This will allow you to send morse code through radio waves to another computer. This could be used as a sort of chat program.
Step 1: Gather Supplies
You will need very few supplies:
-A crystal oscillator (will talk about which kind in the next step)
-A 9 pin serial jack (D-SUB9)
-A little bit of wire
-Something to use as an antenna
-A male to male audio jack (unless you know morse code)
Step 2: Which Oscillator?
To determine what kind of oscillator you want you will need to know what kind of radio you have.
Regular AM/FM radio - We will be using only the AM part. The numbers on the dial are frequencies in KHz. Most oscillators are marked in MHz. You will probally want something in the .3MHz(300KHz)-1.5MHz(1500KHz) range.
Shortwave radio - Go with whatever frequency your SW radio can play. My radio has 3 bands. One is a KC (KHz or AM) band and the others are MC (MHz). I use a 20MHz oscillator.
Step 3: Wire some stuff up
You will need to solder a wire onto pin 4 of the serial jack. This one should be red to make it easier to work with later but not nessesary.
You will also need to solder a (black) wire to pin 5 of the serial jack.
Step 4: Wire up the oscillator
Take the red wire (from pin 4) and connect it to the pin directly above the black dot on the oscillator. Next, take the black wire (from pin 5) and connect it to the pin to the right of the black dot.
Step 5: Add antenna
Now add an antenna to the pin opposite the black dot. You can use pretty much any insulated wire as an antenna, just as long as it isn't too thick. I used a 25 foot telephone wire.