Can I use an LC oscillator to block RF signals?

[geneiusxie]

So I was reading about how wireless energy transfer works - a charger outputs a high power electromagnetic field at a certain frequency and a transformer that is designed to resonate at that particular frequency can absorb most of that energy, even if it would otherwise have a relatively low magnetic coupling coefficient. And I thought, could you put an LC oscillator inside a box to absorb cell phone signals, preventing a phone from taking a call by diverting radio waves to the oscillator instead of letting them reach the phone?

 

[betwixt]

Yes, you can use it to block a cell phone but not in the way you describe.

The oscillator, LC or other type produces another signal which if strong enough will swamp the cell mast signal. It doesn't absorb it or divert it, the idea is to confuse the phone because it can't see the weaker cell signal through the stronger one you are producing. At cell phone frequencies it may not be practical to make an LC oscillator without other circuits to stabilize the frequency as the L and the C will be very small.

Brian.

 

[FvM]

a transformer that is designed to resonate at that particular frequency can absorb most of that energy, even if it would otherwise have a relatively low magnetic coupling coefficient

This is a somewhat simplified description of a resonant receiver that needs explanation.
- the resonator will possibly reduce the field strength in it's vicinity but not cancel it. It can only absorb "most of the energy" if there are no concurrent receivers in the same place.
- concentrating the energy by a resonator will ony work for wireless transmission scenarios with dimensions small relative to the wavelength. It doesn't work for electromagnetic waves.

 

[geneiusxie]

This is a somewhat simplified description of a resonant receiver that needs explanation.
- concentrating the energy by a resonator will ony work for wireless transmission scenarios with dimensions small relative to the wavelength. It doesn't work for electromagnetic waves.

I see. So it wouldn't work for a 900MHZ cell phone signal. But would it reduce the strength of an 1611khz AM radio signal - which has a wavelength of 1862 meters?
But hypothetically, could it work with a 900MHZ cell phone signal if it was right next to the phone?
Btw, I'm just talking about a simple capacitor and inductor connected together. In this case, the "parasitic" inductance of the lead wires of a typical ceramic capacitor would form the L.
I'm not taking about an "active" cell phone jammer that sends high-power RF noise in all directions.

 

[jiripolivka]

I see. So it wouldn't work for a 900MHZ cell phone signal. But would it reduce the strength of an 1611khz AM radio signal - which has a wavelength of 1862 meters?
But hypothetically, could it work with a 900MHZ cell phone signal if it was right next to the phone?
Btw, I'm just talking about a simple capacitor and inductor connected together. In this case, the "parasitic" inductance of the lead wires of a typical ceramic capacitor would form the L.
I'm not taking about an "active" cell phone jammer that sends high-power RF noise in all directions.

In the electromagnetic field, I do not see any possibility that a tuned L-C circuit could "absorb" or suppress the field intensity the way you would wish.
The Yagi-Uda antenna concept utilizes tuned dipoles as "reflectors" and "directors" to focus the wave intensity to a desired point where the "active dipole" is then located. Such arrangement makes a directional antenna which can be effective for waves coming (or going) from/to a desired point.

In agreement with the Yagi-Uda directional-antenna concept, you would observe, with several tuned dipoles around the "active" dipole, field intensity of a wave coming from a particular direction, to be attenuated.

The arrangement, however, is quite sensitive. To screen a device like a cell phone from the electromagnetic field, the best way is to use a conductive "box" around it. A box made from a meal mesh can be used if the holes are much smaller than the wavelength in question.

Oscillators generate RF power, and when connected to an antenna, this power can be radiated. I do not think you could observe any absorption effect like it occurs in atoms or molecules when they selectively absorb IR, visible or UV radiation. If any effect should be seen, you will need really many oscillators distributed in space; try to scale it from the visible light when absorbed by a particular gas, liquid or solid material.

 

[Samaraknell]

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[tony_lth]

The cell phone jammer have many methods to implement. One method is blocking the signalling channel of GSM, so the cell phone can't find the mobile network.