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Re: [TowerTalk] step away from stepIR

To:, "David Gilbert" <>
Subject: Re: [TowerTalk] step away from stepIR
From: Jim Lux <>
Date: Tue, 12 Jun 2007 13:50:54 -0700
List-post: <>
At 01:19 PM 6/12/2007, Rick Karlquist wrote:
>David Gilbert wrote:
> > Rob Atkinson, K5UJ wrote:
> >
> > "Is the element length for 50 ohm Z always the length for best
> > performance?"
> >
>If you read the SteppIR docs you will quickly see that the
>element drive impedance is not 50 ohms but 22 ohms.  There
>is a transformer/balun to get back to 50 ohm coax.  A driven
>element impedance of 22 ohms is quite reasonable in general
>for Yagi design.  The impedance is not the tail wagging the
>dog of the rest of the antenna design here.  That would be
>the case if they tried to get 50 ohms driven element impedance
>to eliminate the transformer.

In fact, I believe there is a fairly elegant paper out there that 
bounds the impedances for a given spacing, so one could, from a 
theoretical basis alone, show that if you put a fixed tuning/matching 
network in (i.e. a 3:2 transformer or similar) that the worst SWR 
across the band is less than some reasonable number.

To a first order, you have 3 dipole elements with characteristic 
impedance of 72 ohms, in parallel.


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