# Resonating Coils

1. Oct 25, 2004

### Jdo300

Hello, I have kind of an odd question here. I was wondering how to make an electric coil that will naturally resonate at a perticular frequency. I am a novice when it comes to electronics so I'm hopeing that someone can explain this to me.

I also have another relate question. I heard somewhere that the earth has an electromagnetic field that resonates at a perticular frequency (I've heard the phenomenia referred to as Schunmen Resonance). Is this in deed true? If so, I wanted to try and make a coil that would resonate with this 'natural' frequency. Any help is appreciated.

Last edited: Oct 25, 2004
2. Oct 27, 2004

### BertHickman

That's not an odd question at all. It turns out that ALL coils will resonate at some frequency. That's because there is always some capacitance between turns and also stray capacitance to ground. The combination of these capacitances and the inductance of the coil form an LC circuit which will resonate. An example is a Tesla Coil secondary - the secondary will have a very sharply defined natural frequency that will be a function of its diameter, length, and number of turns.

On your related question - you are indeed correct. The Schumann resonance is the natural frequency of the Earth - ionospheric cavity. Powerful lightning strokes can excite this cavity, resulting in a degree of "ringing" that can be detected. However, the energy loss of this resonating system is rather high, so the ringing (if you could hear it) would be less like that of a bell and more like a dull "thunk"...

-- Bert --

3. Oct 27, 2004

### Jdo300

Thank you for your reply. Just curious, how would I go about calculating the dimensions of the coil and size wire I should use for a coil that could naturally resonate at the earth's frequency. Could you possibly point me to some formulas or something I could use? My goal would be to make the coil as small as possible. Also would it help if I had the coil wound on part of a circular core like a toroid? This is for a oddball project that I am working on (Not associated with radio antennas).

4. Oct 27, 2004

### BertHickman

Making a coil that self-resonates at 7 Hz would require a coil of truly Herculean proportions - NOT practical. There are a number of Tesla Coil design tools that can estimate the self-resonant frequency of a secondary coil. An easy to use, and accurate, program is Bart Anderson's on-line JavaTC program. You can simply fill in the parameters for a typical secondary coil and have JavaTC solve for the self resonant frequency (and other parameters as well). The program works for helical, flat spiral, or conical coils, but not toroidal coils. The program is here:
http://www.classictesla.com/java/javatc.html

A toroidal coil would provide you with significantly more inductance when wound around a toroidal ferromagnetic core (ferrite, iron powder, or silicon steel laminations). You would probably need to use one of the design programs offered by some of the ferrite or powdered iron core manufacturers to help you solve for the inductance. By adding a discrete capacitor across the coil, you can also lower the overall resonant frequency of the LC circuit, and an LC circuit that uses a discrete capacitor and ferrite core inductor could be designed for 7 Hz using reasonably sized components. The frequency of an LC circuit can be calculated as follows:

F = 1/(2*pi*(Sqrt(LC)) Hz

(where L = inductance in Henries and C = Capacitance in Farads)