I came across this AM broadcast band transmitter project from a guy in Greece. It uses a single transistor to do everything, and a carbon microphone for audio modulation.
There are some major drawbacks here. in Diag.4, the schematic shows the audio jack is connected directly across the R2, which has a few volts across it. The typical experimenter would try to connect this jack to a CD player, not realizing that the current from the transistor could be going back into his player and doing damage to its circuit. The solution is to put a 100 uF electrolytic capacitor in series with the center jack wire to block the DC from going into the player.
Further on, under ‘Component Variations’ he tells how he is going to increase the supply voltage to get more power and greater range. This is a bad idea, and is going to burn out the transistor. The best way to get greater range is to use a longer wire for the antenna (in the U.S. the antenna is limited to 10 feet or 3 meters). For the AM broadcast band, the antenna has to be much longer: at 1000 kHz, the wavelength is 300 meters, so a 1/4 wavelength antenna will be 75 meters, or 250 feet.
To make the transmitter more powerful, raising the supply voltage is a really bad idea, as I’ve said. The transmitter should use a MOPA (master oscillator – power amplifier) design. One transistor is used for the master oscillator, so the circuit will not get hot and drift off frequency. The power is increased with another transistor, and this is where the amplitude modulation is done. With the modulation to the power amp, the signal can be modulated nearly 100 percent, something you can’t do with a single transistor.
He also talks about using a 2N3055 power transistor. A 2N3055 is capable of putting out tens of watts at audio frequencies, but the AM broadcast band is a thousand times higher frequency, and the 2N3055 cannot put out much power, or probably more like no power at those frequencies. For a watt or more at broadcast band frequencies, I would use one or more BD139 transistors, which are very inexpensive. Put them in parallel for more power. But the supply should not be higher voltage, it should be higher current for more power.
There is a lot of pirate radio information on The Web, but much of it is for the FM band. Still, there are plans for AM transmitters that can put out quite a bit of power. The amateur radio transmitters are capable of putting out a thousand watts. Many of these higher power transmitters use vacuum tubes aka thermionic valves. An old 5 tube radio can be converted to transmit on the AM band. The tube that is used to drive the speaker can put out a few watts on the AM band.