I thought the tuner would be set to a certain frequency and only receive that frequency. But when I move the antenna around, it receives different stations and also plays them on top of each other.
Here is a video of it happening: [https://www.youtube.com/shorts/eBlNF2fZOug](https://www.youtube.com/shorts/eBlNF2fZOug)
The radio is set to 89.9. When The antenna is relaxed, it somehow receives 93.1 and 94.7 at the same time and plays it on top of each other, and I can hear both clearly enough to identify the stations. When the antenna is extended at a specific angle, it plays 89.9 like normal.
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An ideal tune would work that way. But in the real world, there is a complex interaction with the surroundings to capacitive and inductive effects, that include your body and the shape of the antenna. It is sometimes said that “Analog circuit design is like black magic” because of how complex interaction is and how the can interact with other objects.
Look at https://www.electronicshub.org/wp-content/uploads/2014/07/FM-Radio-Circuit-Diagram.jpg which is a circuit diagram of an FM receiver. The tuning is done by adjusting the variable capacitance VC1. The antenna is directly above in it the circuit.
Your body will have a capacitive effect with the antenna and it depends on its position relative to you. So there is in in reality another capacitance in the system that is not on the circuit diagram that can change the frequency the oscillation that VC1 and L1 make up. L1 is an inductor and the shape of the antenna and what is around L1 change the insurance.
Radio waves are not just a single signal that goes from the transmitter to the antenna there is a reflection that can both increase and decrease the amplitude of the signal. Diffrent transmitters will have diffrent reflections so what is the strongest signal depend on the antenna position.
You do not need to tune the frequency of the oscillator perfectly it can be a bit off too and you still get the sound out. So it might be tuned a bit off and the combined effect of change on the oscillator frequency and change in the received signal amplitude can result in the jumping station.
If you ever notice the reception change when you get close to the radio the reason is the captive effect and you reflecting radio waves. You could try doing the same thing with the antenna taped to the end of a long stick instead of between your fingers. Use wood or plastic not metal so it has a minimal effect. The result can be that the station change does not longer happen because your body is faster away and does not interact with the receiver.
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