On 11/25/2010 2:30 AM, Jim Thomson wrote:
>
> ## The ARRL HFTA program is flawed somewhere. Apparently it uses simple
> ray analysis in the far field only. In the old w2pv literature /notes,
> w2pv modeled
> a 20m yagi at 150'.... and varied the height of the lower yagi from
> 65/75/85'. The BIP
> gain peaks when using the lower yagi at 75'..and drops off at 65/85.
> HFTA on the
> other hand shows the gain increasing, as the lower yagi keeps getting
> higher and higher.
> Try it, it's bizarre. Gain is lowest, when lower yagi is say 65'...then
> just keeps on increasing, as
> you raise it!
> When the bottom yagi is a 130', it's still increasing. [top one stays put
> at 150']. Since the HFTA
> program does not factor in near field interaction/mutual coupling, etc, that
> might explain the dismal
> results. [ it was w2pv who 1st suggested the BOP mode]. Who knows, maybe
> at the top of the
> next cycle, [ when angles are usually a little higher], BOP may actually
> work...more often.
>
I believe you are correct about HFTA, Jim. From the HFTA manual:
"The internal Yagi model in HFTA is a simple mathematical model.
It does not compute interactions between individual Yagis in a
stack---HFTA
assumes that each antenna is a "point source." For antennas stacked
more than about a half wavelength apart this is not a problem. For
example,
you should be cautious specifying spacings less than about 20 feet
on 20
meters (and proportionately scaled on other bands) because of
mutual-coupling effects between real antennas.
An interesting experiment would be to compare the output of EZNEC with
HFTA (terrain set to flat ground) using the 20 meter stacked Yagi
parameters you give in your example. To get the best comparison it would
be helpful if you had the physical characteristics for the Yagi's that
Dean uses as the basis for his antenna selections in HFTA. I would
expect to see good agreement between the two programs at wider Yagi
spacings with the results diverging significantly as the spacing was
reduced.
73, Mike W4EF...................
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