Quad loop supported by single tower - NEC2 modeling question
 

Hi,

I am modeling a vertically oriented, vertically polarized loop for
160 meters. The loop will be supported at the top of a 130 foot tall
tower with a 6 foot standoff arm. The Bottom of the loop will be 10
feet from the ground. I was modeling the exact physical layout of the
loop with NEC2 without considering the tower structure in the model
at all.

The tower is Rohn 45G with non-conductive guy wires. Not wanting to
duplicate the exact tower model in NEC2 (by hand), I substituted in
essentially a wire with a diameter of 1 foot, using steel for the
conductor. I am modeling the antenna over 'real ground'. Putting in
this wire next to the antenna had a pretty big impact on the resonance
frequency that NEC2 reported. The pattern is slightly altered as
well.

My question is, 1) Am I taking too much of a shortcut to use a single
wire to represent the tower? I just wanted a *rough* idea if this
tower length (1/4 wavelength on 160) was going to have some impact on
the pattern in the real world. Resonance frequency shifted down in
frequency 35Khz in my model.

I've modeled and built loops that I've installed between trees and the
results have matched NEC2 very well, but I don't know what to make of
the support steel tower in this case - am I overstating its effect the
way I modeled it?

-Scott, WU2X

Quad loop supported by single tower - NEC2 modeling question
 

You've done the right thing. The tower features are a tiny fraction of a
wavelength, so the single wire substitution is fine.

To improve the accuracy of the model, use the equivalent radius of 0.421
foot for a triangular tower with one foot sides. This will likely make a
noticeable difference only if the tower is somewhere in the vicinity of
self resonance.

Roy Lewallen, W7EL

wrote:
Hi,

I am modeling a vertically oriented, vertically polarized loop for
160 meters. The loop will be supported at the top of a 130 foot tall
tower with a 6 foot standoff arm. The Bottom of the loop will be 10
feet from the ground. I was modeling the exact physical layout of the
loop with NEC2 without considering the tower structure in the model
at all.

The tower is Rohn 45G with non-conductive guy wires. Not wanting to
duplicate the exact tower model in NEC2 (by hand), I substituted in
essentially a wire with a diameter of 1 foot, using steel for the
conductor. I am modeling the antenna over 'real ground'. Putting in
this wire next to the antenna had a pretty big impact on the resonance
frequency that NEC2 reported. The pattern is slightly altered as
well.

My question is, 1) Am I taking too much of a shortcut to use a single
wire to represent the tower? I just wanted a *rough* idea if this
tower length (1/4 wavelength on 160) was going to have some impact on
the pattern in the real world. Resonance frequency shifted down in
frequency 35Khz in my model.

I've modeled and built loops that I've installed between trees and the
results have matched NEC2 very well, but I don't know what to make of
the support steel tower in this case - am I overstating its effect the
way I modeled it?

-Scott, WU2X

Quad loop supported by single tower - NEC2 modeling question
 

Thanks Roy!

I guess I was confused about the obvious connection the tower base has
to the ground. Will this have any addition affect?

-Scott, WU2X

On Dec 7, 3:34 pm, Roy Lewallen wrote:
You've done the right thing. The tower features are a tiny fraction of a
wavelength, so the single wire substitution is fine.

To improve the accuracy of the model, use the equivalent radius of 0.421
foot for a triangular tower with one foot sides. This will likely make a
noticeable difference only if the tower is somewhere in the vicinity of
self resonance.

Roy Lewallen, W7EL

wrote:
Hi,

I am modeling a vertically oriented, vertically polarized loop for
160 meters. The loop will be supported at the top of a 130 foot tall
tower with a 6 foot standoff arm. The Bottom of the loop will be 10
feet from the ground. I was modeling the exact physical layout of the
loop with NEC2 without considering the tower structure in the model
at all.

The tower is Rohn 45G with non-conductive guy wires. Not wanting to
duplicate the exact tower model in NEC2 (by hand), I substituted in
essentially a wire with a diameter of 1 foot, using steel for the
conductor. I am modeling the antenna over 'real ground'. Putting in
this wire next to the antenna had a pretty big impact on the resonance
frequency that NEC2 reported. The pattern is slightly altered as
well.

My question is, 1) Am I taking too much of a shortcut to use a single
wire to represent the tower? I just wanted a *rough* idea if this
tower length (1/4 wavelength on 160) was going to have some impact on
the pattern in the real world. Resonance frequency shifted down in
frequency 35Khz in my model.

I've modeled and built loops that I've installed between trees and the
results have matched NEC2 very well, but I don't know what to make of
the support steel tower in this case - am I overstating its effect the
way I modeled it?

-Scott, WU2X

Quad loop supported by single tower - NEC2 modeling question