Make Electronics

Experiment 31: One Radio, No Solder, No Power

266 Chapter 5

theory

How radio works

When electrical frequencies are very high, the radiation

they create has enough energy to travel for miles. This is the

principle of radio transmission: A high-frequency voltage is

applied to a broadcasting antenna, relative to the ground.

When I say “ground” in this instance, I literally mean the

planet beneath us. If you set up a receiving antenna, it

can pick up a faint trace of the transmission relative to the

ground—as if the earth is one huge conductor. Actually

the earth is so large and contains so many electrons, it can

function as a common sink, like a gigantic version of the file

cabinet that I suggested you should touch to get rid of static

electricity in your body before touching a CMOS logic chip.

To make a radio transmitter, I could use a 555 timer chip run-

ning at, say, 850 kHz (850,000 cycles per second), and pass

this stream of pulses through an amplifier to a transmission

tower; if you had some way to block out all the other elec-

tromagnetic activity in the air, you could detect my signal

and reamplify it.

This was more or less what Marconi (shown in Figure 5-68)

was doing in 1901, after he had purchased rights to Edison’s

wireless telegraphy patent, although he had to use a primi-

tive spark gap, rather than a 555 timer, to create the oscil-

lations. His transmissions were of limited use, because they

had only two states: on or off. You could send Morse code

messages, and that was all.

Figure 5-68. Marconi, the great pioneer of radio (photograph

from Wikimedia Commons).

Five years later, the first true audio signal was transmitted

by imposing lower audio frequencies on the high-frequency

carrier wave. In other words, the audio signal was “added” to

the carrier frequency, so that the power of the carrier varied

with the peaks and valleys of the audio.

At the receiving end, a very simple combination of a capaci-

tor and a coil detected the carrier frequency out of all the

other noise in the electromagnetic spectrum. The values of

the capacitor and the coil were chosen so that their circuit

would “resonate” at the same frequency as the carrier wave.

Figures 5-69 and 5-70 illustrate these concepts.

Carrier frequency

Audio signal

Amplitude modulation

Figure 5-69. When an audio signal (middle) is combined elec-

tronically with a high carrer frequency (top), the result looks

something like the compound signal at the bottom. In actual-

ity, the carrier frequency would be much higher compared with

the audio frequency, by a radio of perhaps 1,000:1.

Amplitude

modulation

Chopped by

diode

Audible earphone

response

Figure 5-70. When the compound signal is passed through a

diode, only the upper half remains. An earphone cannot react

fast enough to reproduce the high carrier frequency, so it

“rides” the peaks and thus reproduces the audio frequency.

The schematic in Figure 5-71 shows the simple circuit that

you built by wrapping a coil around an empty vitamin

bottle. When a positive pulse was received by the antenna,

it resonated with the antenna and the coil, provided that

the antenna was long enough and the coil was tapped at

the appropriate number of turns.