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Re: [TowerTalk] Symptoms of antenna interaction?

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Symptoms of antenna interaction?
From: "Lux, Jim" <jim@luxfamily.com>
Date: Thu, 25 Mar 2021 07:28:13 -0700
List-post: <mailto:towertalk@contesting.com>
On 3/24/21 6:58 PM, Jim Brown wrote:
On 3/23/2021 12:30 PM, Al Kozakiewicz wrote:
Is this symptomatic of interaction, i.e. feed point characteristics change while resonance frequency remains the same?

If you can do modeling, try modeling dipoles for the bands where you are concerned about interaction, with a generator in only one of them. Open the View Antenna window. Then compute the far field response, and on the View Antenna window, go to View, then Objects, and check the box for Currents. You can check other boxes if you want to see those things. I like to show Axes and Segments to help me tweak the model. Then in the View Antenna window, move the Zoom Current slider to make the currents large enough to see current distribution in the dipole with the generator, and look for current in the one without the generator. If there's no current, there's likely to be little interaction.

Repeat this process as you vary the length of the antenna you're not feeding, then move the generator to the other antenna and repeat the process.

This is not as accurate as full models, which are FAR more complicated, but it will tell you whether to look further. A dipole in NEC is simply a wire with a generator in the middle of it. Always use an odd number of segments for a dipole so that the generator can go in the center. Wires are entered by their x, y, and z coordinates.

73, Jim K9YC

This is exactly the process we are using at JPL for evaluating "is it a potential problem" before going to a more high fidelity model.  Build your models with simple wires - don't try to taper or model mounting hardware. You could model the Yagi as 4 wires.

With modern tools like EZNEC, 4nec2, or AC6LA's it's pretty fast to put the geometry in (and, in 4nec2, which I use, you can define symbols, to make it easier)

I don't know that you really care about resonant frequency in this sort of analysis 5% is probably fine - what you're really doing is looking at the *magnitude* of the coupling, and that doesn't change very quickly if you change the length of the elements.

As Jim says, you visualize the currents on the other antennas, and if it's "big" (where "big" is more than, say, 1% of the current at the feedpoint), there's a potential problem.

This is the HUGE benefit of modern tools on modern PCs.  You can run the iterations quickly, so you can get a more intuitive "feel" for what's going on.  You can, for instance, change the skew angle between two dipoles or antennas and see where the coupling starts to come up (yeah, it's basically cos(theta), but that's not instantly visualizable for small changes).

R.W.Hamming said: "The purpose of computing is insight, not numbers"




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