TUNING THE RECEIVER

Hawke's Bay Tribune, Volume XXII, Issue 273, 2 November 1932, Page 4

TUNING THE RECEIVER

The whole principle of radio communication depends on the ability to tune electrical circuits to any frequency (writes "G.H.T.," in "Listener In"). The transmitter at the broadcasting station is tuned to a predetermined frequency, more commonly known as wave-length; this is determined by dividing the frequency, in cycles per second, into the speed of electricity per second—namely, 300,000,000 metres. The frequencies used for radio communication range from approximately 20,000 cycles—ls,ooo metres—to 600,000,000 cycles—(half-metre)—the latter, however, still being in the experimental stage. To receive a sympathetic response in our receiver the radio frequency circuit must be tuned to the same frequency as that of the station from which we wish to receive. As an analogy of thia tuning, imagine a suspended weight, such as a clock pendulum; this has a natural frequency (the number of complete swings per second), and if given slight impulses at the right number of times per second it will swing naturally. If, however, we shorten the pendulum and thereby change its natural frequency, and then apply the slight impulses at the same rate as before, they will have no effect, for their frequencies are different; in other words, they are not in tune. Capacity is that property possessed by a condenser to store an electric charge. Inductance refers to magnetic properties of a coil. It is the natural characteristic of capacity to make the voltage, or pressure, lag behind the current, while the inductance cadscs the current to lag behind the voltage. If a condenser and an inductance are connected in parallel, and to them we apply an electric charge, the pressure across the coil will rise immediately, but the current will momentarily lag behind, the amount of lag depending on the inductance. In the condenser the current flows, but the voltage lags behind, the amount of lag increasing with the capacity. If the circuit is now broken the condenser will discharge its stored energy. As the current from the condenser falls the magnetic energy is transferred back to electrical energy, so momentarily keeping current flowing and charging the condenser in the re-, verse direction.

When the current from the coil ceases the condenser will again discharge and a similar sequence of events to that described takes place, but in the reverse direction. This reversing will go on at the same frequency per second until the stored energy gradually fades away. Let us now liken this circuit, with its natural frequency, to the pendulum; it should be easily understood that, with slight impulses of the same frequency, the circuit will oscillate in tune with the impulses. By varying the capacity and inductance the frequency can be altered, large capacity and inductance producing low frequencies (long wave length), while small capacity and inductance give high frequency (short wave length).