Radio Broadcast (May 1928-Apr 1929)

AS I HI RROAI )( 5S I KR SEESH KY CAUI The Simplest Receiver , N A recent visit to kdka I had demonstrated tome the simplest and most inexpensive radio receiver in the world. Although generally cautious in the use of superlatives, in this case I use them without fear of contradiction. The cost of the receiver in question is precisely nothing. It is exceedingly compact; you could hide it under either half of your mustache, although it might not be convenient to keep it there. It is portable; you can carry it around the house, or outside, set it up in an instant, and when you don't want it, it disappears. It provides a loud speaker signal of moderate intensity. The loud speaker is contained in the set. There are no knobs, tuning controls, or adjustments of any kind; a child can operate this receiver just as well as a radio engineer. It requires no antenna and no battery or power supply of any description. The maintenance cost, it follows, is nil. In these respects it is certainly the ideal broadcast receiver. It has only two faults, which, in all honesty, I shall now reveal. It will not receive any other station than kdka. The tone is decidedly thin. The technical gentlemen of the Westinghouse Company can produce any reasonable number of these radio receivers on the transmitter grounds on a moment's notice. All they do is to pick up a nail or piece of metal and touch it to a spike in one of the wooden poles which support the antenna. Withdrawing the bit of metal slightly, the experimenter pulls out an arc a quarter or half inch long. He is thus drawing out of the air, according to orthodox radio principles, a few watts of the fairly considerable number which kdka is flinging prodigally over the Pennsylvania hills. As this radio-frequency arc is modulated, it sings shrilly in accordance with the voice or music which is agitating the kdka carrier at the moment. It is a radio receiving set, with all the virtues that I have claimed for it. And when you are through with it you toss the nail away. This receiver possesses one other convenience which, as far as I know, is incorporated in no other instrument of its class, and is found lacking even in $3000 outfits built for barons and millionaires. You can light a cigarette with it! Operation of Broadcasting Stations 20. FIELD strength measurements MOST operators of broadcast transmitters have only the vaguest sort of idea of what their outfits are actually doing, in the way of providing a signal for listeners, even in their immediate neighborhoods. A broadcasting station depends on its listeners just as much as a newspaper depends on its readers, or a gas or electric power company on its customers. The newspaper comparison is closer from the practical commercial angle, while physically the gas or power company analogy is quite6 exact. The broadcast transmitter must provide a certain radio-frequency pressure for the listeners whom it wants to reach, just as the gas company must maintain a certain gas pressure if it wants to sell gas, and the electric power concern must keep a fixed electric potential between its wires. The difference is that the power and gas companies know what that pressure is, whereas the broad caster generally does not. Yet there is no great mystery about the matter. The field intensity of a radio transmitter can be measured, not as simply as gas and electrical pressures, but with an amount of cost and trouble which is certainly not prohibitive considering the fundamental importance of the knowledge gained and the fact that a large investment is often involved. Every broadcast technician should at least know the general theory of field intensity measurements and calculations. The principal articles in the I. R. E. Proceedings are the following: Englund, C. R. : "Note on the Measurement of Radio Signals," Vol. 11, No. 1, Feb., 1923. Bown, Ralph; Englund, C. R.; and Friis, H. T.: "Radio Transmission Measurements," Vol. n, April, 1923. Austin, L. W. and Judson, E. B.: "A Method of Measuring Radio Field Intensities and Atmospheric Disturbances," Vol. 12, No. 5, October, 1924. Jensen, A. G.: "Portable Receiving Sets for Measuring Field Strengths at Broadcasting Frequencies," Vol. 14, No. 3, June, 1926. Friis, H. T. and Bruce, E.: "A Radio Field Vacuum-Tube Voltmeter Ir = 120 TC Is hr hs Is hr hs f / , 40 X 10-5 X (1) Size of loop, 103 cm. square No. of turns. 5 Inductance, 77 microhenries Resistance,3.5 ohms FIG. I Strength Measuring System for Frequencies up to Forty Megacycles." Vol. 14, No. 4, August, 1926. For those who do not wish to consult the sources listed above a brief outline of the subject is presented here. A transmitting antenna, with a certain amount of radio-frequency current flowing in it, sets up a moving field of electric force which is expressed in volts per meter, or in some more convenient units, such as millivolts or microvolts per meter. This simply means that if a receiving antenna is put up within range it will be charged by that field to a certain radio-frequency potential for each meter of its electrical height. The field strength of a 5-kw. broadcasting station, about ten miles away across fairly good transmission territory, may be of the order of 30 millivolts per meter, to give a practical illustration. Then if a listener puts up an outdoor antenna with an electrical height of 3 meters, he will get 90 millivolts from that transmitter to put into his receiver. The electrical height is less than the physical height (one-half is a common ratio). It is a measure of electro-magnetic effectiveness in transmission and reception. Its meaning may be better understood after a study of the basic absorptionless transmission formulas: X d R d R where lr is the received antenna current in amperes, Is the transmitter antenna current in amperes, X wavelength in meters, f the frequency in kilocycles, hr the effective or electrical height of the receiving antenna, in meters, hs the effective height of the transmitting antenna, in meters, d the distance in meters between the antennas, and R is the resistance of the receiving antenna. The transmitting current may be measured by means of a thermo-ammeter in the base of the antenna. The wavelength and frequency are known, if only because the Federal Radio Commission and the Department of Commerce insist on quite accurate data on this point. The resistance of the receiving antenna at the particular frequency in question may be measured by the added resistance method. (See Circular of the Bureau of Standards, No. 74. "Resistance Measurement.") The distance d is a factor which may be set by placing a receiver a few wavelengths from the transmitter, and then Ir will be sufficiently high so that it may be measured in the receiving antenna directly by means of a thermo-galvanometer or thermo-milliammeter, or the voltage across a receiving loop may be found by means of a vacuum-tube voltmeter. Thus the product of the two effective antenna heights, hr hs may be calculated from (1), rewritten as follows: hrhs=l£iARJ; (2) 40Xlsf w Of course this does not give us either hr or hs individually. As stated before, hr may be approximated by taking half the physical height of the flat-top of the receiving antenna above ground, if it is well removed from absorbing objects. Or, in the case of a loop antenna, it may be calculated by a formula which is derived from our knowledge of the mechanism of radio transmission and the nature of radio-frequency pick-up by a loop or coil antenna/which responds to the electro-magnetic component of the wave. With the loop turned end-on to the direction of radiation, and the antenna effect disregarded, the effective height is given by: Area X Number of Turns , , hr = 2X \ (3) Thus hr and hs may both be determined. The quantity hs is also of importance in that it is a measure of the radiating efficiency of the transmitting antenna. This radiation resistance, as it is called, is a part of the total resistance of the antenna. An antenna, like any other energyconverting device, has losses. The ohmic or heat loss incidental to currents flowing in a conductor is one of them. Then there are dielectric losses in the ground or in objects near the antenna. These are actual losses of energy similar co the losses caused by windage, winding resistance, and mechanical friction in an electric motor. But -an antenna is peculiar in that it has one type of loss of energy which it is definitely designed for, which is its reason for existence. It radiates energy, which is purposely lost so that it may be picked up elsewhere for the communication of intelligence. The total power dissipated in the antenna is given by: 102