First, I should state this is not a question about an amateur antenna
system, but I'm hoping someone may know the answer.

Consider a phased array of antennas. There are some 30 or so antennas
all in a line, as below, where "A" is an antenna and "-" indicates a space


A---A---A---A---A---A---A---A---A---A


The overall width of the array is D.

The radiation pattern varies as a function of distance from the antenna
until one is in the far-field. But where does the far-field start for a
phased array? Can one use the normal formula of

2 D^2 / lambda ?

If the width D is large (say 30 m) and the wavelength small (say 0.1 m),
then the far field does not start for

2 * 30 * 30 / 0.1

18,000 m
= 18 km

i.e. the radiation pattern is a function of distance until you are some
18 km (about 11 miles) from the antenna.

If anyone can give me a link to a professional reference on this,
scientific paper etc, that would be useful.


--
Dave (from the UK)

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