How Antennas Transmit

Explore Software Defined Radio — by Wolfram Donat (15 / 30)

👈 Chapter 3 Antenna Theory and Design | TOC | How Antennas Receive 👉

To begin with, all radio transmissions are electromagnetic (EM) waves. To transmit a radio wave, a moving magnetic field induces a current and a voltage in a length of wire. You can test this yourself by connecting a sensitive voltmeter to a loop of wire and wiggling a strong magnet back and forth within the confines of the loop. You’ll see a small voltage and current appear on the voltmeter. It’s a small current — on the order of several microamps — but that’s because you’re not wiggling the magnet fast enough. If you increase the frequency of your magnet-wiggling, the voltage and current will increase in the wire, which in turn will increase the power of the transmission (remember your electrical equations —

, or power equals current times voltage).

Now, let’s pretend that your transmitter is just wiggling a magnet really fast around some wire. This creates an oscillating electric dipole (a dipole is a system where the positive and negative charges are separated by a distance). The oscillation creates an electromagnetic wave that emanates from the antenna (or the wire, in this case). The following image shows a dipole antenna emitting an electromagnetic wave in…