[TowerTalk] coax stubs /bandpass filter

[Dave - AB7E]

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).  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.

Software to design such filters is available free to hams.  Lots of folks use ELSIE, but I also find the free program AADE (http://www.aade.com/filter.htm) to be pretty nice.

73,
Dave   AB7E




------Original Mail------
From: "Richard (Rick) Karlquist" <richard at karlquist.com>
To: "Tower and HF antenna construction topics." <towertalk at contesting.com>
Sent: Tue, 30 Mar 2010 08:36:08 -0800
Subject: Re: [TowerTalk] coax stubs /bandpass filter

I've given up trying to make the lumped element argument to hams.
Too hard to change belief systems.
Coax stub filters are firmly entrenched ham radio folklore,
regardless of whether they make engineering sense.
They are seldom if ever used for filtering in commercial HF radio systems.

BTW, the argument about the Q being "too high" goes away
with the realization of the difference between loaded and unloaded
Q.  The harmonics property is only useful if you need to pass
the frequencies between the harmonics, which is not necessary
for ham use.

Rick N6RK



jimlux wrote:
> Along a similar note..
> 
> The stub is just a moderate Q resonator that's easy to fabricate.  Why 
> not put a lumped LC shunt trap across the feedline?
> 
> Too hard to get the Q low enough?
> Doesn't have the nice harmonics property? (a coax stub for 40 is also a 
> stub for 15)
> 
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