On 21 abr, 10:32, "Dave (from the UK)" see-my-signat...@southminster-
branch-line.org.uk wrote:
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)

Please note my email address changes periodically to avoid spam.
It is always of the form:
Hitting reply will work for a few months only - later set it manually.

http://chessdb.sourceforge.net/- a Free open-source Chess Database

Hello,

Your formula for far field distance (Fraunhofer region) assumes a path
difference between the inner and outer antenna with respect to an
observation point of 1/16 lambda.

The 1/16 lambda (22.5 degrees) path difference is used by many experts
(for example page 39 of "Antennas for all applications" 3rd edition
[Kraus, Marhefka]).

Depending on your application and required accuracy, you can divert
from the 1/16 lambda rule. For example when you taper the power
distribution for your array (to get lower side lobes), the far field
distance is not that far. If you accept a 1 dB error in main lob
gain, you can go down to 25% of the far field distance formula.
However when you want to measure a dip in the radiation pattern of
(for example) -50 dBi, you might need a larger distance.

For your array, you can calculate the effect of distance on radiation
pattern in a spread sheet program.

Best regards,

Wim
PA3DJS