Radio broadcast .. (1922-30)

Record Details:

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STRAYS FROM THE LARORATORY Sun Spots and Radio A correspondent questions the correla- tion between solar activity (sun spots) and radio reception and at the same time wonders if the steadily declining field strengths from distant stations are not due to a "saturation" or overloading of the ether. We sent these doubts and sugges- tions to Dr. Pickard whose speculations on the interesting questions and long con- tinued measurements on WBBM'S field strength have won him great credit. Dr. Pickard's answer is a chapter in the book of fundamental knowledge of radio. Dr. Pickard's curve (Fig. 1) shows such definite relation between solar activity and radio reception as to leave no doubt. Dr. Pickard says: "Signal strength at night from distant stations has been on the down-grade for at least the past three years. But the correla- tion of this decrease with solar activity has been proven beyond any doubt, and irrespective of any hypothesis. Many thousands of reception measurements, taken systematically over periods of years, have shown such high correlation with measures of solar activity that the relation of sun and reception is as certain as the relation of moon and tide. "But 'etheric overloading' is a harder problem. If the entirely hypothetical 'ether' really had the properties of matter, it might, of course, have definite tensile strength, and perhaps also exhibit fric- tional or hysteretic losses, in which case it would either break up or heat under severe loading. But the majority of modern physicists do not believe there is an 'ether'. Instead, our present-day view is that what we call electromagnetic waves are merely energy quanta shooting bullet-wise across empty space. According to this view over- loading has no physical meaning; how can we overload a void by merely projecting more quanta through it? "This reasoning may not be convincing to everyone, particularly those who still cling to their 'ether'. So, avoiding all assumptions as to the existence of proper- ties of the medium, let me present some evidence from the experimental side. First, consider the well-known fact that a station hundreds or thousands of kilo- meters distant can be well received even 60 4O 2O — 0 + 2O 4O 60 WAVE FREQUENCY IN .KILOCYCLES Fig.l Fig. 3 directly under the antenna of an operating high-powered transmitter. Measurements of the field strength of the distant station show no change when the local transmitter is thrown on and off. Yet, the field im- mediately around a high-powered trans- mitter is enormous as compared with that from the distant station, and if there were any such thing as overloading, such a test would show its presence. "But far more severe tests have been made. A pair of metal terminals may be placed in a tube exhausted to the limit of modern vacuum technique, and a field of the order of a million volts per centimeter can be impressed without breaking down the intervening space. In fact, when such a tube finally does pass current, it is because electrons are bodily torn from the metal terminals; not because the space between is in any way overloaded. Recently very great magnetic fields, of the order of a million or more gauss, have been produced by heavy transient currents in small coils. But the space traversed by this intense magnetic field shows no sign of distress. ^20 Such electric and magnetic fields, viewed as strains or loads upon a medium, are mil- lions of millions of times greater than the average loading produced by the radio stations of the world, and still nothing happens. " Finally, consider the space just beyond the sun's surface, traversed by the intense radiation from the photosphere and by the solar gravitational and magnetic fields as well. Heavily loaded as is this space by both waves and fields, during a solar eclipse we find the light from a distant star goes through unchanged, save for the slight Einstinian bending caused by the gravi- tational field." British Heater Tubes An article in January 8, 1930, Wireless World entitled "A New Method of De- tection," while somewhat misleading in title, _since no new method of detection is described, gives some interesting data on English heater-type tubes. American set engineers have long wondered at the high values of mutual conductance secured by foreign tube manufacturers, which cannot all be laid to the fact that tubes "over there " are measured at zero grid bias and not under operating conditions. The Marconi and Osram ML4 is called a general-purpose tube, has a plate resistance of 3000 ohms, and an amplification factor of 6. At a grid bias of 23 and a plate potential of 200 volts the plate current is 19.5 milliamperes. It will turn out about 750 milliwatts. The MHL4 has a resistance of 8000 ohms and a mu of 16. Its resistance is suffi- ciently low that it may be used with a transformer output with about double the voltage amplification secured from out- heater-type general-purpose tube. Both of these tubes are very good, having mutual conductances of the order of 2000 micromhos (at zero grid bias). The heater consumes 1.0 ampere at 4 volts. The MHL4 makes a good grid-circuit de- tector as the curve in Fig. 2 shows. With an r.m.s. input signal of about 2.75 volts it will deliver about 8 volts of a.f. at 50 per cent, modulation. The circuit con- stants are given on the curve. A Correction On page 223 of the February issue curves were given showing the selectivity and sensitivity characteristics of Radiola 21 and 22 receivers. Unfortunately the (Continued on page 349} (j o H 3 O MARCONI AND OSRAM MHL4 HEATER 1-0 AMP. AT 4V. OPERATING POINT" ¥ 0.0001 MFD. 'MHL4 1 2 3 4 5 fl INPUT VOLTAGE (R-M-S-) Fig. 2 RADIO BROADCAST FOR APRIL • 333