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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