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Re: [TowerTalk] coax stubs /bandpass filter

To: "Tower and HF antenna construction topics." <>
Subject: Re: [TowerTalk] coax stubs /bandpass filter
From: jimlux <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Tue, 30 Mar 2010 18:00:21 -0700
List-post: <">>
Dave - AB7E wrote:
> I also don't understand the seemingly single-minded fascination with
> coax stubs.  It takes more than one stub to get the same attenuation
> that a properly designed lumped element filter can provide, the
> optimum location for the stubs is not typically convenient, and stubs
> for the low bands use a LOT of coax (which is both expensive and
> bulky). 

OTOH, scraps of coax are something that a lot of hams have. And one can 
probably use pretty bad coax and still get some improvement.  At last! 
something to use that junk coax you found buried in the grass when you 
cleaned the yard up last year.

  And depending upon the application, the ability of a lumped
> element filter to be either low pass or high pass instead of just
> bandpass can be useful (such as for buffering a low band antenna from
> a single-feedline tribander).  With a little thought, you can pretty
> much design a lumped element filter (or combination thereof) for any
> passband you want.

A lot of folks aren't interested in the "thought" and would prefer a 
"cookbook".  And when it comes to cookbook, coax is easier than 
scrounging up wire and a form to wind an inductor and finding suitable 

One could build a coax stub from dimensions in a webpage in less than 5 
minutes.  I doubt one could do an LC filter in that much time.  It would 
take me longer than that just to find a box to put it in, much less deal 
with connectors, etc.

Don't get me wrong.. I think lumped LC is really the way to go.  Maybe 
there's a market for a simple version.. The commercial products (e.g. 
the NQN or Dunestar, etc.) are in the $60-90 range, and it would be hard 
to beat that price for an assembled unit, by the time you figure 
assembly, packaging, etc.

Maybe, though, someone could figure out a way to do it sort of like 
(gasp) antenna traps.  Wind X number of turns of AWG12 house wire on a 
1" PVC form, etc.

> Software to design such filters is available free to hams.  Lots of
> folks use ELSIE, but I also find the free program AADE
> ( to be pretty nice.
> 73, Dave   AB7E

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