I created a model of a 2-el yagi on 20m with 6' spacing and the gain in
free space is 6.71 dBi (4.56 dBd). F/B is 17 dB. This is with the
parasitic element tuned as a director. The feed Z is 22 ohms, which the
SteppIR transformer matches to 50 ohms. The 2:1 bandwidth is about 240
kHz, but that's not an issue with the SteppIR, of course. I assumed
0.35" diameter copper elements, which are only down 0.06 dB from
lossless conductors, so the efficiency is pretty good (98.6%).
Increasing the spacing to 12 feet gives max gain with a parasitic
reflector. The gain is 6.82 dBi (4.67 dBd) but the F/B is only 6 dB.
Feed Z is 35 ohms (a good candidate for a hairpin match to 50 ohms) and
the 2:1 bandwidth is 410 kHz. Using lossless elements increase the gain
by 0.05 dB, so the efficiency is 98.8%.
It appears the SteppIR 2-el is a good compromise on 20m for limited
space, other than cost.
73, Terry N6RY
On 2009-09-22 12:31 PM, RLVZ@aol.com wrote:
> I had great results with a SteppIR 3-element up 60' when the antenna
> worked as designed. Unfortunately, the Controller was bad upon arrival, and
> then an 80mph windstorm blew two fibreglass elements off the antenna which
> then allowed water into a Stepper motor and it went bad!
>
> Perhaps I'm a glutton for punishment but I'm seriously considering a
> 2-element SteppIR at a temporary QTH in the Midwest as the 3-element SteppIR
> turning radius may not clear the trees. Question: isn't the short 6' boom
> on
> the 2-element SteppIR too short for good results on 20-meters? I am not
> familiar with using modeling programs and don't have the time to learn so
> I'd
> appreciate your thoughts on this.
>
> A 3-element tribander will work at this temporary site as it has about the
> same turning radius as a 2-element SteppIR... and a Tribander like the
> Hy-Gains TH3 might perform as well or even better better on 20-m. (on the
> portion of the band that it is tuned for) as it has a little longer spacing
> between driven element and the reflector. Of course the Tribander would be
> alot cheaper too.
>
> What's your thoughts on the efficiency of a 6' boom length for a 2-element
> 20-m. yagi?
>
> 73,
> Dick- K9OM
>
>
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