> On Aug 8, 2015, at 10:58 22AM, Tom W8JI <w8ji@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
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