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Repeater Duplexers

posted Jan 18, 2016, 9:13 PM by Charles Boling
I was looking at some of those little mobile duplexers, and was amazed that they could do VHF, and get as close as 3.5 MHz for 70dB separation, while being so small.  I became curious as to how they were constructed -- and how tight the frequency adjustment was (i.e. could you use them for a range of frequencies without readjusting).  As it turns out: very tight -- they're 100% notch rejectors, not high/low filters or pass cavities, so both sides need to be pretty exact on the frequency of the other in order to attain a decent suppression of the unwanted signal.  I found a nice little easy-to-understand article on duplexers at EMR.  In fact, they have a collection of 8 articles on repeater filters and such; I'm looking forward to reading the rest of them.    And, of course, if you're looking for info to help you understand repeater construction better, there's always a wealth over at