From: H. Ward Silver (email@example.com)
There is a handy trick when constructing 2-element phased arrays to force
current to be equal in both antennas, regardless of feedpoint impedance
within limits. Feed each antenna with an odd number of
quarter-wavelengths of feedline...the same odd number, of course, to keep
the phase the same in both.
This is a consequence of transmission line characteristics, translating
the voltage across the line where the feeds are tied together into a load
current equal to the same voltage divided by the characteristic impedance
of the feedlines.
If exact pattern symmetry isn't too important, then the feedlines can be
any length, but current distribution may vary significantly. Pattern gain
doesn't vary that much, but the depth of the pattern null is extremely
sensitive to current balance.
The quick thinkers out there will have also realized that a
quarter-wavelength of coax is significantly shorter than a free-space
quarter-wavelength. So if you cut only a single quarter-wavelength for
your feed lines, and space the antennas one half-wave apart, your face
will be red as the feeds will be kinda short. Three quarter-wavelengths
works just as well as one-quarter does.
We used a pair of HF2V's on the beach at W7FR over the weekend and had a
fantastic 40-meter signal. I used an MFJ SWR Analyzer to set up the
coils...it was a snap...took about five minutes for each antenna. The
radial system was a piece of braid and several long wires tied to rocks,
all of which were put into salt water. Yow!
It works well at home, too, you just need a bunch more radials. The
HF2V's are often available for about $100 used and are quite good on both
40 and 80. The 160-meter add-on kit makes them very effective dummy loads
on 160 ;-)
73, Ward N0AX
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