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I've built this coil FM transmitter schematic following these guides which are essentially the same:

I'm able to easily find the transmitter frequency using an SDR and it works fine, the audio quality is good but as many other people I've got this problem: the frequency increases or decreases as I move my hand or my body closer or away from the transmitter.

What is causing that? I think the coil magnetic field is affected by my body static.

How to fix or decrease this effect?

I tried making a shield with aluminum foil around a box and then I put the transmitter inside but the problem persists.

I think it's still there because the antenna which is connected to the coil is coming out of the box through a small hole thus affecting the coil magnetic field anyway.

I also tried removing the antenna so the transmitter is entirely inside the shield box but that only decreases the transmitter range, the problem is still there.

Any suggestion?

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  • \$\begingroup\$ What is the signal in connected to? How has this circuit been prototyped? \$\endgroup\$
    – Voltage Spike ♦
    Commented Jun 7, 2019 at 21:48
  • \$\begingroup\$ 3.5mm jack connected to a smartphone. 5x4mm perfboard \$\endgroup\$
    – Gio Iann
    Commented Jun 7, 2019 at 21:55
  • \$\begingroup\$ It is difficult to make an electrical connection to an aluminum foil shield. Is it connected to ground? Is the battery inside? Add another 10nf capacitor from the top of C1 to bottom of R2 (gnd), otherwise battery leads radiate RF. Calling the bottom of R2 "ground" is fooling yourself in this battery-operated circuit. Even with an effective shield, the antenna will cause frequency shift because of your body capacitance. As @JRE has said, modifying this circuit to make it stable is not worth the trouble. \$\endgroup\$
    – glen_geek
    Commented Jun 8, 2019 at 0:00

2 Answers 2

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it's probably capacitive coupling of the antenna, or traces on the circuit board to your hand.

More stable transmitters like the Amoeba use a separate RF gain stage to isolate the antenna from the oscilator.

http://www.talkingelectronics.com/projects/14%20FM%20Bugs/20%20FM%20BUGS.pdf

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It isn't that your hand disturbs the magnetic field.

It's that these circuits are sensitive to everything.

  • Room temperature changes - frequency changes.
  • You move your hand close - this changes the capacitance between various parts and the frequency changes.
  • You move the device - you change its relation to various metal objects which changes the inductance of the coil and again the frequency changes.
  • The battery voltage drops a little - the frequency changes.

These types of circuits work, but will pretty much never be stable unless you make so many changes to them that they aren't the same circuit anymore.

They look simple, and attract people who want to build something easy. In the end, you spend a lot if time twiddling with a circuit that will never get better.

They demonstrate oscillation and frequency modulation, but they are not circuits you would use for real.

The very function of these circuits is obscure - they depend on poorly defined and unspecified side effects of the components.

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