At 2:32 PM +0000 10/19/02, Dale Hardin wrote:
>I want to use an ICOM AH-4 antenna tuner (HF to 50 mHz) or similar (even
>homebrew) with an appropriate vertical antenna for a multiband vertical
>antenna to go on the flat (gravel armored composite built-up on wood
>trusses) roof of my 4-story condominium....
It's most important to provide a good, virtually solid, ground plane
-- not only as a counterpoise for your antenna but also to
shield/isolate your antenna from the AC power and all the other
wiring within the building. This isolation will keep your
transmitted signal from getting into peoples' telephones, audio
systems, televisions, etc., and will also reduce the amount of crap
that your ham receivers hears from all the light dimmers etc. inside
the building.
Rather than using radial wires, I'd pave the roof with galvanized
"hardware cloth." The trouble with radial wires is that toward their
outer ends the space between the wires is much too great in
comparison with the wire radius. The capacitance of the radial
system per unit (roof) area is much, much less than that of a solid
conducting sheet, so most of the incident electric field lines pass
right through. Think _shielding_.
Unroll the biggest, widest, rolls of hardware cloth that you can
find; use bricks or cement blocks or paving tiles to hold the stuff
down; and then seal the seams (between adjacent strips of hardware
cloth) for RF. You can stitch the seams together with short pieces
of bare wire every foot or so, and then go down the line with a torch
or a big iron and solder them all. To save time, I'd look for a way
to mechanize the stitching with some kind of crimping tool, but
you'll still need to solder the crimped connections to keep them from
corroding, becoming nonlinear, and generating harmonics and intermod
products via the "rusty bolt effect." Keep your antenna and its
wire-screen counterpoise away from metal building parts such as
drains, vents, flashing, roof-mounted air-conditioning machinery,
etc. You don't want RF current flowing on them or their related
pipes, ducts, tubes, cables, etc. One of the lightning-protection
gurus on this list will probably want to add comments here.
At the foot of your antenna, of course, connect the shield of your
coax to the wire screen.
You can make a good vertical antenna from metal tubing or pipe, with
a glass "Coke(R)" bottle as a base insulator. For tubing I'd use
copper water pipe; with standard fittings and the right flux and
solder you can easily solder sections together with a torch. For
minimum visibility you can taper the diameter. The thicker the upper
section, the more visible it will be from the street, but the easier
the antenna will be to tune. Guy the antenna with stranded polymer
fiber fishing line -- which will be invisible from the ground. Be
sure that the line you get is UV resistant; some is not.
I gather that for reasons of visibility you'd prefer to avoid using
traps in your vertical, which is no problem. With an antenna tuner
you can couple coax to lengths (i.e., heights) that are far from the
usual, self-resonant, one-quarter-wavelength. However, if the length
exceeds five-eighths of a wavelength, then the gain of the antenna
toward the horizon will suffer, and if the height is less than about
one-eighth wavelength, then the coupling will be inefficient. Within
these constraints you have a fair amount of room. Make your antenna
3.75 meters (about 12 feet) tall, and it will work well on the 30,
20, 17, 15, 12, 10, and 6 meter bands. For 80 and 40 meters you
could use a second, taller, vertical; or switch in a base loading
coil; or (better) turn your antenna into an inverted-L by connecting
a horizontal wire to its top. Of course, the other end of this wire
will require support. Consider a fiberglass or (better)
graphite-fiber-reinforced fishing pole for this. It'll be thin and
relatively invisible from the street.
73 de Chuck, W1HIS
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