On 2/29/16 3:01 PM, Richard (Rick) Karlquist wrote:
On 2/29/2016 2:29 PM, jimlux wrote:
So, if you had a 5-10 meter tall radiator, being used on 80m or 160m,
there's not a lot to be gained by going longer than 15-20 meters for the
radials.
This often heard concept makes intuitive sense. Kind of
like how the roots of a tree should be of a length related
to the tree height or width.
However, it simply isn't true. Actually, the opposite
is true. Jerry Sevick, W2FMI, wrote an article for QST
a long time ago that famously had a photo of a 40 meter
vertical that was something like 7 feet high, with a top
hat, but worked over a huge radial field. The thing
that was funny about the photo was that the antenna was
made of a clothes dryer with metal arms that served as
the top hat and the pole was the radiator. The concept
was that the less you have in the air, the more you need
on the ground. Jerry did seem to know what he was talking
about.
I'll agree you need to have a LOT more on the ground "close to the
radiator", but did Jerry ever try his 7 foot "Hills Hoist" antenna on a
radial field that was, say, 30 feet in diameter (1/8 lambda radials in
free space, radials twice the height of the radiator), but still much
shorter than quarter or 0.4 lambda.
I'm basing my statements on work with tesla coils - there's not much to
be gained by making the counterpoise under the coil (a very conductive
sheet: chicken wire or solid aluminum foil or similar) have a radius
much greater than the height of the top load above it (e.g. a 4 foot
high tesla coil works about the same with a 8 foot diameter and 12 foot
diameter counterpoise, but not so well with a 6 foot diameter counterpoise).
A single short monopole might be very different than one with a capacity
hat: the current profile on the radiator will look like a ramp with the
straight monopole and more uniform with the capacity hat.
For the tesla coil world, you can analyze it pretty much as an
electrostatic system: everything is a tiny fraction of the wavelength
(operating frequencies in 100s of kHz range).
The antenna case is not quite so simple, but for very short radiators,
maybe it is.
The "capture area" of a short vertical is nearly the same
as that of a 1/4 wave vertical, no matter how small it is
(at least in the absence of losses). Thus there is no
justification for shortchanging it on radials.
I'm not sure that's been actually modeled: I've got a copy of Nec4, so
maybe it's time to try it.
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