>From: Rick Karlquist <email@example.com>
>What Tom says is right on the mark, as long as the last component in
>your amplifier is a capacitor to ground, which is probably the case
>90+% of the time. In Jim's case, the last component is a series
>inductor, in which case Tom's advice needs to be modified to
>placing the stub at the amp, or a multiple of a halfwave away.
>All of this confusion is yet another one of many reasons, IMHO, to build
>filters out of lumped components instead of coax stubs. And
>the filters should be elliptic function low pass filters, not bandstop
>filters. For examples of these filters, study the output filters
>in any solid state transmitter. For example, see pages 3 and 4 of:
I'll go for the lumped design (certainly easier and more predictable), and the
recommendation for Elliptic/Cauer (especially if you need some notches, and
don't care about ultimate rejection far away).
I've looked at transmission lines as a replacement for lumped L and C a bunch
of times (mostly for making phasing and tuning networks), and at HF, I'm pretty
sure that the loss of a TL stub is worse than even cheap Ls and Cs. One
advantage of a C made from coax is that it's pretty inexpensive and stands off
high voltage, and it's empirically adjustable with a pair of cutters. The main
issue with loss is the small conductor size, so the IR losses are there.
In a tuned stub, the low Q might actually be an advantage. You get broader
bandwidth at the expense of ultimate rejection. If you don't need a gazillion
dB of rejection, that might be a good trade. I haven't thought it through,
However, most of the filter designs assume that they're working into a
resistive termination on both ends, and a tuned output amplifier or a resonant
antenna isn't that. I suppose you can do the filter synthesis with a suitable
model of the termination impedances. I guess for sufficiently out of band, the
Z tends to some reasonable asymptote. Or, perhaps, in a practical sense, it
works "good enough" even if the end sections of the filter don't have quite the
Hmmm.. maybe you need an adaptive canceller for the interfering signal?
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