> So - I would encourage those of you who might be able to
> try a phased
> array - but haven't because one (or both) of your antennas
> are gamma
> matched - to give it a try. The trick would be to use a
> hybrid coupler
> and 1/4 wave lengths of feed line - which should force the
> current to
> be in the right phase.
Tree,
The rumor is actually factual Tree. The gamma feed actually
does change what is going on, and a hybrid coupler also is
not generally correct for feeding equal currents. It also
will not assure 90 degree shift if the load is reactive.
Current forcing works on the principle of having the
transmission line in series with the current maxima of the
antenna element. The gamma actually adds transmission line
length and moves the system away from a point where the
feedpoint requires equal currents!
It is still possible to phase the elements, but it gets
tricky. In Rockdale County I phased a 100 foot high open
wire fed 40 meter dipole against a 1/4 wave series fed
tower. I had excellent F/B and had four basic
patterns...broadside...endfire figure 8....unidirectional
endfire.....and of course omni. So a person can certainly
phase any two antennas. It just becomes more complex when
the elements are not fed at a current or voltage maxima. It
is also complicated when the elements are not the same, or
when they are shunt fed.
Equal currents are forced in a series fed element when the
voltage at the end of a 1/4 wave line are equal. Since
mutuals cause the elements to change reactance and
resistance differing amounts when end-firing with a
unidirectional pattern and since each element requires
different power levels, we really don't want a equal power
system. With a traditional 1/4 wave lines and series feed at
the element we want equal voltages at each port of the
phasing system.
A voltage fed system through a 1/4 wave line, or a current
fed system through a 1/2 wave line, would require equal
currents forced at the phasing network. Nothing I'm aware of
off-hand requires equal power in a unidirectional array,
although perhaps a 3/8 wave system might.
What is amazing about phased arrays is they often want to
work in spite of things we do wrong. Traditional 4 squares
with 90 degree hybrids do not operate with the 0/90/180
shift nor with equal currents, but the arrays are forgiving
enough that we are often satisfied with the results. We
really don't care if the null is nearly infinite or only
15-20 dB, if gain drops a dB or so, or if the nulls move off
a bit to the sides.
I'm sure there are cases where a shunt fed system through
some lucky circumstances will work. They certainly all can
be made to work with proper planning. I just wouldn't bank
on being able to use a gamma matched system with traditional
phasing and/or a hybrid coupler. It's stacking the odds more
and more against being lucky.
73 Tom
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