From: "Jeff DePolo WN3A" <[EMAIL PROTECTED]> > > >Even pass cavities don't necessarily attenuate harmonics produced by a >circulator sufficiently. A 1/4 wave cavity will have a very good pass >response at 3/4 wave (3rd harmonic).
A long time ago I tuned a 10" diameter Motorola VHF pass can to a 2 meter freq., then tried to use it at exactly 3x that freq. It wasn't even close to resonance. I suspect there was some capacitive loading or overmoding taking place that was pulling the 3/4 wavelength resonance. I'll try repeating that test some evening & report my findings. If this is true, that would make a pass cavity more effective at suppressing odd transmitter harmonics than one would normally expect. >Even at other harmonics, or between >harmonics, a bandpass cavity isn't necessarily going to give you adequate >attenuation of the harmonics. A pass can is always good practice on any >transmitter, but a one-stop-shopping cure for isolator harmonics it is not. Whether the higher-order resonances are pulled or not, I would expect a pure pass cavity to have excellent rejection at antiresonant frequencies such as even harmonics. Yet another evening experiment to try. > >And be careful of the varieties of "harmonic filters" out there. Some of >the cheaper ones are just 2nd harmonic traps. They'll knock down the 2nd >harmonic by 40 dB or so, but do nothing for the 3rd and higher harmonics, >which can really be a problem on highband (3rd harmonic ends up on UHF). A >real low-pass filter is what you should use. Agreed. However, I thought the biggest harmonic threat from an isolator was the 2nd. Bob NO6B Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
