This project is the AM transmitter the simplest and most basic possible, an oscillator and modulator combined with a transistor.
An ordinary AM radio is used to hear the output of this transmitter. Power is necessarily low to limit the distances at which it can transmit, but you should be able to transmit up to about 50 feet. This should be enough for one side to the other of the home or across the yard.
The transistor operates as a modulator oscillator. Remember there are two requirements for a circuit oscillates: a higher gain than 1, and a retroactive coupling output to the input to produce a regenerative response.
The transistor has a much higher gain, then he meets this requirement. The signal voltages between the base (B) and the emitter (E) are amplified more than 100 times and appear as an output voltage across the collector (C) and the emitter (E).
The requirement for the released either retroactive coupling to the input is met by appropriate connections through the parallel resonant circuit which is used to control the oscillation frequency.
For a regenerative response, the transistor must have a signal supplied to its base, and which is phase-shifted by the signal at its collector. This out of phase condition is achieved simply by connecting the collector and the base at opposite ends of the parallel resonant circuit formed of the ferrite coil and the tuning capacitor.
The 0.001 uF capacitor in this circuit connects the base to the lower portion of the parallel resonant circuit, because RF frequencies than at issue, it appears to be a near short circuit. The emitter of transistor must at least appear electrically (if it is not really) connected between the voltages of the collector and the base.
Often taking on the coil is used for this purpose, but this circuit, the transmitter does that seem connected electrically due to the 100 uF capacitor that works in conjunction with the emitter resistor 1K and the internal impedance of the transistor between the base and emitter.
The resistance of 47K provides a base bias current to turn the transistor on for proper operation as an amplifier or oscillator. The earpiece is used to modulate the transistor base voltage to obtain the modulation of the RF oscillation frequency.
The crystal earphone is very inefficient in this circuit as voltage generator, so do not expect a powerful modulation. The best results are obtained by tuning a receiver near the end of the high frequencies of the dial where there are no stations. Now slowly turn the transmitter until it can be heard in the receiver. Have fun!