I did see a useful diagram on how this is done, but seem to have mislaid it. Here is a brief description I found on the web:
Spark gap transmitters were banned early, early, early in radio
history as they;
1) Typically transmitted thousands of miles, and
2) Since there is no tuning feature other than antenna length,
they were heard all across the radio spectrum.
Think of lightning strikes. Every time one occurs, you get static
on your radio. At least if it's nearby (50-100 miles). Now think
of a man made device that is designed to make static
intentionally. You can get a LOT of range out of it.
I could make one with an ignition coil, a spark plug (optional,
as any gap will work), the car battery, and a length of wire, the
longer the better.
Run a piece of wire up the highest thing around and secure it at
the top.
If it's a tree or anything wet, make sure it doesn't ground out
anywhere. This is the antenna. At the other end of this wire,
connect it to the pointed end of the spark plug, i.e., where the
spark plug wire attaches.
Take a short piece of wire and run it from the screw threads on
the spark plug to the high voltage output on the ignition coil.
Take another piece of wire and run it from the negative terminal
on the ignition coil to ground.
I mean, stick the wire in the ground, the more the better. Bury
it. If the soil is wet, better still. Take another piece of wire
and connect it from the negative terminal on the ignition coil
(yes, this is the same terminal that the last piece of wire
connected to) to the negative terminal on the battery.
Take another piece of wire and connect it to the positive
terminal on the ignition coil. Leave the other end unconnected.
At this point the transmitter is complete. Connecting the
positive coil wire to the battery will energize the coil and
*may* produce a spark at the plug.
Don't worry if it doesn't. When you disconnect the wire, a spark
*should* be produced. If it doesn't, check all wiring
connections. Make sure the antenna isn't grounded.
Make sure the ground wire is. You may have to change your spark
gap as well. Be advised that making and operating a transmitter
of this type will bring the wrath of the FCC and possibly other
government bodies down on *YOU*.
Rapidly making and breaking the battery connection with the
positive wire will enable a person to send a morse code type
distress signal. Doing this as long as possible will alert the
appropriate authorities and give them ample time to triangulate
your position. I would continue doing this until someone arrived.
Doing this in a non emergency situation (testing) will get you
busted big time. Fines are usually $3000 - $5000 for a first
offense and can go a lot higher.
