Topband: Best wire antenna for roof top location

W2RU - Bud Hippisley W2RU at frontiernet.net
Sat Aug 8 13:36:22 EDT 2015


> On Aug 8, 2015, at 10:58 22AM, Tom W8JI <w8ji at w8ji.com> wrote:
> 
> Large buildings are not towers or poles. Buildings have a significant amount of large conductive metallic things and noise generating junk inside.
> 
> If the building has wiring and large connected metallic things under the horizontal antenna, it will act like a reflector. If the antenna is somewhat low to the roof (less than 1/4 wave or more above the roof), the elevation pattern won't be much different than a low dipole over flat earth. Most of the radiation will be beamed straight up.
> 
> Even with a 400 ft high building, a horizontal antenna a fraction of wave over the roof can be very disappointing.

Many years ago, at the start of my sophomore year in college, a classmate and I decided to operate CW Sweepstakes from my room in a 5-story dormitory.  The dormitory footprint was a long, skinny rectangle with brick parapets rising 10 feet above the rooftop at the four corners of the building.  We had easy access to the roof, which was flat with a tar and gravel surface, so we strung an 80-meter center-fed dipole diagonally across the roof between two opposing parapets — 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.

We had a single-813 transmitter that ran 500 watts input at a time when the legal limit was 1 KW input.  It was far more power than I was accustomed to, since my home station transmitter at that time was a Heath DX-40, running about a tenth as much power.  I was expecting “great things” in this contest, but it was one of the most disappointing outings I’ve ever participated in — we struggled for every QSO the entire weekend!

A few days after the contest we learned from the head of the college maintenance department that underneath all that tar and gravel was a solid sheet of copper! 

Moral of the story:  Believe what Tom tells you — especially his final sentence above!

Bud, W2RU  


More information about the Topband mailing list