Posted by
Ron D'Eau Claire-2 on
Apr 11, 2005; 4:16pm
URL: http://elecraft.85.s1.nabble.com/Non-resonant-attic-loop-tp376882p376888.html
Gene, KC0RXY wrote:
I'm needing to install an antenna in my new home. An attic loop is a
necessity ... can't run an outside wire easily.
I'm assuming that an 88-ft loop will be non-resonant in all bands 80-10,
and fed with ladder line to the tuner should work.
Other suggestions?
----------------------------------------
For a number of years I was an apartment dweller who only once had
permission to run an outdoor antenna. My antenna of choice in each situation
was a doublet using whatever length I could manage. The feeder was open wire
line made up of two small white wires (since apartment walls are invariably
white) that went straight from the ATU on the operating table up the wall
(held to it with a couple of small staples), through two small holes in the
ceiling where it meets the wall and up to the center of the doublet. Once
above the wall, a couple of lightweight insulators made from bits of scrap
plastic (cut from film cans) were used as spacers. The spacing is not
critical.
It worked very well on all bands where I could get at least 1/4 wavelength
of wire up into the attic. That is, with about 35 feet of wire (17 feet each
side of the feeder) hung in the attic just under the roof so it was sort of
inverted V, I could work on all bands 40 meters and up. The loss of
efficiency of a short doublet doesn't really show up until the radiator gets
to 1/4 wavelength or below.
Of course, it would also load on 80 and even radiate. Shoot, an 8-foot
mobile whip will radiate on 80. But I never seriously tried to operate on 80
(or 160) from an indoor antenna because of the electrical noise. Lamp
dimmers, blow-dryer motors, every sort of appliance seems to be designed to
emit maximum electrical noise on 80 and 160 meters!
Only one time did I find my performance rather disappointing from such an
attic antenna, considering its low height (usually about 20 feet since I was
usually in a two-story building). It worked okay on 40 meters but
performance on 20 meters and up was dismal compared to other QTH's. The roof
was tile. Up close it appeared to be a colored concrete with a lot of air in
it, so while the tiles were waterproof, they weren't as dense and heavy as
normal tile or concrete. I found a broken piece in the garden, evidently
left by someone who had done some repairs on the roof.
On a hunch I took the piece of roofing tile into the house and put it in my
microwave oven (with a glass of water to protect the magnetron). After a
short run the water was warm and the piece of roofing tile was blistering
hot! Minerals or something in the tile mix was definitely not "transparent"
to radio waves - and its ability to soak up RF probably went up with
frequency.
So the sort of roof you have really does matter!
The open wire feedline made of white wires attached to a white wall was
virtually invisible - a nice feature since I often had my operating desk in
the living room. Friends visiting would have to be shown the "feeder" before
they'd see it on the wall from a few feet away.
Ron AC7AC
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