RADIO NOTES.

Northland Age, Volume 25, Issue 16, 13 August 1925, Page 2

RADIO NOTES.

(By “Electron)” We will now consic! r what happens at the receiving end using a crystal rec-iv r. The aerial here is l I so elaborate as at the transmitting end and usually comprises one wire between about seventy and I2oft long and about thirty feet high. The lead in is taken through a coil shunted by a condenser or other tuning arrangements and then to earth. This condenser is variable, that is one set of plates is capable of being moved in between the other set or clear of them as required. From the leadin end of the coil a lead is taken to one side of the crystal detector. From the other side of the crystal detector the signals pass through the telephones to earth. It will be seen that signals have a choice of two paths, through the coil or through the crystal, telephones and to earth. The crystal and telephones offer a fair amount ot resistance to signals while the ceil possesses very little resistance so through the coil the signals go.

We saw that the waves of the transmitting plant made impressions in the ether and now the electricity in the aerial is moulded to the impressions in the ether made by the waves in the transmitter. This may be likened to a dentist making a set of teeth from an impression taken of your mouth and not from your mouth itself. Every signal from every station within range is surging from one end of the aerial to the other through the coil to earth and then back again not at a frequency determined by the length of the receiving aerial and size of the coil and condenser but at the same number of times a second as in the transmitting aerial where the wave length was determined. To make the signals we want to hear pass through the detector and telephones we must in some way block the waves from passing through the coil and force them through the crystal and telephones in their endeavour to reach earth before starting theirffiackward surge. By using a suitable coil and varying the moving plates of the condenser from the out position to the in position there will be a certain point where the aerial exactly'fits the wave length of the station we wish to receive. At this point the coil offers very high resistance to this wave length although letting all other wave lengths pass through as usual, with the result that signals pass through the crystal and telephones. I said before that the signals had to be deciphered. The waves in the aerial are surging in one direction and then in the other at 1,000,000 times a second (wave length being 300 metres). The surges or oscillations in one direction completely cancel the oscillations in the other direction so that nothing would nominally be heard. The work of the crystal is to allow the oscillations of one direction only to pass through which will operate the telephone receivers and the speech or music would be heard exactly the same as it was picked up by the microphone at the transmitter. A receiver, such as described above will receive up to about twenty miles from a broadcasting station. (Single valve sets next time). The writer ot these notes will be pleased to answer questions on the subject.