[TowerTalk] Using Stubs to Reject Harmonics

[Richard (Rick) Karlquist]

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?
> 
> Jim

No you just need a multielement lumped filter built from the handbook.

Rick N6RK