Make: Electronics by Charles Platt (classic books to read TXT) 📗

Figure 5-62. Each exposed section of wire is twisted into a loop using sharp-nosed pliers.

Figure 5-63. The stripped end of the wire is secured through the holes drilled in the bottle.

Figure 5-64. The completed coil, wrapped tightly around the bottle.

Your next step is to set up an antenna. If you live in a house with a yard outside, this is easy: just open a window, toss out a reel of 16-gauge wire while holding the free end, then go outside and string up your antenna by using polypropylene rope (“poly rope”) or nylon rope, available from any hardware store, to hang the wire from any available trees, gutters, or poles. The total length of the wire should be about 100 feet. Where it comes in through the window, suspend it on another length of poly rope. The idea is to keep your antenna wire as far away from the ground or from any grounded objects as possible.

High Voltage!

The world around us is full of electricity. Normally we’re unaware of it, but a thunderstorm is a sudden reminder that there’s a huge electrical potential between the ground below and the clouds above.

If you put up an outdoor antenna, never use it if there is any chance of a lightning strike. This can be extremely dangerous. Disconnect the indoor end of your antenna, drag it outside, and push the end of the wire into the ground to make it safe.

If you live in an apartment where you don’t have access to a yard outside, you can try stringing your antenna around the room, hanging the wire from more pieces of poly rope. The antenna should still be about 100 feet long, but obviously it won’t be in a straight line.

Hook the free end of your antenna to one end of your coil. At this point, you also need to add a germanium diode, which functions like a silicon-based diode but is better suited to the tiny voltages and currents that we’ll be dealing with. The other end of the diode attaches to one of the wires leading to a high-impedance earphone. A normal modern earphone or headphone will not work in this circuit. The return wire from the headphone is connected to a jumper wire, the other end of which can be clipped to any of the taps in your coil.

One last modification, and you’ll be ready to tune in. You have to ground the jumper wire. By this I mean connect it to something that literally goes into the ground. A cold-water pipe is the most commonly mentioned option, but (duh!) this will work only if the pipe is made of metal. Because a lot of plumbing these days is plastic, check under the sink to see if you have copper pipes before you try using a faucet for your ground.

Another option is to attach the wire to the screw in the cover plate of an electrical outlet, as the electrical system in your house is ultimately grounded. But the sure-fire way to get a good ground connection is to go outside and hammer a 4-foot copper-clad grounding stake into reasonably moist earth. Any wholesale electrical supply house should be able to sell you a stake. They’re commonly used to ground welding equipment.

Figure 5-65. The simple pleasure of picking up a radio signal with ultra-simple components and no additional power.

Figures 5-66 and 5-67 show the completed radio.

Figure 5-66. A signal from the antenna can pass through the coil to ground. If the jumper wire is attached to an appropriate tap on the coil, it resonates with the radio signal, just powerfully enough to energize the earphone which is wired in series with a diode.

Figure 5-67. The real-life version of Figure 5-66.

If you’ve managed to follow these instructions (one way or another), it’s time to tune your radio to the nearest station. Move the alligator clip at the end of your patch cord from one tap to another on your coil. Depending on where you live, you may pick up just one station, or several, some of them playing simultaneously.

It may seem that you’re getting something for nothing here, as the earphone is making noise without any source of power. Really, though, there is a source of power: the transmitter located at the radio station. A large amplifier pumps power into the broadcasting tower, modulating a fixed frequency. When the combination of your coil and antenna resonates with that frequency, you’re sucking in just enough voltage and current to energize a high-impedance headphone.

The reason you had to make a good ground connection is that the radio station broadcasts its signal at a voltage relative to ground. The earth completes the circuit between you and the transmitter. For more information on this and other concepts relating to radio, see the upcoming section “Theory: How radio works.”

Enhancements

The higher your antenna is, the better it should work. In my location, this is a major problem, as I live in a desert area without any trees. Still, just stringing the wire out of my window and tethering it (with rope) to the front bumper of my car enabled me to pick up a faint radio signal.

To improve the selectivity of your radio, you can add a variable capacitor, as shown in the following section. This allows you to “tune” the resonance of your circuit more precisely. Variable capacitors are uncommon today, but you can find one at the same specialty source that I recommended for the earphone and the germanium diode: the Scitoys Catalog (http://www.scitoyscatalog.com).

This source is affiliated with a smart man named Simon Quellan Field, whose site suggests many fun projects that you can pursue at home. One of his clever ideas is to remove the germanium diode from your radio circuit and substitute a low-power LED in series with a 1.5-volt battery. This didn’t work for me, because I live 40 miles from the