C2 is only 100pF. It is a capacitor so it does not short the base to the emitter at DC, it stops the preamp transistor from amplifying the RF from the output of the transmitter. 100pF has a reactance of 80k ohms at 20kHz so it has no effect on the signal from the microphone. At 100MHz its reactance is only 16 ohms.
I believe C12 is a coupling capacitor, how come it's not the same value as C1 and C2. Is that because the audio frequency will be different after it passes the the first RC circuit?
I believe C12 is a coupling capacitor, how come it's not the same value as C1 and C2. Is that because the audio frequency will be different after it passes the the first RC circuit?
C1 and C3 are 330nF (330,000pF) and pass audio frequencies from 30Hz to 15kHz.
C12 is 30pF and passes radio frequencies from 88MHz to 108MHz. The reactance of C12 at 100MHz is only 53 ohms so it passes the radio frequency very well.
Q2 is the radio frequency oscillator and its output to C12 is near 100MHz.