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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