Topband: Shared Apex Loop Array

[Tom W8JI]

> Is is a two element crossfire vertically polarized array made from loops.
>
> Personally, if I were going to build one, I would spread the loops apart. 
> I would make the loop centers as far apart as possible, not to exceed 1/4 
> wavelength on the highest band.
>

Let me expand a bit how it works. Because the SA array has such close 
spacing, the center vertical element wires cancel. They just vanish from any 
significant pattern effect. The result of the shared apex is the loops 
function as two short ground independent verticals that are phased.

The axis null in a loop forms because all opposing sides carry 180 degree 
out-of-phase currents.  If we push two small  loops together, the vertical 
center wires carry mostly opposing currents, and go into transmission line 
mode. They are too close for any useful pattern control, so the loop changes 
mode.

The reason I suggest wider spacing, if I was doing all that electrical 
hardware, is pushing the loops apart keeps the loop elements in loop mode, 
so far as pattern is concerned. This will add two nulls through the loop 
axis. Although the nulls are rather small area, any new null or additional 
null not in signal direction is a good null.

The no free lunch rule is, as usual, in full force. We simplify by using one 
support and decrease length, and we lose other nice things.

We have two short verticals phased, instead of what would be three or four 
with wider spacing.

73 Tom