On Sat,8/8/2015 10:36 AM, W2RU - Bud Hippisley wrote:
but, with the usual wire sag, the feedpoint was about 5 feet above the gravel.
We weren’t worried, because the roof was at least 70 feet above the surrounding
terrain.
Hmmm! Let's remind ourselves of Nuradi's situation. The roof is 110m
high, 45m x 33 m. Corner to corner is less than a wavelength on 80M,
more than a wavelength on 40M, but the distance to a corner from a wire
strong between the two corners is less than a quarter wave on 80M, less
than a half wave on 40. Assuming an ideal conductor on the roof, it's
going to act as a reflector going upward, but the low angle pattern will
be determined in the far field.
I've not worked before with two ground media, so I pulled out W7EL's
instructions for doing so. I built a very simple model attempting to
roughly simulate Nuradi's situation. I'm running NEC2 with EZNEC Pro5.
The first ground medium is sea water, with a radius 120 ft (it's a
rectangular building so that's an approximation. The second medium is
Very Poor: cities, industrial, and it's at -360 ft. Yes, I'd like to
elevate the first medium and have the second medium at 0 ft, but EZNEC
won't let me do that. I simulated 40M and 80 dipoles in the range of
20-30 ft. What I got was a two lobe vertical pattern -- a VERY strong,
very narrow low angle lobe, and a broad upward lobe whose strength
depends on the height of the dipole above the roof. Very low (5 ft)
makes the bottom lobe VERY narrow and VERY low (about 2 degrees) and
makes the high lobe a lot weaker. Yes, it's a poor antenna -- IF its low
to the roof. But if it's up 20-30 ft, a horizontally polarized wire
looks like a nice DX antenna.
What am I missing?
And, like I said before and several others added -- all that stuff on
the roof is likely to be mondo noisy.
73, Jim K9YC
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