Hey Tom,
Yes, I should have mentioned the station is just Northeast of
the LA basin in the foothills of the mountains. There is another
set of hills about 1 or 2 miles south of the Mesa (approximately the
same elevation), then it is all downhill to the Pacific Ocean. My
main concern is the effect of the nearby ground on the performance
of a horizontal antenna, especially on 160 where the dipole is less
the 1/8 wavelength above the plateau. I think there is no question that
a good vertical with an efficient ground system would be effective here,
but that would be a lot of work to install (the site is quasi-public, so
the radials would have to be buried for safety reasons).
Anway, the general question that I am asking is the following: is
it valid to ignore the effects of the ground immediately underneath
the antenna, if it is in the near field of the antenna, when calculating
the far field elevation pattern? IOW, when you are sitting on the edge
of a 400' mesa with the antenna only 1/8 wavelength above the plateau,
does the far field pattern look like a high dipole (low angle of radiation)
or a very low dipole (big null at low elevation angles). Also, it would
be interesting to know if programs like K6STI's TA handle this case
effectively (I have been thinking of buying a copy).
The same question of course would apply to an 80 meter inverted vee
installed at the top of the 60 foot tower, but too a lesser extent (this
antenna would be almost 1/4 wavelength above the ground underneath
so it would be decoupled more effectively from the local ground than
the 160 antenna).
Anyway, mother nature has given us a freebie, as normally you have to
spend a lot of money to get your antenna that high above average terrain.
I would like to make the most of it with the right antenna. There is a helipad
close to the site so going higher than 70' with the towers is out of the
question
(we are right at the derated height limit with the current installation).
73 de Mike, W4EF.........................
----- Original Message -----
From: <n4kg@juno.com>
To: <W4EF@dellroy.com>
Sent: Thursday, January 11, 2001 6:47 PM
Subject: Re: [TowerTalk] Re: Radials over salt water
> Are you on the West Coast?
>
> 60 ft sounds pretty low for an 80M Delta Loop
> since an equilateral triangle for 80M would be
> a little over 80 ft tall. With the apex at 90 ft,
> the bottom wire is still less than 10 ft over ground.
>
> A moderately high horizontal dipole (> 70 ft)
> fairly close to the edge might be interesting.
> (Fairly close = Tower height or less)
>
> 73, Tom N4KG
>
>
> ________________________________________________________________
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