How to build a radio
To kick off our adventures in radio, Adam Murphy set out to build one, with the help of demo maestro Dave Ansell...
Adam - Radio is everywhere. And in a pandemic, people are finding renewed comfort in the little wireless box. The equipment is simple and easy to get a hold of. And you can indulge in the radio while doing a myriad of other things. Radio waves, which carry the signal are electromagnetic waves, just like light or x-rays, only there are a lot longer. Where light has a wavelength in the billionths of a metre, long radio waves can be kilometres long. Given the ease, you can get a hold of a radio, I thought it should be relatively straightforward to put them together, myself but... Oh, this is so many turns so much copper this is taking so long.No, just stay! Ow, copper gets hot when you heat it Murphy. So I decided to get some help from Dave Ansell. The first bit of the instructions were to wrap a load of copper wire around a tube. So what's that all about?
Dave - So it's doing two things. The first one is it's acting as an aerial. Radio waves are electromagnetic waves, which means they produce electric and magnetic vibrations in space. And there's little magnetic fields and if you have a magnetic field through a coil, it will cause a voltage to be produced on that coil. If an electromagnetic wave goes past your coil, it will cause a varying voltage on the coil.
Adam - Any EM wave going past my coil then can induce a little bit of current in it, which through a circuit, you can get a signal which is a radio, but why so many turns, why so many coils,
Dave - There's all sorts of radio waves going past your coil at the same time, but you only want to listen to one of them. That's one at a certain frequency, a certain pitch effectively. And so you want to build a circuit, which is really sensitive to that pitch, but not sensitive to any other. So the way you do that is you build a resonance circuit. So a bit like if you push a swing, it wants to swing at a certain frequency. The circuit will want to have currents going through it at a certain frequency. And you do that with a coil, which behaves a bit like the inertia of the swing. A bit like a mass on the end of the swing and a capacitor, which is a bit like a spring.
Adam - So with the right combination of capacitors, along with my coil, I can shoot into a specific frequency, a specific station. I've got a little tunable capacitor on this, which has a knob that you can turn to change the capacitance and then change the exact frequency or station that you're tuning into. But that's not the end. My little circuit had a lot of other components, but the other key one is called a rectifier. And that has an important job of its own.
Dave - The right. So the radio waves you're picking up are at a much, much higher frequency than the frequencies we use in speech. You could feed those into a loudspeaker, but you wouldn't hear anything because they're millions of cycles per second. Whereas we hear in thousands of cycles per second. And so you end up with a wiggly line that gets bigger and smaller and bigger and smaller at an audio frequency.
Adam -So one of the things the rectifier does is taken a that's too high in frequency, too high in pitch for you to be able to hear and essentially averages out by just looking at the changes in the amplitude or the loudness, which is in a frequency you can hear, but it's not done yet.
Dave - You can put that into a loudspeaker. You can't hear anything because it averages to zero. So to be able to detect it, you've got to cut away the bottom half of the waves. So you put it through, effectively, a diode so you see the top half of the wave. And then when that's averaged out by the loudspeaker, it will turn into a sound wave at a frequency we can hear