Hertz's apparatus was shown in Fig. 6.3. It worked. It was the first one that worked; nonetheless it is not the best for explaining the underlying principles. From a more modern viewpoint, an apparatus that will radiate electromagnetic waves may be represented schematically by the block diagram of Fig. 6.4.

Obviously, there must be a source of power: you can't get something out of nothing. The next element is an oscillator. Its function is to produce a fast-varying current as shown in Fig. 6.5(a). If it just goes on indefinitely it can't possibly carry any information. The information is added by the modulator which, for wireless telegraphy, causes the current to be switched on and off, yielding for example the output shown in Fig. 6.5(b). The fourth element in the diagram is the transmission line needed to lead the current to the aerial, and the aerial is there to radiate the power. In Hertz's experiment there was no modulator (he had no intention of transmitting any information) and there was no clear distinction between oscillator, transmission line and aerial, but later all these components became separate.

What did the oscillator consist of? In its simplest from it was just a resonant circuit fed by discharging an inductor or a capacitor. The trouble with these spark-transmitters was that they radiated a wide range of frequencies. A number of different types of oscillator followed in the first decade of the twentieth century, but none of them was entirely satisfactory until the advent of the vacuum tube oscillator invented more or less simultaneously in Germany and in the US. This oscillator produced a single frequency output with the aid of a resonant circuit.

The disadvantages of radiating a wide range of frequencies may be appreciated by looking at a simple example. Let us assume that four ships are in the same area and they want to communicate with each Fig. 6.4 a block diagram of other at the same time. To be precise, let us say that ship A wants to a radio transmitter. send a message to ship B and similarly ship C wants to send a message

Fig. 6.5 (a) Continuous alternating current. (b) Pulsed alternating current.

to ship D. The result would have been ships B and D receiving both messages superimposed without a chance of separating them.

Once transmitters with single frequency outputs became available, there was a simple way of separating the messages. The receiver on ship B was tuned to the frequency of the transmitter on ship A and, similarly, ship D received only at the frequency emitted by ship C.

The next block in Fig. 6.4 is a transmission line. Its role is to carry the electromagnetic wave to the aerial. The aerials were long pieces of wire at the time. Unfortunately, they could not support themselves when standing upright so masts had to be erected. Balloons and kites were used as temporary measures but not of course in regular transmission or reception.

The block diagram of the simplest receiver is shown in Fig. 6.6. The signal comes in at the aerial and travels on the transmission line to the detector (demodulator) where the presence of the signal is shown in the form of sound, or it is printed on a tape.

0 0