Hi, ref questions about elevated dipoles:
> I wonder what the difference is, at those heights, between
> a GP as you describe, and a vertical dipole ? Seems to me
> that if you take half a standard dipole for the top bit, and
> half a fan dipole for the bottom bit, you've really got an
> elevated GP...
Here is an excellent photo of exactly the sort of antenna you
you might have to recopy the above URL into your browser
"open file" space, it is a bit long for the email.
This is the 15 mHz phased array here on Kauai out at
WWVH. The array is designed to proved a cardioid's
pattern, with max power to the West or out to the far
Western Pacific ocean areas.
Note that many ground mounted radials fanning out all
around, you can see them in the lower foreground of
the photo. Even though this is a vertical dipole, with
the downward sloping (and note the insulators to
provide the correct resonant radial length) the ground
radials were absolutely necessary to recover the
ground loss power, and to obtain the total predicted
field strength out to the far West of the Pacific.
You just can't turn a dipole vertical with the lower end
any where near ground and get high efficiency without
the use of the ground radials, hi. I suppose if you were
able to get the lower end of the dipole well above even
1/4 wave length in height, you would be better off,
but at WWVH it was far more economical to use the
ground radials to terminate the high E fields from the
lower ends of those down sloping fanned radials
(which, by the way, provide the shielding for the
coax feed which goes up the center of the lower
portion of the supporting tower structure).
The antenna structures were built by Rohn some
30 years ago, and now need replacing since they
are sitting right on/at the beach here on the island.
When asked, Rohn would not bid again on such
an elevated and center insulated structure; anyway,
now, I guess, Rohn is gone.
73, Jim KH7M
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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