Radio Broadcast - 1927, May - 61 Pages, 4.9 MB ... - VacuumTubeEra

Some Facts About Coil DesignThe Oscillating Circuit as a Form of Voltage AmplifierThe "Qain"in the BroadcastSpectrum of Some Typical Coils The Essentials for a Well-Designed CoilBy ROSS GUNNTHE oscillating circuit, as applied to radio of a magnetic field, the capacity serves to store when one potential reaches its instantaneousreception, has been so thoroughly discussedin the past that itthe energy in the form of an electrostatic field, maximum, the other potential isgoing throughmight appear and the resistance serves to transform this stored its instantaneous zero value. These two potentialshave a certain interesting relation in termscertain point of view is always most fruitful in happens as the energy shifts from the condenser of the circuit constants. One of these potential?the matter has been completely exhausted. A energy into heat. The physical picture of whatthe consideration of oscillating circuits, especiallyin giving one a thorough understanding of familiar to the reader, and will not be given at and the other as the generated potential. Theto the coil, and back again, is undoubtedly will be referred to as the applied potentialthe physical phenomena that take place. In this time. In case the reader is unfamiliar with applied potential is the potential introduced intothe first part of the present paper an approach tothe circuit from some outside influence, such asthe problem is made from a particular angle, andan oscillator or an antenna. This applied potentialisit will be shown that an oscillating circuit is notD OSS GUNN,only a device to select a given frequency butthe author of this in phase with the high-frequency currentarticle,*may also be considered which is the first of two on as a type of the flowingsubjectin the oscillating circuit. The other potential,or the generated potential, is producedamplifier.of receivingIn the second part of the paper, the set radio-frequencyinductance and research within the its relation to engineer for the United States Aircircuit, by the action of the oscillat-inductors, was formerly a radioselectivity Service. Mr. Gunn is now attached to the Shane ing current, and is 90 degrees out of phase withand gain in a tuned amplifier, considering theLaboratoryoscillating circuit, is discussed. Finally, theat Yale the current and theUniversity and has devotedapplied potential.Theconsiderable attention toprime factors that should be considered to makethe currentproblems invokedflowing in any alternating currentcircuit is determined byinup the the ideal coil are design and use of coils in high-frequencythe applied potentialand the circuitgiven.oscillatingThe resistance or "loss" in an circuits. The present article doesconstants, in the mannernotoscillatingcircuit is made up of several components, and pretend to be revolutionary, for the basic given by the well known equation:theoryinvolved is atit is convenient to divide the circuit resistanceleast 50 years old. The author,instead of discussing power factor and= 1decrement,which have littleinto two parts, namely, the part due to coil resistance,and the part due to condenser to many read-resis-meaningers, has chosen to use atance. Modern factor which has a simplecondensers are relatively efficientsince they have resistances of only a fraction physical interpretation. The idea that there are where I is the current, Ea is the applied potential,L is the inductance, C the capacity, R thetwo potentials in anof an ohm oscillating circuit and thatat broadcast frequencies, while theordinary coils have resistances of from 5 to their ratio is the power factor, is of course well50equivalentknown, but the useohms. It is then obvious that, even if a manufacturerof the bestof the factor "G" series resistance, f the frequency,to explain and it is a numerical constant of value 3.1416.good condensers would succeed ratio of inductance to capacity, has If the circuit is resonated to ainrarely been pointed out. The definite frequency,casual reader willcutting the resistance of his condenserthe reactances arein balanced andhalf,equation (i)findthe total circuit resistance would be but this story an excellent discussion of circuitslightlyproblems whichaffected since the condenser resistance are is, in by no means reduces to:abstract, andgeneral,but a small percentage of the total circuitbetter informed should be (2)those who are interestedin the work Mr. Gunn and his assistantsresistance. On the other hand, if the coil resistancewere cut in half, the circuit resistancein which where I, Ea, and R are the same it is pre-as previouslyhave done, and in the waymentioned.sented. THETheEDITOR.greatest interest, however, iswould be cut almost in half, and the signal impressedon the grid of the amplifier will beduced by the applied potential. Considering nownot in the current flowing, but in the signal pro-double what it would be with the original poorthe application of the oscillating circuit to acoil. An improvement of this magnitude would this process, he is referred to almost any good radio set, it is assumed to be connected to a vacuumtube as shownbe well worth while, and may be accomplished elementary textbook on physics or electricity.by giving sufficient attention to the design of When in Fig. i, and so neutralizedthat there is no appreciable regeneration.the theory of oscillating circuits is examined,it is found that, in any series resonant Since the vacuum tube is a device controlledthe coil used in the oscillating circuit. Attentionis directed to the coil particularly, because the circuit, there may be considered to be two highfrequencypotentials which are approximately are able to apply to the grid, the greater will besolely by potential, the greater the potential wegood modern condensers are so much betterthan the best coils that there is no particular 90 degrees out of phase with each other. That is, the output signal from the tube.gain made by the use of the highestTHE "GAIN" OF A COILgrade condensers. The coil, then, shouldbe the object of considerable attentionT^HK diagram of connections in Fig.*and study until its losses are reducedi shows that the condenser and theto a minimum.inductance are in parallel across theIn the following discussion it isgrid and filament of the amplifyingshown that an oscillating circuit tube.mayTherefore, any potential thatbe considered as a type of voltageappears across the inductance or condenserwill be impressed on the gridamplifier, and a physical interpretationof the relation of the circuit constantsto the gain or desirability of01when an alternating current flowsof the tube. It is well known that,any given circuit will be C3given. TheLJthrough an inductance, there is generateda potential which is deter-physical interpretation that thismethod leads to, simplifies to a greatmined by the following equation:extent the perplexing questions thatEg = 2TfLI (3)arise, when one tries to state definitelywhere Egjust what is desired in an is the potential generatedoscillatingacross the coil,1 is the current in thecircuit.coil, L the inductance, and f the frequency.Similarly, the potential gener-Every electrical oscillating circuitLOOSE COUPLINGconsistsof an inductance, capacity, anda resistance (with usually an ated across the condenser is given by:oversupplyof the latter). The inductanceserves to store the energy in the form FIG. I where the symbols are as before.