[TowerTalk] baloon supported wire verticals

[Chuck Counselman]

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