## Design Procedure for Passive Phased Arrays

Here's the basic strategy:

Determine the element currents to produce the desired radiation pattern. A
simple strategy will probably work fairly well, and there's no point in optimizing
the phase to the nearest 0.1 degree, since you can't build the antenna that
closely. There is an intuitive argument that for basically identical radiators
(often the case in phased arrays), you want the magnitudes of the element currents
equal. NEC does this quite nicely, especially if you use one of the modified
versions (I like 4nec2 from Arie) that has current sources (modeled as two port
networks that transform voltage to current).

Determine an estimate of the mutual impedances. A good idea is to vary antenna
dimensions by small amounts to determine how much variation there is in Mutual
Z. Wild variations means difficulty in tuning, and maybe a better physical design
is needed. Again, NEC is your friend here, since you can just excite each element
in turn, and measure the current in each feed point. You can either work with
admittances or impedances.

Calculate feed point voltages to get the desired element currents. This is
as simple as multiplying the desired currents by the Z matrix.

Calculate apparent feed point impedances. Divide the feedpoint voltage by the
feedpoint current.

Synthesize a network to partially cancel the feed point's reactive component
over the band. This is done to reduce the amount of reactive power flowing in
the feed lines, which reduces a lot of the loss. Particularly for arrays where
the elements aren't resonant at the frequency of interest (a 20m array, working
at 40m, for instance), cancelling the reactive power at the feed point goes
a long way towards improving efficiency. I'm not so sure about whether it's
worth trying to get the magnitude of the impedance to match the transmission
line.

Synthesize networks to distribute power among the
feed lines with the required phase and magnitude. You'll need to take the feed
line length into account, and some mismatch.

Model the entire system and determine actual pattern

Systematically vary parameters to determine sensitivity of pattern to variations
in parameters

radio/antenna/phased/design.htm - 1 Dec 2002 - Jim Lux

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