"Sean H. Breheny" <shb7@CORNELL.EDU> wrote:
> 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?

I'm going out on a limb, as RF isn't really my forte (where's "RF Jim" when
you need him?), but here goes ...

As I understand it, in the near field (< 5*wavelength or so) you need to
consider E and M fields independently, but in the far field, you can assume
they're fairly strongly related by the permittivity of free space.

I also gather that antenna analysis programs concentrate on currents
(M fields) for their far-field analysis, as these are simpler to combine
(vector addition) to get the far field results.

I think the elements of an all-driven antenna can be considered to interact
weakly, in the sense that the induced currents from mutual coupling are
orders of magnitude less than the currents produced by the driving signal.
Therefore, the analysis simply needs to add the fields resulting from the
element currents using the correct phase angles, which are dependent on
both the relative positions of the elements and the phase of the driving
current. There will be cancellation in some directions that counteracts the
gain in others, conserving the total energy.

I hope that helps. Anything more detailed than that and I'd have to go
consult people who know more about the subject than I...

-- Dave Tweed

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