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
