I want to make a simple wireless device (radio?) that consists of only two copper coils where changing magnetic flux in primary coil causes a current flow in the secondary. I tried, once, with LEDs and 3-Volt DC batteries, but failed. So any insights or plans are appreciated. I would also like to give it some range (like, 2 meters).

Moreover, how would this device of mine will be different than a radio? Because, to my understanding, the changing flux here also causes electromagnetic waves to travel away from the coil. So does this mean that the basic principle behind the both is same?

My own knowledge about all this, as this writing would suggest, is very limited. I could not find any designs or schematics on the internet simple enough to follow and understand. Any help therefore is appreciated.

  • \$\begingroup\$ As you have noted, it is the changing flux that causes voltage to be induced in the coils. You need a circuit to induce a continuously changing current, to cause changing flux. The principle is that of a transformer. Only a small amount of the flux generated in one coil will be coupled into the other 2m away, so lighting lLEDs at that range is not possible. \$\endgroup\$ Commented Jan 27, 2016 at 22:02
  • \$\begingroup\$ It is the a magnetic field that the coil produces. A changing voltage creates a varying electric field. The two are related and at a distance from the source, the fields combine into an electromagnetic wave, this is a radio wave. When you are close to the emitting coil, the emission is known as near field, and is predominantly magnetic. \$\endgroup\$ Commented Jan 27, 2016 at 22:13

2 Answers 2


A hearing aid audio induction loop is such a system and quite easy to implement (provided you have a hearing aid).

It's most basic setup is to feed an audio signal into an audio amplifier and have this drive a loop of wire around the listening area. A coil, oriented correctly, inside the loop will pick up the audio signal and, if amplified, can drive an earpiece.

This article, My Story: Magnetic Communication Project, gives a good idea of how it works and how to make one.


So does this mean that the basic principle behind the both is same?

It is the same basic principle but, it's sufficiently dogged in differences that in the case of mutually coupled coils, the H-field falls with distance cubed whereas with a full EM wave the H-field falls linearly with distance. That means a fundamental difference to the range one can get for a given power input. Here's the Biot-Savart law applied to a coil: -

enter image description here

The formula at the bottom of the picture becomes an inverse cube law when Z>>R but, you could build big coils at about 1m diameter - do you envisage this as a possibility to consider?

The reason why a full EM wave doesn't "fall" as rapidly as cube law is that the E-field produced by "small" coils is nowhere near enough the right magnitude compared to the H-field. A full-fat EM wave has E to H in the ratio 377:1. Regard it is volts per amp or resistance i.e. the resistance of free space is 377 ohms.

To get the coil to be "E" resonant means it has to have dimensions of circa one wavelength. 30 MHz has a wavelength of 10 m and 10 MHz has a wavelength of 30 m so, depending on what frequency you envisage, they may need to be quite large.


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