Nuts and Volts - September 2015

6 September 2015

Radio Propagation

QI am in the sixth grade and am working on a project for school about how radios work. Can you help me by explaining how the radio signals can be received through the air?

Julie Gutierrez Redding, CA

ASixth grade is a great time to start learning about one of the most fascinating technologies of our times. Radio signals are traveling electromagnetic waves which can move through air or even the relatively empty reaches of outer space (Figure 1). Radio waves are used to carry information (such as music, voice, telephone, computer data) from a sender (a.k.a., transmitter) to a receiver.

The theory behind radio stretches back to the Persians who developed batteries by using an iron rod and copper cylinder in a clay jar with some acid material to produce low voltages as shown in Figure 2 (you can do the same thing in a cup using a steel plate, a copper plate, and lemon juice). Unfortunately, the Persians had no use for these batteries since the electric light bulb, MP3 players, and TV would not be invented for several thousand years.

The Greeks discovered static electricity (what makes your hair stand up when you slide down a plastic board) by rubbing fur on a piece of tree resin called amber. I like the static charge which causes the plastic wrapper to jump back out of the trash can and attach itself to your hand. Benjamin Franklin discovered that lightning was a moving stream of electrical particles. These discoveries were neat, but had no practical use. In the 1600s through the 1800s, scientists discovered many more things about electricity. Some of these discoveries were useful, and eventually lead to the development of radios. Charles Coulomb discovered like charges repel and unlike charges attract (Figure 3). The positive charges were atoms missing negative charges, which were named electrons (Figure 4). Michael Faraday discovered that if you move a magnet through a coil of wire or vice versa, you can generate a voltage (the principle of the electric generator) as shown in Figure 5. Voltage pushes electrons through a wire just like pressure from a pump pushes water through a pipe (Figure 6). The moving stream of electrons is called current; the wire tries to keep the electrons from flowing by its resistance.

Georg Ohm (yes, this is the correct spelling) discovered that the current through a wire (I) is equal to the voltage between the ends of the wire (V) and the wire’s resistance (R). As a formula, this is represented as I = V/R, or what we call Ohm’s n WITH TIM BROWN Q & A

In this column, Tim answers questions about all aspects of electronics, including computer hardware, software, circuits, electronic theory, troubleshooting, and anything else of interest to the hobbyist. Feel free to participate with your questions, comments, or suggestions. Send all questions and comments to: Q&A@nutsvolts.com.