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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