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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