I've built this coil FM transmitter following these guides which are essentially the same:
- https://www.instructables.com/id/The-Ultimate-FM-Transmitter (without the mircophone)
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.