On Fri, 20 Apr 2007 00:46:07 -0700, Roy Lewallen
wrote:

Richard Clark wrote:

Hi Owen,

And you have already allowed that superposition does not fail. Thus
there must be some other failure to be found in the choice of antenna.
From other correspondence, it is asserted that a gain antenna, by
virtue of its size, cannot be placed in null space (that point wherein
all contributions of energy sum to zero) which is planar and
equidistant between sources (there being two of them for the purpose
of discussion).

Have I described this accurately?

I think it might be more fundamental and perhaps subtle than just a
limitation of size. If the null space is a whole plane, as with the two
radiating elements of my example, you have an infinite area on which to
construct your antenna, although it would have to have zero thickness.
But even allowing infinitely thin elements, I don't see any way you can
construct it entirely on the plane so it will be more sensitive to
signals coming from one side of the plane than the other.

Hi Roy,

I presume by your response that it affirms my description. Moving on
to your comments, it stands to reason that the reduction of the
argument proves you cannot build an antenna with directivity within a
very specific constraint - the null space. As there is zero dimension
on the axis that connects the two sources, then no directivity can be
had from a zero length boom as one example. Other examples would
demand some dimension other than zero along this axis is where I see
the counter-argument developing.

... But if I'm
right, then there's no way to do as Owen originally proposed, namely to
determine entirely from a null space that the null is the sum of
multiple fields, let alone the nature of those fields -- at least with a
directional antenna. It has to extend out where it can a sniff of the
uncanceled fields to do that.

This then suggests that there is something special about null space
that is observed no where else. That is specifically true, but not
generally. What is implied by null is zero, and in a perfect world we
can say they are equivalent. Even a dipole inhabiting that null space
would bear it out, whereas an antenna with greater directivity along
that axis would not.

However, if we open up the meaning of null to mean a point, or region,
within which we find a minimum due to the combination of all wave
contributions, then I would say a directive antenna is back in the
game, and that it exhibits Owens proposition (if I understand it - but
I still need to see Owen's elaboration).

73's
Richard Clark, KB7QHC