Spark gap transmitters have been illegal for a long time, but with only 50 volts, nobody will hear you. Of course, if you start with 12 volts and use an automotive ignition coil to generate the spark, you will get somewhere.
First of all, your transmitter will not sound like the continuous tone of regular radio transmitters; it will sound like a series of ticks, like the old fashioned telegraph sounders. So you will need a start tick for each character. After a while, you can train yourself to read these.
Diversion: a history lesson. The first Americans captured in the Vietnam war were not communication Einsteins. They did not know the morse code, and thought it would be impossible to use because a continuous tone can not be sent by tapping on the prison walls. They invented the Vietnam code, a 5 x 5 grid of letters, with K missing. (further proof of stupidity, they could have merged U with V and I with J, for only 24 letters, just like in ancient Rome). But with their code, using taps, they could communicate.
A . . B . .. C . ... F .. . G .. .. Y ... ... Z ... ...
The enemy forbade comminication, but on one occasion put the prisoners to work on a rockpile. They turned it into a giant broadcast transmitter while the Viets commented on their wonderful enthusiasm and progressive socialist attitude with that great rhythm.
So, even with ticks alone you can send morse code. It is a lot more efficient than Vietnam code because the most frequent symbls are the quickest to send. For example, the most frequent letter E is a single dit. So here is how you change the morse code for ticks alone:
Letter Standard Symbol Ticks only
A dit dah .. .
B dah dit dit dit . ...
C dah dit dah dit . .. ..
All you do is add an initial tick to each letter. It is not hard to get used to hearing and decoding.
Now for the receiver: I recommend an ordinary AM broadcast band receiver set between stations below 600 kHz.
Transmitter: For the transmitter, you can use a 12 volt car battery with an automotive ignition coil. Just wire the battery in series with a telegraph key and the primary of the ignition coil. The high voltage output of the coil goes in series with TWO spark plugs or other spark gap to the antenna. The antenna will be HOT, this will discourage stray cats from messing with it. Back in the day, I was able to communicate 5 miles with a similar rig, using a 150 foot wire in a tree for an antenna, a resonant transformer circuit that tuned both the spark gap and the antenna, but at that time I did not know about the advantages of two spark gaps in series.
It is illegal to transmit this way, so keep your antenna no larger than needed to prove your project to the teacher. Do NOT let your teacher near the antenna when you are transmitting, or you will get a very nazi surprise.
It is good to know how to do this, because if civilization collapses, we will be able to keep some low bandwidth global communication going. I have a plan using carfully timed pulses so that you are listening for a pulse only when it is expected. I have a modified code that includes a parity pulse: all symbols have an even number of pulses. It is a little harder to learn. Also, I was considering the use of quartz crystals in series with the antenna to limit interference with other stations, but I will worry about that after civilization collapses. If you can grind a telescope mirror, you can grind a quartz crystal.
If you want to know more, call me, K6YVL. You can find me through the radio amateur callbook. I am in southern California.
