On 4/24/19 4:42 AM, john@kk9a.com wrote:
Great information Dan!
I have always just guess when converting a tower to a wire diameter. The
original poster seemed concerned that his tower would effect his
horizontally polarized HF beams, I have not seen that occur.
John KK9A
I think the question would be about the SSV/BX style tower which is
larger at the bottom than the top. Rohn 25 or 45 are "small" compared to
a wavelength in the horizontal direction, so they can be modeled as a
"fat wire" - just like a cage dipole element, for instance.
The tower in question is 7.5 ft at the bottom and 2 ft at the top 80 ft
high.
The OP was asking about a 20m Yagi to be mounted at 60 ft, where you'd
effectively have big square loops that are about 3 1/2 ft on a side (14
ft total perimeter) near the antenna, as well as diagonal struts of
some length.
The wavelength is 60-70 ft, so those squares are about 1/4 wavelength in
perimeter. If they were 1/10th wavelength, I'd say "model it as a big
wire", but that's big enough that there might be some interaction,
especially since they will be effectively "inside" the Yagi.
Dan Maguire AC6LA wrote:
Perhaps here:
https://en.wikipedia.org/wiki/Antenna_equivalent_radius
As a comparison, the lower-right of this image shows both the Leeson
and Wikipedia equivalent diameter for a Rohn 25G cross section:
https://ac6la.com/adhoc/4sq1a.png
AutoEZ users, the Leeson and Wikipedia formulas are on the Variables
sheet tab for the "Bent Dipole 4-sq" *.weq model files as described in
the first section here:
https://ac6la.com/aecollection8.html
Dan, AC6LA
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