Make magazine's Super Simple FM Transmitter
Make magazine published this FM transmitter circuit http://makezine.com/projects/super-si... and I had a go a making it.
From the initial component values, I was able to achieve oscillation around 136Mhz (as measured on my oscilloscope). The critical components that influence the frequency are the two 0.01uF (=10nF = 10,000pF = "103"pF) capacitors and the 4-turn (1/4 20 bolt former with #18 wire) inductor. While I used #16 wire, which will have influenced the inductance, I needed to alter these components in order to lower the resonant frequency of the carrier wave:
I used larger 0.1uF = 100nF = 100,000pF = "104"pF capacitors and I used 6-turns of the inductor, spaced out to roughly 16mm or so (I failed to measure this width, so tweak it if needed). As they say, " your mileage may vary".
With these larger oscillating components, the frequency was lowered to 99.65MHz as measured, and the results work well at 99.70MHz on the radio dial.
The transmitter is 'rough', but can be made to sound clearly. I needed an antenna from the ground plane to radiate enough power to overcome a stronger commercial station at 99.7FM. Hand-capacitance was strong too, so you might need to listen with some correct body positioning near the circuit.
Lastly, my MP3 player suffered a broken mono-jack when I tried it as a source. I needed to open the player to get the jack free, and that caused more static while playing.
Overall, a nice circuit as a demonstration, and with so few components, it offers a good learning tool to understand what is going on. However, I am still struggling to think through the processes and events in the circuit, so any comments on how this thing works are greatly appreciated!
The original Japanese website for this circuit is here by Tetsuyo Kogawa http://www.translocal.jp/radio/micro/...