Science, Maths & Technology

# Challenge: Transmit and Receive a Signal

Updated Monday 28th January 2008

The science behind transmitting and receiving radio signals, part of the BBC/OU's programme website for Rough Science 2

WARNING: It is illegal to transmit unregulated broadcasts in the UK

With just a basic tool kit, some copper wire a piece of coke, a car battery, a bottle of rum, some old kitchen pots and pans would we be able to transmit a signal across the island and successfully decipher the message?

Firstly, what sort of signal are we going to transmit?

Since the beginning of time people have been trying to communicate over distances greater than the human voice can travel. More rudimentary attempts included the use of smoke, fires and waving flags. Mirrors were also used to flash the image of the sun to distant objects.

Copyright: Used with permission After the discovery of electricity, a revolution in communication took place. Wires were stretched from one point to another and an electric current passed through them, controlled by a switch called a telegraph key. A light or buzzer could be turned on and off over great distances and using a simple code, such as the Morse system, people were able to communicate almost instantly over distances that had previously required days or weeks by horse or train.

In 1901, Marconi sent a signal across the Atlantic using the recently discovered radio waves. It paved the way for today's complex wireless communications systems such as satellites and mobile phones.

How are we going to transmit our signal?

Feeling ambitious, we decided to go wireless. We may not to able to make a mobile phone system but our very own radio station with a transmitter and receiver might just be possible.

What is a transmitter?

Radio can be incredibly simple. All you need to transmit a signal is a spark. Radio waves are composed of electric and magnetic fields. The exact way the two fields are brought together in the radio wave is rather complex but the physical theory that describes these waves means that a changing electric or magnetic field will create radio waves directly.

The tiny spark produced in a light switch when you turn it on also produces radio waves which is why you hear a click on the radio when you turn nearby electrical items on and off.

If we can maintain the spark (rather than just for an instance when the switch has been operated) you can convert the 'click' into a 'buzz' and use it to send a Morse code type message.

Why does a spark produce radio waves?

Air is normally a very good insulator and so does not conduct electricity. However, if the voltage is high enough (for a given amount of air) the electrical energy causes the air atoms and molecules - composed of electrons, neutrons and protons - to let go of their electrons. This is called ionisation.

Because the electrons are negatively charged they are immediately attracted to the positive electrode and this movement produces an electrical current. The electrons bump into other atoms along the way causing further ionisation and therefore greater current.

The ions are constantly moving about and this movement of charge produces changing electric and magnetic fields that create the radio wave energy. Most of the energy is produced as heat and light but a little goes to form the radio wave energy we need for transmitting. The spark is not a very efficient way of generating radio wave energy but it is the simplest!

A conductor in its natural state - negative electrons (blue) and positive ions (red) evenly distributed along its length.

The same conductor in a state of imbalance where all the negative particles (blue) have been forced to one end.

How can we make a spark transmitter?

To create a spark you need a large voltage difference: the bigger the voltage difference, the bigger the spark. And the more radio waves created, the further you can transmit a signal.

To create a voltage difference we need a supply of electricity and on the island we made use of a car battery. The battery produces a voltage difference of 12V, which is not really enough for what we want. To increase the voltage we made a transformer.

How does a transformer work?

A transformer is a device that can increase or decrease voltage and it does this by decreasing or increasing the corresponding current. We can understand this best by considering the power through the system.

The power is given by the voltage x current. The power in and out of the transformer must be the same (ignoring heat losses) it's just that the transformer converts one set of voltage x current to another set of voltage x current.

For example, 1 Volt x 6 Amp = 6 Watts. This is the same as 6 Volt x 1 Amp = 6 Watts.

The transformer can convert 1 Volt to 6 Volt (a multiplication of 6) if the corresponding current is converted from say 6 to 1 Amp (a multiplication of 1/6th).

