"gudmundur" wrote in message
...
I do sound work at horse shows, and would like to provide my audio source
to vehicles parked at the show. I have a real nice 100 milliwatt input
1.6mhz transmitter which shows about 60 milliwatts of output at 50 ohms.

I would like to build a transmitting antenna that would be 4 feet in
length
and having the 3/8 24 thread at the bottom so I can fasten it into my
hamstick
mag mount base.

I am thinking a 1 inch diameter wooden dowel 4 feet long, and wrapped
evenly
from bottom to top with perhaps #22 wire.

Any idea how much wire it would take to begin to act like a 50 ohm
impedance
at the base? Would you solenoid wind it bottom to top, or would it work
better
with maybe a 4 inch high multilayered coil at the bottom and 3.5 foot
steel
whip antenna.

1) 60 mW RF out for 100 mW DC input sounds like suspiciously high
efficiency.

2) To a VERY rough approximation, I'd aim to put a quarter-wave (at 1600
kHz) of wire on the dowel. That's almost 200 meters / 660 feet. I doubt
that you can fit that much on the dowel. In any case a tophat on the dowel
will partially compensate for too little wire. Even something a foot or two
in diameter might help a little.An example: a broadcast station near 1600
kHz has a tower only 67 degrees tall (less than 90 degrees / 1/4-th wave).
At the top they stick 4 or 6 or 8 10-ft-long rods straight out to the sides.
They claim this top-hat is the equivalent of 10 degrees more tower height,
bringing them up to an equivalent of 77 degrees. With this they claim to
meet the FCC's required minimum antenna radiation efficiency for their class
of station. Top-hats like this are pretty common for AM stations. Sometime
they use the top few feet of the guy wires.

3) Putting the loading coil at the bottom of the antenna is the worst
possible choice. It's the current flowing in the antenna that radiates, and
the current is highest at the feedpoint and zero at the tip, for
quarter-wave and shorter antennas. The high current points would all be
hidden in the loading coil and only the high-voltage/low-current part would
be "visible". Look at the short CB antennas. They will be have most of the
turns of wire near the tip.

4) Your mag mount won't have enough capacitance to the vehicle to be a
really good connection at 1600 kHz if you manage to make the antenna
impedance low, like 50 ohms.

5) In any case, the impedance looking into the base of any antenna _I_ think
you can fit in 4-ft length will be much higher than 50 ohms. You should
build a matching network. The less wire you can fit on the antenna rod, the
higher the input impedance. Look in the ARRL Antenna Book for networks for
feeding short antennas on the 160 meter ham band. The coil in a matching
network should be wound with rather hefty wire for low loss.