary wrote: Dan Kagabine, the chief engineer at TX-RX systems use to always say that "once you have enough isolation to overcome any desense, then any more is a waste of money as it does nothing for you". "If you only need 70 db then a 100 db duplexer does nothing more for you than a 70 db duplexer". While I'll agree that more isolation, then what is needed to insure no desense is a waste; if this gentleman is suggesting that isolation in reserve is a waste, I strongly disagree. Why? Operating conditions can change - snow and especially ice on the repeater antenna can detune the system and isolation in reserve will allow the repeater to operate without desense until the reserve is used up. Kevin Custer