The transformer is made up of two coils of wire (a primary and a secondary coil) wound around each other usually on a metal former to make it more efficient. The multiplication factor is dependant on the ratio of the number of turns of wire in the two coils.

Copyright: Used with permission If both sets of coils have the same number of turns then the voltage and current that goes in will be the same as the voltage and current out. If the secondary set of wires has 100 times the number of the primary then the transformer will in principle step up the voltage by 100 times.

However, a transformer will not work with the direct current (DC) from a battery - it will only work with an alternating current (AC - one that changes constantly).

To convert the DC voltage of the battery from DC to AC we built a buzzer. This is a device that constantly interrupts the direct current flowing and produces a crude AC current. This current was then fed to the transformer that boosted the voltage producing a much bigger spark.

How did we make our transmitter on the island?

The Rough Science transmitter had the following parts: 12V car battery, Morse key switch, buzzer and transformer mounted on a wooden board and an antenna.

The Morse key is a switch that connects and disconnects the 12V power to the rest of the circuit. It has a wooden knob on the top, which makes tapping out the Morse code easier.

The buzzer consists of the electromagnet with a saw blade switch forming the buzzer device. The saw blade switch has two connections: one connected at the fixed end and a second made by a screw contact on the flexible section.

Copyright: Used with permission The screw could be adjusted to get the best contact. In a very crude way the screw also sets the buzzing speed and therefore the pulse frequency to the transformer. The interrupted current from the buzzer then went into the primary of the step-up transformer.

The transformer was made by winding about 100 turns of thin insulated wire onto a bundle of iron nails and then the much thinner insulated secondary was wound on top of this - about 10,000 turns.

How did we send our message?

The buzzer sparked quite a lot but it was a bright white spark: lots of heat and light but not much radio energy. The high voltage output from the transformer was blue. The radio could pick up the buzzer spark but the high voltage output spark was louder.

When the Morse key was held down the buzzer buzzed, the current pulsed to the transformer which stepped up the voltage to several hundred volts producing a nice bright spark. The radio waves created by the spark (arc) were then radiated by the antenna which was connected electrically to the arcing terminals (saw blade) of the output of the transformer.

Often we found that we could not get a reliable arc (probably due to the humidity) and so we wired the output from the transformer (using a common connection) onto the buzzing switch which created a lovely reliable blue arc.

The simplest radio is a crystal set. This consists of an antenna, coil and capacitor, a detector (the crystal) and headphones or earpiece.The antenna picks up the radio waves and converts it into a tiny electrical voltage which is passed onto the coil and capacitor.

Copyright: Used with permission These two devices form what is known as a tuned circuit that filters out most of the radio signals and just lets one through. There is no battery in this simple receiver - it gets all its power from the original radio wave that induced its voltage into the antenna.

In the Rough Science radio we made the receiver even simpler by not including a capacitor. This meant that the receiver was not tuned which helped to pick up as much signal from the broad spark transmission (the spark is hard to tune) as possible. The antenna was a long piece of wire connected to the coil detector junction.

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Did we successfully complete the challenge?

In three days we just about managed to make apparatus that could send a signal about 20m. It was a struggle to hear the magic word above the noise of the wind and crashing waves nearby on the shore.

Copyright: Used with permission The sound of the transmitter picked up by the radio was a little indistinct - a sort of rasping buzzing sound and very feeble. In addition, because the radio was not tuned, it picked up radio stations in the background.

On our maiden test run with the equipment, we did manage to send the word "YES". It was a very fiddly job to get reliable signals with the equipment outside, with wind, rain and all the other distractions of our fabulous location but we succeeded with our challenge.

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The Evolution of Communication - from the ThinkQuest site

How to Contruct an Efficient Wireless Telegraphy Apparatus at a Small Cost - by A. Frederick Collins, reprinted from Scientific American Supplement,1902

Antique Wireless and Scientific Instruments - from the collection of John D. Jenkins

Building a Radio - from the Science Toys site