[TowerTalk] MODEL FOR TOWER

[jimlux]

On 4/24/19 4:42 AM, john at 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
> 
> _______________________________________________
> 
> 
> 
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk