Jim Lux wrote:
> 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.
A rule of thumb is that an inductive stub has the Q of a coil of the
same diameter. Thus RG-8 coax is not even as good as a coil on a
1/2 inch diameter form. The Q of capacitors made from coax is
not as bad, but still fairly poor compared to lumped capacitors.
> 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, though.
No, low unloaded Q is never an advantage. If you want broadband, lower
the LOADED Q.
> 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 terminating impedance.
> Hmmm.. maybe you need an adaptive canceller for the interfering signal?
No you just need a multielement lumped filter built from the handbook.
TowerTalk mailing list