[TowerTalk] step away from stepIR

[Jim Lux]

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.