Phased Array using a Passive Element Beamformer
This is not easy as has been amply discussed in sources like the ARRL Antenna
Book. The schemes described there rely on fairly simple arrangements of
antennas, and further, rely on the quarter wave transmission line current
forcing properties. The Antenna Book does mention the broadcast approach
in passing, that is, using separate LC networks on each element, then separate
phase shift networks , and finally suitable power dividers, and then goes
on to say that it is too complex, and that setting the array up for various
directions is too difficult. Here is the basic idea:
Why is the broadcast approach so difficult?
So, the Antenna Book, and other ham literature, has concentrated on solutions
using limited flexibility: limited numbers of elements, restricted numbers
of phases to drive the elements. Arrangements like the 4 square with 90
degree phase shifts are popular because with a half dozen relays and one
LC network you can make it work, and it gives you a reasonably good null
in the back.
There are multiple networks needed to determine the pattern and they all
interact, so there isn't a simple way to have a big rotary switch, or a
few relays to control the array direction.
The interactions are quite complex, and not amenable to hand calculation.
A couple of developments in the past few years has changed the situation.
All of a sudden, you can have a single knob to steer the array, with the
computer taking all the work out of setting up all those L's and C's.
The first development is the availability of cheap computer controlled
LC tuning networks, as described in QST, and as available from LDG
The second is that computers capable of doing the necessary impedance and
matching calculations are cheap and readily available.
The key components are
passive1.htm - Revised 16 June 1999, Jim Lux
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