Mike, nice ambitious process you've started. Been flying the balloons
since about 1966.
1. You can use #18 wire for the vertical.
2. If you have a pair of trees you can hang a rope from, that is to say
wind up with a horzontal rope supporting X number of feet, that works
well.
3. As to size, you should try to get the largest diameter balloon you
can, check the Army/Navy stores. To inflate a 1/2 wave vertical, you
need to blow up the sucker to about 3.5 to 4 feet. Tie it off with a
good strong kite string about 2 feet long, and tie the top of the
vertical to the other end.
4. It helps considerably if you also tie about 20 lb. fishing line to
the balloon to lower it.
5. The impedence at the bottom will vary from about 600 to as much as
3000 ohms depending on wire size. You can use a very simple LC circuit
mounted inside a Tupperware plastic container. Here's how it goes.
The bottom end of the vertical, the top of the coil, and the hot side
of the cap all are tied together. The ground side of the cap, and the
other end of the coil are grounded. ( BTW, you can use RG8 for your
cap... ) Now, here's how I do it. I mount a double sided piece of PC
board into a 8x14" plastic something. Mount a ceramic feedthrough in
the top ( end actually ) of the box. Mount 2 SO239s in the bottom of
the box. ( well, only 1 if you're not using coax for the cap. ) This
239 will be the input from your coax. Ground the coil to the PC board
with a nut and bolt. Now mount a ground rod attachment ( the dohicky
which you use to connect your station ground wire to the ground rod )
to the bottom of the box. You'll need about 5 inches of 3" diameter
8TPI coil, and start off with about 300 pf of capacitor. ( about 8-10
feet of RG8 coiled up will do nicely ) After the coil is mounted, use
a small screwdriver and push in slightly every other turn of the coil.
( gives you something to solder to )
4. To tune the beast, let it fly ( and I highly recommend you have the
vertical attached to the matchbox before you do ) Or if you wish, you
can fly the whole thing and then touch the end of the vertical to see
how much voltage is there. Use a ground rod with about a foot sticking
out of the ground, and mount the box to the rod just as you ordinarily
would. Hook up your MFJ or whatever to the feed coax, and, using a short
jumper wire from the center of ther SO239, tap into the coil about
8 turns from the bottom. When you reach the lowest SWR by changing the
tap, you're halfway there. Now adjust the cap ( or trim the coax )a
little at a time and you will work the antenna to the frequency you
want. There is considerable high RF voltage at this point, so don't
try using a cap which wont handle at least 4000 volts. ( this at the
KW level of course. ) You'll be able by changing the tap slightly and
varying the cap to bring it down to 1:1 in no time.
You asked about beverages...they are necessary and should be as far
away from the vertical as possible, otherwise, it will couple noise into
the beverage.
This antennas will kick ass and take names!
And there you are; Everything you always wanted to know about a 1/2
vertical but were afraid to ask.
73
Ed
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