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Re: [TowerTalk] MODEL FOR TOWER

To: towertalk <towertalk@contesting.com>
Subject: Re: [TowerTalk] MODEL FOR TOWER
From: Steve Maki <lists@oakcom.org>
Date: Wed, 24 Apr 2019 20:03:09 -0400
List-post: <mailto:towertalk@contesting.com>
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

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