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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: "Richard (Rick) Karlquist" <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Tue, 30 Mar 2010 19:24:06 -0800
List-post: <">>

Tom Osborne wrote:
> Hi Rick
> Doesn't it take a lot more 'tuning stuff' to make lumped elements than it 
> does to make stubs?  All I have here in the way of test equipment is an MFJ 
> 259B analyzer and a VOM :-)  Works good for making stubs.
> I remember when I made some mark/space filters for some TU's that I built 
> some years back, I needed a signal generator, o'scope, and a frequency 
> counter.  All that stuff is gone now.  73
> Tom W7WHY

Tuning an LC tank is just as easy as tuning a stub.  Obtain the
design capacitance as a combination of off the shelf capacitors,
and then connect to the coil and measure resonant frequency with
MFJ259 or whatever.  Move tap on coil to get resonant frequency correct.
If you overshoot, you can go back, unlike trimming a stub where you
start over if you overshoot.  In a multipole filter, there is a
known algorithm where you can tune one pole at a time by shorting
the neighboring poles.  Another algorithm uses a 1 port network
analyzer like the MFJ259 and tunes for best reflection coefficient.
I better you understand filters, the less test equipment you will
need.  I would say the MFJ259 is going to be sufficient in most cases.
You can predict coil inductance vs size and turns to within a few 
percent with Wheeler's formula, found in any handbook.  OTOH, with
coax, you have to measure velocity factor.

Rick N6RK

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