Jorge Diez - CX6VM wrote:
> Hello,
>
>
>
> What´s the minimum distance between antennas to start playing with HFTA for
> a 3 stack of 40-2CD´s?
>
>
>
> HFTA is doing well with a distance of 33 ft and also 16 ft! but this is too
> close.
>
>
HFTA doesn't take into account interactions in the near field between
the antennas. It just looks at the far field pattern, assuming a
particular pattern for each of the antennas in the stack.
As to where the assumption of "antennas are isolated from each other"
starts to break down? The classic "it depends".. certainly, anything
with a half wavelength or so will have some coupling. Antennas with a
lot of gain have a lot more stored energy in their near field, which
makes them more susceptible to interaction.
The only way to know for sure is to model it in something like NEC. It
doesn't have to be a really accurate model (e.g. you don't need to know
taper schedules, boom corrections, etc.).. what you are looking for is
the magnitude of change when you put the second antenna in. Model it
with single conductors of some reasonable compromise diameter.. Run the
pattern at some frequency you're interested in (you don't really care
whether the antenna has good vswr, at this stage)..
You DO want to check the pattern and make sure that the F/B and F/R
ratios are in the ballpark you expect (they'll be different, because
your model is pretty sketchy, but it should be close).. You could
change the modeling frequency up and down to get where you want (or
scale all the element lengths by some small fraction).. the idea is to
get relative element currents about the same as they are in the real
antenna.
That way, the magnitude and phase of the near field being modeled is
going to be comparable to what the real antenna has.
Now, throw in the second antenna, and compare the patterns. If the
side/back lobes change by a bunch, bingo, you've got interaction, and
then you can decide if you want to improve the accuracy of your model,
or approach it a different way.
Or, you can look at the element currents and see if they change a lot.
You can look just at the current at the center of the element (put
excitation sources at the center of each element, but set to very small
voltages, and you don't have to hunt through the segment currents to
find it.. it will be in the table of feedpoints)
In particular, look for significant changes in the phase of the element
current, especially if that element is carrying a current that is, say,
more than 20% of the driven element.
Small changes in phases and magnitudes have almost no effect on the
forward gain, but will have a much bigger effect on side and back lobe
levels. Turning a -20dB null into a -10dB null doesn't take much power
in the wrong place. That same 10% of the total power is only 0.5 dB in
the forward direction.
The other thing to do with a NEC model is to not drive the second
antenna (i.e. set its feedpoint excitation to 1 millivolt or something
small) and only excite the first antenna. Then see how much current is
induced in the second antenna's feedpoint. If there's no interaction,
then there won't be any current. If the current is <10% of the current
in the primary antenna, then you're looking at any "errors" from the
second antenna being 20dB down, and probably negligible.
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