Radio Broadcast (May-Oct 1922)

Record Details:

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io8 RADIO BROADCAST electric currents flowing in the aerial-to-ground circuit. These currents will alternately flow up and down, the frequency of complete (double) reversal being identical with the frequency of the arriving radio wave. The currents will last as long as the radio waves continue to strike the aerial; when the wave ceases, the currents will rapidly die away. It should thus be evident that if a stream of radio waves having a length of 360 meters, and Rigid Support Spring Adjustable Weights Fig. 2 therefore a frequency of 833,000 cycles per second, impinges upon the aerial of Fig. i, there will be set up in the aerial wires an alternating current of the same frequency, viz. 833,000 cycles or complete reversals per second. This current will flow between aerial and ground through the tuning coil and meter shown, and, if the meter is of proper delicacy, will register its passage by moving the pointer. Similarly, if a 375 meter radio wave strikes the aerial, it will generate an 800,000 cycle current which will also flow through and be indicated by the meter. Supposing that we desire to receive the signals carried by the 833,000 cycle wave and current, and to exclude the signals of the 800,000 cycle interfering wave, it is clear that we must find some method of augmenting the effects of one while reducing those of the other. There is a practical scientific way of selecting electric currents of any one frequency at the expense of those having different frequencies. The method is based upon electrical resonance or tuning, and is analogous to the phenomenon of "sympathetic vibration" which is so well known in the field of music. 1 1 depends simply upon securing an agreement between the frequency of the driving forces (the radio waves, for instance) and the most-easily-assumed or "natural" frequency of the driven system (in our example, the antenna-to-ground circuit). When these frequencies are alike they are said to be tuned to or resonant with each other. A digression will, perhaps, aid in securing a vivid idea of this natural or most-easily-assumed frequency of vibration. It is easy to grasp the thought of natural frequency of mechanical vibration in, for example, such an arrangement as is shown by Figure 2. Here a weight is supported by a coiled spring. At rest the weight takes the position A, where it is shown in full lines. If it is pulled down to position B and released, it will bob up and down between B and C, the path of travel gradually growing less and less until, finally, it will come to rest at the original position A. Perhaps the most interesting thing about such a system is that the number of times the weight will bob up and down again per second or per minute (in other words, its natural frequency of vibration) will be the same for every swing, regardless of the distance the weight moves in any one vibration. The most effective way to change this natural frequency of vibration is to change the stiffness of the spring or the mass of the weight. As can be easily seen, the greater the mass, the more slowly the system will oscillate; similarly, the greater the "flimsiness" of the spring, the more slowly will the weight move up and down. As these two factors (or either of them) increase, therefore, the frequency of natural vibration decreases. By changing either the weight or the spring one can make the frequency of the system anything he desires, within structural limits. Suppose that when disturbed and left to oscillate the weight rose and fell, or executed a complete up and down movement, four times per second. The system would then have a natural frequency of four cycles per second.