Radio Broadcast (May-Oct 1922)

238 Radio Broadcast Thus the circuit can be adjusted so that its natural frequency is any value desired (within the limits of the set.) Tuning then means that you adjust the natural frequency of your receiving circuit so that it is equal to the frequency of the radio waves you desire to receive. Why must this be done? Because the amount of energy in the received radio wave is so small that, if the receiving circuits were not tuned, there would be no effect produced by the waves. It is a case of resonance. We are all familiar with resonance effects though we may not call them by that name. When you swung your playmate you used the principle of resonance. You timed your pushes so that they would come just at the right instant. By doing this you were able to make the swing go very high, using only slight pushes. The swing had a natural period of oscillation; by timing your pushes (tuning them, so to speak) you got a large eflfect from a small amount of energy. Examples of resonance are numerous. The fool who rocked the boat knew about resonance. He timed the swaying of his body to the natural frequency of oscillation of the boat, thus overturning it. He secured a large effect from a small amount of energy. Have you noticed, very often when a piano is being played that, as a certain note is struck, the glass in a picture frame or some other object will rattle. This is because the object has a natural period of vibration equal to that of the note. The two are "in tune" and thus the small amount of energy in the sound wave produces a large effect. Thus, then, when any system which has a natural period of oscillation or vibration of its own is acted upon by a very feeble force that has the same frequency, the effect produced by the feeble force is large. The radio wave from a distant station has only a feeble energy when it reaches your station. It has a certain definite frequency. If you desire that your receiving set be affected by the feeble energy in the radio wave, you must adjust your circuit or circuits so that their natural frequency is equal to that of the radio wave. The receiving circuit and the radio wave are then in resonance, hence the latter produces a comparatively large amount of energy in the receiver. This process of adjusting the natural frequency of the receiving set to equal that of the desired radio wave is called tuning. Now we are ready to get a better understanding as to why we can have more than one radio message in the ether at the same time without interference. Suppose one station is transmitting on a wavelength of 300 meters and another on a wavelength of 600 meters. Their frequencies are then 1,000,000 and 500,000. Now if you adjust the natural frequency of your cirruit to be 500,000, it will be in resonance with the 600 meter wave and out of resonance with the 300 meter wave. The 600 meter wave will affect your receiving apparatus; the 300 meter wave will not unless the latter is very close by. You are tuned to 600 meters. You did it by adjusting the values of the capacity or inductance in your receiving set. From the above explanation it might be thought that your receiving set will respond to one frequency (wavelength) only. Unfortunately this is not the case. There is a hand of wavelengths to which your receiving set will respond. The width of this band depends upon the receiving set. It may be that if you tune to 600 meters your set will respond almost equally well to any radio wave whose length is between 510 and 690 meters. This is a variation of i 5 per cent, on either side of the 600 meters. This is not very selective tuning. It is to be noted that any station transmitting on a wavelength between 510 and 690 meters would cause interference if the waves had nearly the same amount of energy as the waves you were trying to receive. Some receiving sets are capable of better selection — sharper tuning. A set that will respond to wavelengths between 570 and 630 meters (5 per cent) when tuned to 600 meters is a fairly sharply tuned set. Such a set diminishes the possibility of interference. This then is a limitation on the number of ether waves that can be utilized without interference. Another limitation lies in the fact that a transmitting station radiates an impure wave. That is, instead of radiating its energy at one wavelength, it radiates a band of wavelengths on either side of its main wavelength. Thus we have the transmitting station radiating a band of wavelengths and a receiving station responding to a band of wavelengths. This limitation upon a limitation greatly reduces the number of messages that can be in the ether without producing interference at the receiving station. Rapid progress is now being made in the reduction of the width of both