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Re: [TowerTalk] Using Stubs to Reject Harmonics

To: Jim Lux <>
Subject: Re: [TowerTalk] Using Stubs to Reject Harmonics
From: "Richard (Rick) Karlquist" <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Tue, 02 Feb 2010 20:25:58 -0800
List-post: <">>
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 

> 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

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