How to tune the frequency of a coil?

Question

I'm doing a scientific fair project. I want to do it about wireless energy transmission.
In practice consists of two coils NOT physically connected. One is the transmitter of energy, which is connected to a source of energy. The other (s) is the receiver, which receives the energy through magnetic fields. The main characteristic is that both coils are designed to have the same frequency, since this way the amount of energy transferred is maximized. So my question is, how can I do, to equalize its frequencies? Keep in mind that I am a high school student and you could explain it to me in more practical terms to carry out the project.
PD: I investigated, and according to this, I must obtain "the maximum output" to pass the point of resonance, but I have no idea what this means

hyportnex · Answer

Normally, the resonant frequency of the antenna can be tuned with a variable capacitor (varactor) using a bias voltage. In direct inductive coupling of coils there is no resonance involved although capacitors may still be used to match the driver amplifier or receiver detector to the coil. The word antenna in this context is probably a misnomer for there is no radiation involved, rather the coils act as a pair of loosely coupled inductive transformer. You can find the details and with design formulas in Umar Azad, Crystal Jing, Ethan Wang: "Link Budget and Capacity Performance of Inductively Coupled Resonant Loops" IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 5, MAY 2012

Answered by hyportnex on May 11, 2021

R.W. Bird · Answer

A coil does not have a frequency. A circuit with a coil, a capacitor, and resistance has a resonant frequency: $ω^2$ = 1/(LC). The frequency in your transmitting coil will depend on the source of power.

Answered by R.W. Bird on May 11, 2021

How to tune the frequency of a coil?

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