Electromagnetic waves

The conversion of sound waves into electric currents and the amplification¹ of these currents are two basic electronic techniques required for record players, different electronic techniques.

There are many reasons why it is not practically to transmit radio waves of very low frequencies, but for one things² the transmitting aerial will be impossibly large. It is therefore not possible to broadcast electromagnetic waves of audio frequencies, and for long-distance transmission it is necessary to resort to higher frequencies and therefore shorter wavelengths; the wavelengths actually used vary about 1 metre to 2000 metres.

In general, electromagnetic waves are propagated through the Earth's atmosphere in two ways: travel over the surface of the Earth, and waves, which leave the surface of the Earth. They are reflected back ionized layers of the outer atmosphere between 60 and 400 kilometres above the Earth's surface. This ionized region of the atmosphere is called ionosphere.

Long waves arc transmitted as over ground waves and therefore lose a good deal of their energy in passing through ground obstacles ³; they consequently have only a limited range. The shorter waves escape more easily from the surface of the Earth. They can be received over greater distances owing to their reflection back to the Earth by the ionosphere. Very short waves pass through the ionosphere so they can be used for long-distance communications. This is why the range of TV stations is limited. This problem has however been overcome ⁴ in recent years by the use communication satellites. If the satellite simply reflects the radiation, it is called a passive satellite, where as it is receives the signal and retransmits an amplified signal, it is called an active satellite.