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I can not find the frequency from an FM receiver... I am not sure for the coils Henries... I tried to make on of my own and I used: 19 cm of 0.8 cm wire I winded 8 turns around a 0.6 cm drill bit and I stretched it until it reached 1 cm long I have no oscilloscope so I can not find the exact value... Is something wrong with the circuit? Why can't I transmit to FM frequency?

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    \$\begingroup\$ Sorry, when you neither have components of known value nor any way to measure a circuit, then you're basically just stumbling around in the dark. Also, around 100 MHz, parasitic effects become significant, so I guarantee you, this circuit that you've shown isn't showing all the capacitance and reactance that is in your physical circuit. \$\endgroup\$ – Marcus Müller Jan 3 '18 at 20:48
  • \$\begingroup\$ Even if you had an oscilloscope, it would have to be one with a very high bandwidth (which few hobbyists can afford) to be able to check the right oscillation frequency. For a circuit like this to work you need quite some luck. Especially if you have little experience then it will be quite hard to make a circuit like this work. \$\endgroup\$ – Bimpelrekkie Jan 3 '18 at 20:57
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    \$\begingroup\$ Oh, and by the way, without a way to measure anything, you really shouldn't be building radio transmitters, especially if their power output isn't properly limited. You can produce nasty surprises for legal radio devices, and that in turn means you might get nasty visits by your local radio regulatory body. Depending on your local laws, that means confiscation of your tech, fines, and if you endangered something by jamming critical systems, potentially even jail. \$\endgroup\$ – Marcus Müller Jan 3 '18 at 21:10
  • \$\begingroup\$ Inductance of your coil should be close to 200nH electronbunker.ca/eb/InductanceCalc.html Should have a an rf bypass capacitor across the power supply, close to Q1 buildcircuit.com/simple-steps-for-making-fm-transmitter \$\endgroup\$ – Bruce Abbott Jan 3 '18 at 23:27

Your system - if not dead - is working under the usual FM band. I suspect about 50MHz. Change C1 and C2 to 5pF and experiment with smaller coil, say 5 or 6 turns. It shoud be easy to cut off one turn at a time from the middle and solder the ends. Coil wire should be without insulation and half to one millimeter thick. Be sure that all RF parts are in smaller area than 5cm x 5cm with minimal wire lengths. Your audio amp seems overly complex. Usually one transistor, 2 resistors and 2 capacitors are enough.

Add a 330pF capacitor in parallel with R1 if you already haven't rf decoupling. Essentially this is already suggested in comments (=rf bypass by user Bruce Abbott).

Do not even think this to work if it's built onto a breadboard. See, how tightly 100MHz circuits are in radio receivers to keep parasitic capacitances and inductances low enough. It means = not an extra cm of wire anywhere.

  • \$\begingroup\$ I am not using a breadboard of course I shall try to change my coil and I will tell you the results \$\endgroup\$ – DrunkProgrammer Jan 5 '18 at 16:50

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