At 3:50 PM -0800 10/31/02, Ted Bohrer wrote:
>...So here's the hairbrained scheme...
>I float a Happy Birthday helium balloon 30 - 40 ' up holding a
>vertical wire antenna which is tied to my rear bumper (but not while
>driving).
>...So if this contraption is to work, how long should the antenna
>wire be at 14.200?
I've experimented with balloon-lifted antennas, and also done some
modeling. The simple (without loading or phasing sections) base-fed
vertical antenna having the best power gain toward the horizon has a
height of 5/8ths wavelength. I'll leave it to you to calculate the
wavelength for f = 14.2 MHz. My mental calculation says that your
vertical should be about 43 feet tall.
The driving-point impedance of this vertical is negative-imaginary,
in other words capacitive, about like a 1/8th-wavelength vertical
whip; but the resistive component is much greater, maybe around 90
ohms IIRC, although my memory is shaky. If your rig (or its antenna
tuner) can match an 8-ft. whip, it can probably match a 5/8ths-wave
vertical.
The tricky part is keeping the wire vertical! Verticality matters,
because a 5/8ths-wave wire has gain perpendicular to the wire, and if
the wire slopes much, your gain will be pointed the wrong way. The
slightest breeze will carry the balloon(s) downwind. Use balloons
with a total volume of at least 6 cu.ft., preferably more, and keep
the antenna light. I suggest using lightweight synthetic fiber
string (for strength), with very light wire loosely twisted around it
and tied to it every few feet with dental floss. IIRC, I used 7x40
Litz wire because it provided higher RF conductance per unit weight,
but you could get by with (say) 26-gauge stranded wire. The wire
should be so light that the string is necessary for strength.
The way to keep the wire vertical is to attach two or three very
lightweight lines (like fishing line) to the balloon and run them to
ground like guy wires, to keep the balloon from going anywhere
horizontally.
What this has to do with towers is that a balloon-lifted wire can be
used to extend the top of a tower electrically, so it becomes
feasible to make (say) a 5/8ths-wavelength vertical for 160 meters.
If you tether the balloon to the ground, you'll have trouble with the
wind running it into trees. The tower gets you above the trees.
Finally, be sure to have the antenna grounded for DC. The wire can
pick up quite an electrostatic charge in the breeze, and you can get
seriously jolted. Of course you'll be far from electric power wires.
73 de Chuck, W1HIS
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