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[TowerTalk] sources

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Subject: [TowerTalk] sources
From: (L. B. Cebik)
Date: Wed, 10 Dec 1997 06:49:29 -0500 (EST)
> On another note, I finally got my own copy of the Communications Quarterly
> article nixing quarter wave verticals.  VERY interesting stuff!  However, I
> ended up wondering: if avoiding radiation from the radials is the goal, why
> wasn't anything said about the radiation introduced by sloping/drooping the
> radials.  I thought that also caused the radials to radiate.  I think that
> the 1/8 wavelength radials for 7mhz and above elevated verticals could be
> brought out perpendicular to the radiator without too much mechanical
> difficulty if radiation from them is so important to avoid (the construction
> would look similar to a capacity hat, with stiff non-metallic struts going
> from the radiator up to the ends of wire radials, or even use aluminum
> tubing as the radials and support them from above with kevlar.  Perhaps this
> wasn't considered because most of the serious researchers are interested
> only in 80 and 160?  But even on those bands compromise 'loaded' radials
> could be brought out perpendicular without too much effort (considering how
> much effort these guys are willing to go through anyway).

Yes, the drooping radials beneath a quarter-wave upper vertical do
radiate.  The polarization can be thought of as 2 components, horizontal
and vertical.  The horizontal component is self-cancelling due to symmetry
of the radials (and near-symmetry is close enough to keep that radiation
30 dB below the main lobe).

Vertically polarized radiation is roughly proportional to the slope of the
radials--non-existentent when they are horizontal and max (vertical
dipole) when they are vertical.  Relative to the latter case, the
distribution of current among the radials is least significant the closer
to vertical they are--most significant the more horizontal they are (as a
disturbance to symmetry and hence incomplete cancelling of fields).

It pays always to think of the radials we attach to verticals as part of
the radiating system and then ask what the radiated field is based on the
conditions under which the radials do their work.  Then you can integrate
into your thinking all kinds of variants:  standard buried radial 1/4 wl,
elevated horizontal radial 1/4 wl, shortened radials with a perimeter wire
1/4 wl, shortened radials with either a loading coil below the feed point
and 1/4 wl above or a lengthend vertical section fed at electrical center
or lower and off center, etc.  All of the variants can be analyzed as
modified 1/2 wl vertical dipoles, with the radials being part of the lower
1/4 wl section--unless you turn it upside down and make the radials what
used to be called a capacity hat--which you can place on both ends and get
an efficient shortened vertical dipole.

Hope this helps.



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