Home » Radio » Shortwave Radio » Electromagnetic Waves Work

# Electromagnetic Waves Work

While avoiding Maxwell’s Equations, we need a basic understanding of how electromagnetic waves work. We can’t live without them.

You all know that radio signals travel through space as electromagnetic waves, composed of electric fields (volts/meter) and magnetic fields (amps/meter). Transverse electromagnetic waves contain E and B fields perpendicular to each other. How are EM waves created?

Current flow in any conductor sets up a magnetic field. Variation In the magnetic field creates an accompanying electric field. Variation in an electric field stimulates a new magnetic field. And so on.

Scientists discovered the relationship between magnetic and electric fields using a form of Ohm’s law. His law defines the AC or DC relationship between voltage, current and resistance, or E=I·R in a circuit. For AC, resistance is replaced by impedance, which accounts for reactive losses at higher frequencies.

Wave impedance, or the impedance of free space, is 377 Ω. Not surprisingly, you will find this is just the ratio of magnetic to electric fields in the EM wave. Technically, this value arises from the magnetic permeability  and electric permittivity of free space, which are constants.

So how does all this work? Let’s say you have a typical weak signal with a field strength of 1 μV / meter. Using ohms law, you can calculate the strength of the magnetic field as 2.7 pico Amp / meter.

Magnetic field current = Electric Field Voltage or 1E-6 volts / Wave Impedance or 377Ω = 2.7E-9 amps.

Your takeaway from this calculation is that if you are going to play with magnetic loops, you are dealing with very, very small currents!

## Electromagnetic Waves Work – Radiation Resistance

Antennas work by flinging tons of free electrons into space. In order to break free of the conductor, these electrons need to be accelerating or decelerating. If you think about the shape of a sine wave, it is obvious that charges change velocity rapidly at the peaks of the sine wave.

You have probably read that Radiation Resistance is a virtual resistance because it does not create any heat or loss in the circuit. But RRAD is real in the sense that it represents work done by the free electrons as they leave the conductor. It is energy lost at the atomic level when you form electromagnetic waves.

### One comment

1. Not Sure says:

Fascinating. Came here looking for your coherent interference flow graph, and wound up reading this (twice).

This site uses Akismet to reduce spam. Learn how your comment data is processed.