>
> 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 pass can to a
> 2 meter freq., then tried using it at exactly 3x. Wasn't
> even close. Apparently overmoding or some capacitive effect
> pulled the higher-order resonance. Perhaps some evening I'll
> try that experiment again & see how far down the 3rd harmonic
> response is. If it was due to overmoding then it's possible
> that this effect will not be present on the narrower (4 to
> 6") VHF cavities.
The coupling would be the only reason it wouldn't be well matched/resonant;
the cavity itself would be resonant. Ever see Decibel or Telewave 3/4 wave
UHF bandpass cavities? They are identical to the same brand's VHF 1/4 wave
cavities. The loops are smaller, but otherwise there are no differences in
construction, inside or out. Whatever differences in pass performance they
have at 1/4 wave (VHF) versus 3/4 wave (UHF) would likely be attributable to
only the coupling.
Some funny resonances can appear at frequencies well outside the design
range of some filters when they are not operating in their native design
range. "Tip capacitance" from the end of the center conductor to the bottom
of the cavity, coupling loop size/shape, and overmoding (when the cutoff
frequency is exceeded) can all play a part in these odd responses. I have
some fat 3/4 wave 900 MHz pass cavities that will tune up as 2m pass cans!
Go figure.
> I thought that a pass cavity would have maximum attenuation
> at even multiples of resonance. The only real issue should
> then be the 3rd & maybe 5th harmonic of the isolator output.
Not necessarily. If the coupling technique was perfect in that the only
energy that passed from input to output was that which was coupled from the
input loop, to the center conductor, and back out the other loop, then you
could argue that were true. But in reality, there is some coupling between
input and output loops aside from the desired mode. As such, you end up
with a circuit with more than one path from input to output. So looking at
it from strictly an impedance/resonance sense, there's no guarantee that at
even harmonics that the insertion loss would be the highest, nor return loss
lowest.
You're right in that the 2nd harmonic tends to be highest, but when looking
at harmonics on highband, the 3rd is probably the one you want to be most
wary of in terms of interference to other co-located services on UHF.
--- Jeff
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