WHEN TUNING

Evening Star, Issue 21079, 16 April 1932, Page 4

WHEN TUNING

“ How does my set separate the stations?” This is probably one of the most frequent questions asked of the radio man, and one of the hardest to answer in a word (states the ‘ Listener In ’). In order to carry the “ sound waves from the transmitter to the receiver, a wireless station first sends out an electric current along which these waves may travel. There must bo water before there can bo water waves—-there must be air to carry the sound waves from one place to anotherj this “carrier wave,” as it is called, is sent out from the station in order to carry the waves which are transformed ultimately into sound waves in our wireless gear. When a set is working very close to a transmitting station, naturally the signals from that station are very strong indeed. As we travel farther from the transmitter, the signals begin to weaken, until, if we go far enough away, we would not be able to hear anything at all, even with a powerful set. Once we get the idea of stations sending out waves of a different length, it is easier to see how we can separate them. If we have two cog wheels which have teeth of the same size and the same distance apart, they will fit snugly into each other, and, what is more, they will not fit into a cog which has te-th of a different size. The same thing happens in broadcasting. The transmitter sends out waves of a definite length. This length is determined by the values of the coils and condensers at the transmitting end. Now the receiving set has coils and condensers which change their value ■when the controls are operated, in such a manner that the wave lengths to which they are in tune at any one moment can be changed at will. Thus, if we wish to hoar a station whose wave length is 200 metres, we turn the dials of the set until the set is in step with any signals at all on a 200-metro wave length, but is out of step with wave lengths which are not 200 metres long. If we change our minds and want a station on 400 metres, we turn the dials again until the set is in tune with_4oo- - signals. The 200-metre signal will now bo very much out of step, and we shall not be able to hear it at all. If we think back to our cog wheels, we will see how two cogs to whoso teeth fit properly will run smoothly, but they will not grip if the teeth are of widely different sizes.

Unfortunately, if wo are very close to a transmitter, the power radiated from the aerial is so great that it forces its way through the set even if we are tuned to a wave length considerably different from that of the nearby station. Even the most powerful and carefully-made sets arc frequently unable to deal with such a condition. Even if we are several miles from a strong station we often find it coming in on top of another. In a case like this, we have either to reduce the amount of energy our set receives or buy a set which has a large number of tuning circuits, which will help to filter out the strong signal, and keep it in its right place. With a smaller set we must cither shorten our aerial, make our aerial coil smaller, or fit a wave trap which will absorb or reject the energy from the nearby station before it has a chance to swamp cut the signals wo arc wanting to hear.