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Re: [TowerTalk] MODEL FOR TOWER

To: towertalk@contesting.com
Subject: Re: [TowerTalk] MODEL FOR TOWER
From: jimlux <jimlux@earthlink.net>
Date: Fri, 26 Apr 2019 12:52:29 -0700
List-post: <mailto:towertalk@contesting.com>
On 4/26/19 10:45 AM, Gedas wrote:
One thing I have not seen mentioned (maybe I missed it) is the fact that a whole lot of time & effort is going to go into generating a super accurate model for a complex tower and antenna system but then, what of the grounding of the tower itself at the base? Grounded or un-grounded or sorta grounded are all going to give different results.

That's easy to test in NEC. Just put a resistor between tower and ground.

I actually don't think it will have a huge effect. the tower is basically a bunch of vertical wires. They'll change the tuning of the Yagi a bit, and there will be some current in the area of the Yagi. But 60 feet away, there won't be much current.

I'd worry more about the horizontal and sloping cross members. They're more "aligned" with the segments in the antenna.

However, NEC is a good way to test this. You put the wires in, and see if there's current. If there's current, that's something to think about.

Even in NEC2, which isn't great about connections between antennas and ground, you could do a quick check. If you have NEC4, then it does great at modeling wires penetrating the ground - that was one of the big improvements from NEC2.

To be honest, I don't know why NEC4 is still export controlled, other than LLNL has no incentive to re-examine it. It's probably not a huge money maker (and they're non-profit anyway), but as long as someone has to vet the recipient, they're going to charge for it. And it probably helps some statistics - Gerry Burke's organization at LLNL probably gets some kudos for "number of licenses issued" (that's the way it works at JPL) and that helps justify their funding from DoE.



There are going to be deviations from real world results if one models with the limitations of objects touching the ground with NEC-2 and even with NEC-4.

NEC4 is pretty good at modeling things penetrating or beneath the soil surface. The key is that at the point of contact, you need to have a segment boundary. The recommended (in the manual) way to do this is to have different wires for the above ground and below ground parts of the structure. That guarantees that there's a node at the surface.

What NEC4 doesn't do well is model a wire laying on and just touching the surface. When I model, what I do is model the wire at 10cm, 5cm, 1 cm, 1mm, -1mm, -1cm, -5cm, and -10cm and see what changes. (assuming the wire is <2mm in diameter).

To be honest, people have spent their lives studying the problem of a wire just touching, or partly immersed, in a dielectric boundary. J.R. Wait at NIST has lots of papers on this.

The big uncertainty is the soil dielectric properties.



Gedas, W8BYA

Gallery at http://w8bya.com
Light travels faster than sound....
This is why some people appear bright until you hear them speak.

On 4/25/2019 7:40 PM, jimlux wrote:
On 4/24/19 5:03 PM, Steve Maki wrote:
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



Here's what the latest NEC documents say:
"The wire radius a relative to {lambda} is limited by the approximations used in the kernel of the electric field integral equation. NEC uses the thin-wire approximation, neglecting transverse currents on wires and assuming that the axially directed current is uniformly distributed around the segment surface. The acceptability of these approximations depends on both the value of a/{lambda}  and the tendency of the excitation to produce circumferential current or current variation. Unless 2*pi*a/{lambda}  is much less than 1, the validity of these approximations should be considered."


So NEC does not model transverse currents in a conductor - so while you can model a tower as a wire of comparable diameter to the tower, the model will only work for (mostly) fields that are vertically oriented.


A further hiccup in modeling a lattice tower might be the "short segments forming loops" problem.

NEC2 doesn't deal well with very short segments. NEC4 deals with them just fine.

However, for loops where the circumference is <0.002 wavelength, the results may not be valid.  IN practical terms.. if you've got a triangular tower with face width 1 foot (perimeter 3 feet), if the wavelength is >1500 feet, you might have a problem.  Topband and Cheap TV antenna lattice *might* get into trouble.

Modeling 4" reinforcing mesh or a dense rebar lattice might also run into troubles.


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