I did try it with two shunt fed towers on 160 meters 1/4 apart using a
Wilkinson power divider and had little success >>>
Because of mutual coupling, antennas with unidirectional patterns have
grossly dissimilar impedances at each element. The elements, provided they
have reasonable loss, have nowhere near similar impedances and nowhere near
the impedance as a single element.
Elements in two-element arrays require equal currents to have deep nulls.
Since the impedances are grossly different, there isn't any divider that
will supply the design phase and current ratios to low loss elements. It
takes a phasing system custom designed for the actual impedances involved.
<<and then read in ON4UN’s Lowband book that phasing shunt fed tower was
very difficult if possible at all. >>
That is not correct.
I successfully phased a G5RV (100 ft high as a "T") against a 130 foot
shunt fed tower. It had extremely deep nulls with three patterns, and I had
four patterns. I had unidirectional NE, unidirectional SW, bidirectional
NE/SW with deep nulls NW and SE, and broadside (which was almost omni
because of the close spacing).
Shunt fed towers do not behave like normal tower because the shunt wire acts
like an additional length of transmission line. This alters feedpoint
requirements, and the elements no longer require equal currents at the
feedpoint for deep nulls like a current maxima fed element, or equal
voltages like a voltage maxima fed element. They require something between a
voltage fed element's requirements of equal voltages and a current fed
elements equal currents, the exact requirement dependent on the shunt
characteristics as a transmission line. The shunt also introduces phase
shift that must be allowed for.
While it is not an easy cookie cutter task, it is not nearly impossible. It
can actually be pretty fast to set up, if the phasing system is adjustable
in delay and ratio.
Wilkinson's and similar only supply the required ratios and phase when
terminated in the design impedance, and that does not happen with
unidirectional patterns in small endfire arrays (unless significant loss is
added). When the division and phase errors caused by mutual coupling
creating dissimilar impedances is combined with the shunt feed adding phase
shift, and the shunt changing required ratios in voltage and current, it
probably would not work very well. That doesn't mean phasing shunt systems
cannot work, or even that the job is difficult. It is just different.
73 Tom
All good topband ops know how to put up a beverage at night.
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Topband Reflector
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