Topband: Best wire antenna for roof top location

Jim Brown jim at audiosystemsgroup.com
Mon Aug 10 01:56:48 EDT 2015


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


More information about the Topband mailing list