A Simple Introduction to Radio

This article explains how simple circuits can be used to tune into different radio signals.

Robert Lomas

Making Crystal Sets

As a short-trousered school boy I couldn't afford to buy a radio, not on my restricted pocket money, so the only way to get one was to make my own. It needed to be simple and it was. I made a crystal set.

Used toilet rolls, army-surplus wire, a condenser taken from a broken radio set my grandmother had thrown away, were all pressed into service. All I needed to buy from a radio junk shop was a cats-whisker, which was the old name for a crystal diode, and a pair of ex-Army headphones.

I made wooden base to hold the bits in place. Next I wound 250 turns of enamelled copper wire round an empty toilet roll centre, borrowed my school's only soldering iron to connect the condenser across it, and glued the bits to the wooden base. My rather dashing headphones were connected through the 'cats-whisker' across the toilet roll tuning coil. Now I needed an aerial and an earth. I buried two square yards of chicken wire netting in the flower bed under my bedroom window and connected it to bottom of the coil. This was my earth connection. To the other end of the coil I connected 50 feet of 'Army surplus' insulated copper wire, which ran from my bedroom window to the tree at the bottom of the garden. Just one thing left to do. I connected a 'by-pass' condenser, salvaged from the broken radio set, across the terminals of the headphones. Now I had my own radio set. One coil, one variable capacitor, one fixed capacitor, a rectifier, a pair of head-phones, an earth connection and a length of wire slung out of the window to the nearest tree. And it worked first time.

As I slowly turned the knob on the tuning condenser I could hear the BBC Light Programme, then the Home Service and even the Third Programme. How could a simple collection of bits and pieces of ex-War Department junk be turned into a working radio receiver? It�s because the way of radio works can be simple. If you make your aerial pretend to be the right length to fit the radio waves of the station your want to hear, you will be able to listen to it. But to me it was just a way to listen to Sub-Lieutenant Phillips as he told Petty Officer Pertwee to put his 'left hand down a bit' to run HMS Troutbridge into the dockside. These Sunday afternoon editions of 'The Navy Lark' heard on a crystal set still make me smile.

I soon found there were other things to hear on the radio, stations my simple crystal set couldn't tune into. If I twirled the dial of the radiogram right down, as far as it would go, I heard people who had their own private wireless stations. They were telling each other about the vast distances their �short waves� had gone and bragging about the distant stations they had �worked�. I wanted to know more - and curiosity drove my quest for a cheap short-wave radio of my own.

It took me months to save up enough money to buy myself a bit of war surplus kit, a PCR2 communications receiver, which tuned the short waves band from 1.8 to 30 Mhz. Now I was set up to really hear the world.

PCR2 stood for Pye Communications Receiver Model 2 and it had been made by the Pye Electrical Company for army use in World War II. It was a general purpose radio set which tuned the most useful short waves from 10 metres to 150 metres and many other short-wave broadcast bands. It had a socket for me to plug in the headphones from my crystal set and what was even better, a small built in speaker.and a jack-plug to fit and even bigger one, which of course I did.

It was wonderful. Now I could tune the local broadcast stations clearly, including the all important Radio Luxembourg. I could listen to Horace Batchelor offering to make me rich by selling me a system to win the football pools. His address, spelt K A N E S H A M, for those of you old enough to remember his adverts were a regular interruption to the pop music. I am still puzzled as to why he wanted to sell his secrets to me, why he didn't just use it win a fortune for himself if it was so good?

My new radio set had a better tuning system and a more accurate dial than the crystal set it replaced. It still worked in a similar same way though, by fooling the radio waves into thinking that the aerial was the right length to fit the signals I wanted to listen to. I was using the same aerial I had put up for my crystal set but I could now �trim� it. My new set had an aerial matching control and this enabled me to tune in very distant stations. For the first time, if I listened at the right time of day I could hear short wave stations in the United States, and from all over Europe.

To tune into the Light Programme, which was broadcast on a wavelength of 1,500 I had fool the aerial into thinking it was 375 metres long. This is why the aerial trimming knob was useful, it slowed down the radio waves so they were felt as strong as if I had really put up a much longer aerial.

There are two ways to tune a radio receiver. You can adjust the length of the aerial to exactly a quarter of the wavelength of the transmitter signal. Or you can do some electronic fiddling with the aerial signal so the set only hears the frequencies you want it to.

How Radio Tuning Works

I have been casually using the term wave-length to talk about different radio stations so here I explain just what I mean by the wave length of a radio wave.

