[TowerTalk] quad

[Tom Rauch]

> Sorry, but I've to insist on this point.
> The sides of a 1Wl loops are NOT end loadings in any case, this is
> conceptually wrong.
> The current and voltage distribution along sides is varying like in a half
> wave dipole or, eventually, like in a quarter wave transmission line. Cut
> the 1 Wl loop at  voltage nodes and discard the dipole without feeding.
> This is a bent half wave dipole with no end loading but with a linear
> current and voltage distribution. Now feed also the other half loop (the
> specular one), in phase. This is another half wave dipole with a linear
> current and voltage distribution. Now, beeing the bent sides 180° out of
> phase the far field from them is cancelled exactly like it happens with
> the loop.

No, that isn't so. Broadside to the bent dipole, radiation from the 
bent ends cancels. 

In a line going through the ends, you have two vertically polarized 
sources spaced 90 degrees apart driven 180 degrees out-of-phase.

A half-wave dipole, bent at the ends, has very little cancellation of 
radiation in line with the antenna off the ends...because the spatial 
phase and source phases total 90 degrees. The radiation is 
vertically polarized.

The quad antenna has two out-of-phase sections carrying exactly 
equal currents at each end. From any azimuthal angle along the 
horizon, the vector sum of the two fields is zero for a horizontally 
polarized antenna.

At higher elevation angles there is some phase error in the arrival of 
the two fields at a distant point, but it is mostly cancelled by the 
field from the opposite side of the quad.

The quad antenna, unlike a bent dipole, has very little radiation in 
the farfield from the vertical sides at any angle or direction.

Because of that, the sides of the quad only serve two purposes. 

1.) They end-load the 1/4 wl long "dipole" areas that do the actual 
radiating

2.) They end-feed the second dipole that is not broken and fed at 
the center with the proper phase delay.

The sides in a quad behave very much like they are not there at all, 
so far as farfield energy is concerned. The bent ends in a 
conventional dipole radiate a vertically polarized field directly off the 
ends of the dipole, and that field continues into the farfield. 

If you feed the center of both dipoles in a quad, and replace the 
vertical wire at the end with a capacitance hat of the proper size, 
the result is an almost identical pattern to a quad element at any 
angle or direction.

If you do the same with a simple bent dipole, the pattern changes.

That is why it is proper to consider a quad element two stacked 1/4 
wl long end-loaded dipoles, when predicting pattern. You cannot do 
that with a bent dipole.


73, Tom W8JI
w8ji@contesting.com

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