Topband: PHASING SHUNT FED TOWERS

[Tom W8JI]

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