At 2:36 PM +0200 4/4/03, Sibilja Mirko wrote:
>our club (S58U) is planning to put up top band vire vertical in one
>of next contests. Baloon is meteorological one, filled by He.
>Advices and hints would be greatly appreciated....
I have experimented with balloon-supported wire antennas for 40
meters, but I believe that my conclusions apply equally to a 160-m
antenna:
1. Use _Litz_ wire rather than solid or stranded wire, to minimize
weight and wind resistance while retaining the necessary RF
conductance.
Use the thinnest/lightest possible Litz wire, subject to the
constraint that the antenna's feedpoint resistance due to "copper"
loss should be less than, say, 20 or 25% of the radiation resistance.
The mechanical strength of the wire is unimportant, thanks to...
2. Twist high-strength (at least 100-lb. test) _stranded_ (not
monofilament) polymer (Dacron? Kevlar?) fishing line (e.g.,
"SpiderWire") loosely around the Litz antenna wire. To keep the
loose twist from unwrapping in the wind, tie the wire to the polymer
line with _waxed_ dental floss at intervals of 2 to 3 m. After tying
each knot, trim the ends of the dental floss. The wax on the floss
will keep the knot tight and will keep the tie from slipping along
the antenna wire.
For equal diameter and especially for equal weight/mass per unit
length, stranded polymer fishing line is _very_ much stronger than
copper wire. Having the polymer line take virtually all of the
tension makes it possible to use very thin and very light wire,
subject only to the RF conductance requirement.
Always wear gloves when handling "SpiderWire" or similar (stranded
polymer) fishing line. This line is so thin and so strong that it
cuts through skin and flesh very easily.
3. Use the largest possible balloon. The vertical lift of a balloon
is proportional to its volume, which is proportional to the cube of
its radius. The horizontal drag force due to wind is proportional to
the area of the balloon, which is proportional to the square of its
radius. It follows that the tangent of the vertical angle of your
antenna will be directly proportional to the radius of the balloon.
I would not consider using a balloon of radius less than five feet
(diameter equal to 10 feet, or about 3 meters). A "blimp" (a balloon
whose shape provides aerodynamic lift) would be better than a weather
balloon, but I understand that you already have a weather balloon;
and blimps are expensive.
You will be surprised and disappointed by how far downwind your
antenna tilts, even in a light breeze. A nearby tree will snag your
antenna. Do not "fly" a balloon-supported antenna anywhere near an
electric power line!
Most "weather" balloons do not retain helium very well. (Metallized
Mylar balloons are very good.) Be prepared to add helium at least
daily.
4. _If_ you are able to keep your antenna close to vertical, make it
more than one-quarter-wavelength tall, and base-feed it through an
"antenna tuner" or matching network. It will radiate much more
efficiently because the current in its counterpoise or "ground"
system will be much reduced. The best possible antenna height is
nearly 5/8ths wavelength.
5. Especially if your antenna is only 1/4 wavelength tall, its
counterpoise or "ground" system will be important and will limit its
efficiency.
6. The antenna wire must be grounded for DC (not for RF) at _all_
times, including while it is being erected, while it is being used,
and while it is being pulled down. Even if there is no lightning in
the area, the wire will acquire a _dangerous_ electrostatic charge.
Dangerous not only to electronics, but to _humans_.
73 de Chuck, W1HIS
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