Here is a diagram of the carrier wave of a radio station. As you can see it repeats, first creating a growing positive voltage and then reversing to create negative voltage before starting the whole cycle over again. Radio waves travel at a constant speed of 300,000,000 metres a second. So if the voltage went round the full voltage cycle of starting from zero, positive swing, back to zero, negative swing and finally back to zero in one second, the wave would have travelled 300,000,000 metres in that time and that would be the length of the wave. You could have chosen any point on the cycle to measure the wavelength as I've shown in the drawing. That's rather a long wavelength but the faster the wave completes its cycle the shorter the wavelength. For example if the wave completes a million cycles a second (what we call 1 Mhz) then the wavelength will be 300,000,000 divided by 1,000,000 which is 300 metres. So the faster the wave oscillates the shorter the wavelength of the signal. I mentioned when discussing dowsing that a water molecule will vibrate at about 98 Ghz (98,000,000,000) which gives a wavelength for its signal of about a quarter of a millimetre. So the faster the rate of vibration the shorter the wavelength of the electromagnetic wave.

But let's stay with simple radio tuning circuits and aerials.

Every aerial has a natural frequency it tunes, rather like the string on a piano, the longer the string the deeper the note. In the same way the longer the aerial the lower its frequency of reception will be. My original aerial of 50 feet (about 24 metres) had a natural reception frequency of 12.5 Mhz if it used its full length but radio waves don't work like that. So what wavelength of signal would I hear if I listened to the natural reception frequency from the end of my aerial? This diagram will help understand. It shows the natural reception wavelength of an aerial.

The signal which would give the strongest voltage response to my aerial would be one four times longer than the wire, making it resonant at a quarter of its wavelength. So listening directly to the output from the aerial I would hear the strongest signal which would be any broadcast station with a wavelength of 96 metres, or frequency of about 3 Mhz.

This circuit is called a rejector by engineers The capacitor has to be there to compensate for the electrical interaction of the coiled wire I had added to the end of aerial, as I will explain shortly. I choose to use a rejector aerial tuning circuit to make make my crystal set. This is what it looked like.

The variable capacitor had two sets of interleaved plates, one set fixed the other movable and by turning the centre shaft the value of the capacitance could be changed. Here is a drawing of what the circuit components looked like as they lay on the bench.

The aerial will capture signals from many different wavelengths, the strength of the signal being much higher near its natural reception frequency and much less at other frequencies but this circuit can be used to reject all the signals you don't want to hear and just allow the chosen one to make it to the earphones. It does this using a property called resonance which is a natural effect that amplifies small movements and makes them larger.

To understand resonance, imagine a child on a swing: the first push starts the child moving to and fro; as the swing changes direction, the child stops completely for a moment. If you continue to give the swing a small push each time it pauses just before it starts a new cycle, then the speed and movement will become faster and faster. Indeed, you will soon have to stop pushing to avoid the child being pushed right over the top. This increase in the amplitude of the swing by making small carefully timed pushes is called resonance and it is how this coil and capacitor circuit works.

The cycle of events goes like this. When the current from the radio wave hitting the aerial first beings to rise on the upper connection of the coil-capacitor circuit, the capacitor is not charged. All the current is sucked into the empty capacitor to charge it up. No current flows to the coil because the uncharged capacitor looks like a short circuit and takes in all the available electrons to charge it up.

Once the capacitor is charged up, the current can start to flow to the coil. As it does, it makes a magnetic field around the turns of the coil, as well as an electric field along the length of the coil. This combined electro-magnetic field is a radio wave that travels off in all directions. Imagine the waves like ripples that spread out on a pond when you throw in a stone.

By the time the current in the coil has built up, the alternating current from the aerial that was feeding the circuit will be starting to die away, getting ready to reverse its direction for a new cycle. As the supply current dies away in the coil, the stored charge in the capacitor starts to flow. This discharges the capacitor into the coil and keeps the electro-magnetic field in the coil going for much longer. Finally the capacitor will discharge and the whole cycle will start again.

The speed at which this happens depends on the number of turns, and the size of the coil, as well as the size of the capacitor. Different pairs of coil and capacitor will respond at different frequencies. In this way the circuit only magnifies the radio signal which matches the resonant frequency of the circuit and it rejects all the others, hence engineers call it a rejecter circuit.

This is not the only why to choose just one signal from the cacophony of radio stations continually pulsing the aerial. I could have arranged the same components like this drawing of an Acceptor Tuning Circuit for a Crystal set.

If the natural resonance frequency of the coil and capacitor match the transmitter frequency, the current from the capacitor worked with the coil and magnified the current. If the natural frequency did not coincide, the transmitter current would be over-ridden by the capacitor current and they would cancel each other out.