Radio Broadcast (May-Oct 1922)

Increasing the Selection Power of a Radio Circuit 2 I 1 conjunction with, the radio telegraph and the radio telephone. And, at any rate, it will be just as well for Science, to add to its records of the original radio phone, which attracted scientific atten tion the world over long ago, those first words uttered on that eventful Sunday, February 15, 1880: "Mr. Bell, Mr. Bell, if you hear what 1 say come to the window and wave your hat!" Increasing the Selection Power of a Radio Circuit By JOHN V. L. HOGAN Consulting Engineer, New York; Fellow and Past President, Institute of Radio Engineers; Member, American Institute of Electrical Engineers WE HAVE seen that by correctly coordinating the amount of capacitance and inductance in a freely vibrating radio circuit, we are able to secure an agreement between the most easily attained or natural vibration rate of the circuit and the received radio waves.* Each radio wave has a definite predominant frequency of vibration; the standard broadcasting wave oscillates at the rate of 833,000 cycles per second. By adjusting the capacitance of an intercepting or receiving aerial and the inductance of the tuning coil connected to it, we may make the natural oscillating frequency of the circuit from aerial wires to ground exactly the same as the frequency of the arriving wave; in this case there will be produced the greatest possible amount of current in the receiving aerial system, and consequently the loudest possible signals will be heard in an associated receiving telephone. SOME LOGICAL CONCLUSIONS A NATURAL conclusion to draw from the fact that agreement of natural and received wave frequencies results in maximum current is that disagreement between these frequencies would cause a reduced flow of current. That is, we would expect to hear weakened signals if we adjusted our antenna capacitance and tuning coil inductance to correspond to a circuit frequency differing from the received wave frequency. That is exactly what does happen when the experiment is tried. * "Tuning the Radio Aerial System" by John V. L. Hogan. Radio Broadcast, June, 1922, p. 107 But when we change the circuit frequency to a value different from the tuned or resonant value, at which it is in harmony with the received wave, how much will our signal strength be reduced? For example, how far must we de-tune the circuit (or how great must be the disagreement in frequency) before the current is reduced to one-half its maximum or resonant value? This is the question whose answer explains the matter of sharpness of tuning or the selective power of radio receiving instruments. To understand why one resonant circuit will tune more sharply than another, we must consider a little more closely what happens while such a circuit is oscillating. As we have seen, when a charged condenser is connected into a circuit including an inductance coil, the electrical energy stored in the condenser will discharge as an electric current through the coil and circuit. The current does not die away and vanish unless the circuit is poorly conductive; it "overshoots" and recharges the condenser in the opposite direction. Immediately thereafter, the condenser discharges backward through the coil and circuit, the electrical momentum or overshooting action of the coil causing a third recharge of the condenser. This time, however, the direction of the charge is necessarily as it was when the current oscillations were started. It is not hard to see that such successive reversing discharges of the condenser will generate an alternating current in the system, and that the frequency of this alternating current will depend upon the size of the condenser and the coil. The larger the capacitance of the condenser (just as the greater the flimsiness of the spring in a mechanical