Topband: contesting in a field

Tue Nov 15 19:33:11 PST 2011

For one of the early ARRL Topband contests in the 1970s I, then as WB2URU, rented a tank of helium and inflated a World War 2 surplus balloon to support a quarter-wave vertical.   Everything went well until the balloon reached a height of about 80 feet.  At that altitude there was enough wind to whip the balloon around violently in all directions, and nothing I could do on the ground had any effect.  Unable to exert any control whatsoever, I hauled it down, gave up and switched to my tree-hung inverted L.

I later concluded that the balloon's spherical shape was the worst choice I could have made for aerodynamic stability; even a cube would offer six flat surfaces over which wind could pass in a straight line.

There is the kitetoon (a composite of kite and balloon) with a somewhat elongation to a  sphere that simulates a fuselage.  It tapers, with three fins attached to its more slender end.  One sometimes can see these flying over used car lots.  They can be hired.

The kite is a much less costly sky hook.  A most important point is to never attach the top of the antenna to the kite itself.  Kites fly with greatest stability at heights above 200 or more feet.  Kites fly at varying angles to true vertical.  A kite at 500 feet with the antenna attached to its line 300 feet below will be stable and very close to truly vertical.

Kites may be roughly grouped into two categories: cellular, where the lift results from air passing through cells open at both ends, and airfoils where lift results from the different air pressures between top and bottom surfaces, exactly like an airplane wing.  Cellular kites are best for higher wind speeds and the box kite is a simple cellular structure that performs well.  Airfoils do best under lighter wind conditions, and the delta, a fairly recently invented kite, is  very stable and can fly almost vertically.  Of course there are variations on these two basic types of kites which attempt to combine the advantages of each.  Such combinations are really compromises, and one usually does best using either one or the other of the two basic types.

It is perfectly possible to raise a center-fed dipole with a kite, and I have seen this done with good results.  A base-fed antenna is attractive and 0.75 wavelength high will give a convenient low feedpoint impedance and a high-angle radiation pattern; either or both may be desirable.

A 0.5 wavelength  will give a high base impedance and low radiation angle.  Inserting stubs or lumped circuits to make a vertical collinear is not a good idea because of the mechanical complications and their weight.  What I deem a real killer antenna would be a 0.5 wavelength base section decoupled through just a capacitor to a 0.375 wavelength top section; total length 0.875 wavelength.  Very similar is the AM broadcast antenna of WHO in Des Moines, IA, where he total length is a bit shorter, about 0.83 wavelength I believe.

If the kite supported antenna is for temporary operation, its design should be one requiring few radials.

The internet has a lot of material on this subject.  One such place is blueskylark.org.

73,

Charles, W2SH          

> Date: Tue, 15 Nov 2011 00:52:46 -0600
> From: ahhyup at gmail.com
> To: topband at contesting.com
> Subject: Topband: contesting in a field
> 
> So I have a new (to me) travel trailer, and a family farm, and I've made
> maybe two 160M contacts ever. I don't get to play radio much, usually just
> sweepstakes phone every year, and I missed last year. I find it fun to do
> kinda off the wall stuff, its fun.
> 
> So put this all together and I'm thinking I'll park the travel trailer in a
> field, during a 160M contest, and play radio. The missing piece is the
> antenna.
> 
> I've done some reading, kites, balloons etc. I'm fine to build a big
> lifting kite, I've got a hunk of tyvek somewhere, but that's not the point,
> the point is what I'd lift. Balloons aren't interesting, I can't make
> helium in the wood shop or sew a balloon.
> 
> What I'm after is some thoughts on the antenna. Looks like a kite lifted
> 1/4 wave vertical should be doable, but what about the radials? Try to
> elevate them? I'm only 100w, so I'm interested in some reasonable (or a bit
> more than what one might think reasonable) efforts which will help me be
> successful. I read about bleeding static but don't have specifics.
> 
> That being said, if there is no wind, and no antenna being lifted, success
> is not only in the result but also in the prep. I'll be ready for the next
> opportunity. A good story is worth something also.
> 
> So what do you all think? I've got barbed wire fencing, lots of space,
> copperweld and electric fence wire, some 50-60' or higher trees here and
> there along the edges of the fields. Hilly pasture even for that matter,
> but really limited in access for the travel trailer (not that it has to be
> part of the setup, but I've got it, might as well use it).
> 
> Thanks,
> Chris
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