On 04/24/19 8:40 AM, jimlux wrote:
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.
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.
In the scenario where you have a large enough tower that a nearby
horizontal antenna is impacted by the tower's horizontal members - is
there a fundamental difference between a lattice tower compared to a
cylinder of like diameter?
I've assumed no, but now you have me wondering.
-Steve K8LX
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|