> -----Original Message----- > From: Repeater-Builder@yahoogroups.com [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of Nate Duehr > Sent: Saturday, June 02, 2007 3:03 AM > To: Repeater-Builder@yahoogroups.com > Subject: Re: [Repeater-Builder] Duplexer tuning - sanity check > > > On May 31, 2007, at 7:13 PM, Bob M. wrote: > > > The duplexers on UHF are typically set for 5 MHz > > spacing, although closer spacing is often available. > > The purpose of the duplexer is to allow the receiver > > and transmitter to share one antenna. They offer very > > little filtering of other nearby signals. You need a > > good, selective receiver front end to handle that. The > > transmitter can often be cleaned up by adding an > > isolator to the output; some units may already have > > that feature, especially commercial base stations. > > Just to clarify a point here, I think I see what you're saying, but > it's slightly misleading for those who haven't used them... > > An isolator does nothing to "clean" a transmitted signal. It's a > circulator with a 50 ohm resistive dummy load on one or more ports > coming back toward your transmitter. > > It keeps reflected RF in your antenna and feedline system (hopefully > you don't have any, but there's always some...) and OTHER > transmitters out of yours. (Also very helpful if your antenna falls > off the tower and virtually all of the RF is being reflected... your > PA might still survive... if the load can handle the power.) > > Since they're ferrous devices, they can actually CREATE broadband > noise and IMD. > > So I guess you could call keeping external RF signals from coming > down your feedline, and out of your TX -- thus keeping all those > frequencies from MIXING in your transmitter -- "cleaning" -- but > that's not what most people think of when they see that word... > > From your description, folks might think an isolator is something > like a bandpass filter (which it's a good idea to have PAST an > isolator) on the TX side of things. That's definitely not what they do. > > -- > Nate Duehr, WY0X >
Mostly correct but an isolator could be thought of as a device to "clean up a transmitter" too. An isolator provides a flat 50 ohm load to the transmitter no matter what kind of load the antenna or duplexer may present. Sometimes transmitters get a little "squirrelly" and generate spurs when not seeing a flat 50 ohms. A bandpass filter is not only a good idea to have after an isolator, it is a must have device. Or a low pass filter in place of the bandpass filter as isolators generate strong harmonics that need to be filtered. 73 Gary K4FMX
