Kogawa's simplest FM transmitter

 After the last unsuccessful attempt at building this thing, I've finally got it working.
Some helpful tips:
Normal output (note the awful frequency drift-off)
Output with base overdriven
Signal's doppelganger to the left
  • Things like size of the pads (exactly 5x5 mm.), lead length (minimal) and component placement really DO matter. If you assembled a transmitter like this one and it doesn't seem to work (or it does, but the signal is noisy with lots of out-of-band emissions) you probably should check the layout.
  • Signal frequency drifts off with decrease of battery voltage, and really fast. Therefore, it's not practical to use transmitter as it is now. One should add a voltage regulator to power the circuit in order to stabilize output frequency.
  • Too large of an amplitude of the signal will saturate the transistor's base and "bloat" the output frequency band, rendering the sound almost unrecognizable.
  •  Almost any sort of transistor will do, such as renowned 2N3904. Look in the datasheet for fT (Current Gain - Bandwidth Product). It shouldn't be less than 170MHz (Kogawa himself uses 2SC2001 with this figure, 2N3904 is even better with 300MHz).
  • The transmitter, other than the intended RF signal, will produce it's weakened undesired copies outside the band. Search for the strongest signal.
  1. Stabilize the power supply to prevent frequency drif-off (add a simple 7809 voltage regulator and increase the power supply voltage).
  2. Add band-pass filter at the output to supress out-of-band noise.
  3. Add antenna to invade the neigborhood 8-)