# What is the theory behind the Arduino UNO being able to create AM waves at 800 kHz

I am doing a school project and saw someone use an Arduino and a piece of wire as an antenna here: https://forum.arduino.cc/index.php?topic=88422.0

I've tested this experiment and it works, but I have searched the internet for some time now for some background knowledge or theory on why this works, and how does it?

If anyone knows anything about the arduino creating simple AM waves, could you please let me know about it or redirect me to another source.

For some context, in this school project I've got to explain the theory behind creating basic AM waves and amplitude modulation in general, then prove the theory by conducting this experiment which I prove it correct.

• That code just generates a 800kHz PWM signal, that is then turned on and of to greater the clicking sound you get. May 2, 2017 at 8:41

Strictly speaking it's not AM, it's OOK (On-Off Keying) which just turns a carrier frequency (800kHz PWM, 50% duty cycle) on and off.

Many cheap 443MHz (eBay) modules do the same thing - turn the carrier frequency on and off.

The presence of the carrier frequency is seen as a logic 1 and the absence as a logic 0.

The fact that it's using PWM to generate a square wave means that the "cleanliness" of the signal will be incredibly horrible. You really want to get the carrier frequency as close to a pure sine wave as possible. A square wave has lots of harmonics at 3x, 5x, 7x, 9x etc the frequency. So your 800kHz signal will also have a 2.4MHz signal, a 4MHz signal, a 5.6MHz signal and so on, each at a lower level. That causes interference in other signals.

To understand more about why it works you first have to understand radio transmission theory, and that's not something we can cover on an Arduino site (it's an entire college course all by itself).

• How do you get 2.4, 4.8, and 7.2 MHz from 6x, 6x, and 9x? May 2, 2017 at 14:20
• @JamesWaldby-jwpat7 Through the careful use of a typo. Should have been 3x 6x 9x. Actually there should be 5x and 7x in there too. And 6 is wrong. May 2, 2017 at 14:24
• I thought that might be the technique used! So useful sometimes :) May 2, 2017 at 14:32

It works because everything is wrong. A perfect sine wave of 800 kHz would not transmit very well with such a small wire as antenna.

The output signal is a digital pulse (PWM signal) with very sharp edges. Those edges create many harmonics as Majenko wrote. It is those high harmonic frequencies that can use that small wire as antenna. The receiver might get help of those harmonic frequencies to combine them with the base signal to be able to receive the base signal of 800kHz. The receiver might also receive the electrical field coupling (capacitive coupling) instead of really receiving radio signals. The electrical (capacitive) coupling might even be main part of what is received.

You could add a large antenna or a large open coil or a tuned circuit of a coil with a capacitor. That should increase the range, because that will transmit real radio signals.

The Arduino board with the wire can not be used to explain AM radio signals, because it is not a AM radio signal. You can also not use it to explain AM modulation, because there is no modulation (only on/off). Do you know someone who has equipment to make modulated AM signals ?

Last but not least: it is illegal to transmit at 800kHz.