Hi Dave and Russell,

Thanks for the replies.

I am familiar with such programs (like the class of NECs that Russell
mentioned). I have EZNEC, which sounds like the one you mention, Dave. They
seem to work very well.

However, I don't think I explained my question very well because that's not
really what I'm asking. I know that you can accurately model an antenna
system if you go all out and specify every detail of it and use a
sophisticated software package.

What I'm really asking is this: I've always heard (and read) that if you
want to determine the gain of an active antenna array (not a parasitic one
like a Yagi, but one where you drive all the elements), then you can do so
fairly accurately by assuming that the elements do not interact and just
doing simple EM interference calculations. In reality, they will interact
somewhat, but the result will be close to what you get by assuming that
they do not interact.

I'm wondering what theoretical justification there is for this assumption.
It seems to me that they must interact rather strongly, because if they did
not, you would be able to violate conservation of energy quite dramatically
even for elements more than a wavelength apart (and most examples of active
arrays given in books show elements only a quarter or half wave apart).

It seems as though the only way they could interact without drastically
altering the resultant pattern would be if the individual pattern shapes of
the elements were not affected but the magnitude of the E field was scaled
by a uniform constant in all directions. This seems to be a strange kind of
interaction to have.

Ideas?

Thanks,

Sean

At 07:54 AM 1/15/2003 -0800, you wrote:
>Sean Breheny <shb7@CORNELL.EDU> wrote:
> > After the longwinded intro, my question is this: Is there a well-known
> > way of estimating how much the far-field E field of an antenna must
> > change when other elements are placed in its vicinity? This would seem to
> > be a very fundamental quantity that you would need to know because it
> > affects the estimated gain of an antenna array.
>
>A ham friend of mine showed me an antenna analysis program that seemed
>quite sophisticated. As I understood it, you gave it descriptions of all of
>the elements of the antenna (both driven and passive) in terms of size and
>position. It would divide each element into short segments (wavelength/10
>or less) and then model each segment as a piece of "current". It would then
>solve the resulting system of discrete equations based on Maxwell's laws to
>come up with the correct value of current (phase and magnitude) for each
>segment. Based on this, it could then produce fairly accurate far-field
>pattern graphs.
>
>If you want, I'll write to him and ask him what that program was.
>
>-- Dave Tweed
>
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