I don't know if you are still getting TT or not Gary. There are some good
ideas in this post. 73, Dan
----- Original Message -----
From: Chuck Counselman <ccc@space.mit.edu>
To: <towertalk@contesting.com>
Sent: Friday, April 04, 2003 6:31 AM
Subject: Re: [TowerTalk] baloon supported wire verticals
> 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
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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