[TowerTalk] quad

[Maurizio Panicara]

Tom,

I took the time to model an 80m loop and two 80m half wave, face to face
bend dipoles feed with half power and one delayed 180°, like it happens in
the one WL closed loop.
I did test both models over real ground (half wave from loop center) and in
free space.
In both cases (two bent dipole or 1 WL loop) horizontal and vertical fields
are the same, the plots are nearly overlapping.
In both cases the current distribution (shape and phase) in the vertical
sides is the same.
It's a fact that vertical sides in 1 WL loop do not radiate much, but the
reason is other than beeing a sort of loading device like a capacitive end
loading.
Probably, it is more adequate to look at loop vertical sides like a sort of
big transmission line.

73,
Mauri I4JMY




----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: <towertalk@contesting.com>; <n4kg@juno.com>; "Maurizio Panicara"
<i4jmy@iol.it>
Sent: Tuesday, August 15, 2000 1:20 AM
Subject: Re: [TowerTalk] quad



> 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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