Dave (from the UK) wrote:
Jerry Martes wrote:

Hi Dave

I'm curious about two things.

1 - Do you intend to actually make and record measurements of the
radiated field, or do you want to determine the minimum distance at
which the measurements can be made?


yes


An interesting problem. What you're presumably trying to do is
determine how far do I need to be to bound the uncertainty on a
measurement in an arbitrary direction. Or, another way, at what distance
is the collective effects of the phase error for each of the signals
(due to path length differences) smaller than your measurement
uncertainty (so you don't care anymore).

This can be quite challenging if you want to worry about -40dB nulls,
for instance, because a very small phase error can result in a -40dB
null becoming a -30 dB null.

Complicating it a bit is that what you're probably really concerned with
is a statistical problem.. you've got multiple sources, a random
direction of observation, (and practically speaking, some propagation
uncertainties between antenna and observation point).

You might want to look for a paper by Dybdal and Ott: "Coherent RF
Error Statistics", IEEE Trans MTT, v34,n12, Dec 86, pp1413-1420 which
discusses this in some detail, and, as well, provides some nice
approximations that are useful in practical systems.



2 - What prevents the use of a computer modeling program to
predict the pattern?


nothing. I think that will be done. But a theoetical analysis would be
nice if possible.

One can come up with a "bound" for the performance from analytical
means, and a Monte Carlo analysis can give you some statistics.

Jim