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.
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.
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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