Rainey, Challenor W.

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 The Knowledge Bank at The Ohio State University Ohio State Engineer   Title:  Radio Frequency Current Creators:  Rainey, Challenor W. Issue Date:  Feb-1929 Publisher:  Ohio State University, College of Engineering Citation:  Ohio State Engineer, vol. 12, no. 4 (February, 1929), 10-11, 26. URI:  http://hdl.handle.net/1811/34550  Appears in Collections: Ohio State Engineer: Volume 12, no. 4 (February, 1929)         10 T H E O H I O S T A T E E N G I N E E RRADIO FREQUENCY CURRENTC. W. RAINEY, '32If the question, "What are the kinds of electri-cal current?" were put to the average laymanslightly versed in electrical nomenclature, theanswer would probably be, after some thought,alternating and direct current. Through scantknowledge other statements might be erroneouslyadded. Seldom will radio frequency current beadded as a distinct type. It would be futile to at-tempt to give a definition and the significance ofradio frequency current in a single statement andexpect the reader to grasp the meaning, unless hewas well acquainted with the subject. The sub-ject is too obscure to attempt this. Also, theorieshave not existed long enough to completely standthe tests of research.The electrical engineer of a very few years pastknew nothing of radio frequency current. Thenecessity for research on the subject was apparentwhen, in the design of insulation and equipmentfor radio use, radio engineers found a lack ofready information on the subject. Henceforthexperiments were conducted, some of the resultsof which are to date puzzling and but partly ex-plained. Radio development might be said tohave passed through a "low loss" period. "Lowloss" refers to low electrical losses through in-sulation of radio frequency circuits. Radio de-signers found that they could not utilize insula-tion that would pass criticism in the use of suchcurrents as low potential alternating or directcurrent. The "low loss" period developed andbrought into use such remarkable insulators asPyrex, Insulantite, and Bakelite. During thisand previous periods engineers were beginningto become interested in the prospects of such afield for research. Their investigations are justbeginning to materialize. Investigators at firstwere hindered by the lack of instruments. Neces-sity brought forth many new and improved in-struments such as vacuum tube voltmeters,thermalcouple meters, and methods for accurateresistance measurements.HOW RADIO FREQUENCY CURRENT IS GENERATEDThe contents of this article would, perhaps, beclearer if, first, a brief description of the theoryand manner in which radio frequency current isgenerated would be given. Let us first considerthe heart of the circuit, the vacuum tube (prop-Fig. 1—Radio frequency generator.Fig. 2—Alternating current curve.erly termed the triode tube). This tube, as im-plied in its name, triode, contains three elements.One element is maintained at a high temperatureso as to emit electrons; another acts as a screento control the electrons by its polarity charge; thethird element, due to its positive charge, receivesthe negatively charged electrons. The first ele-ment is termed a filament, the second, a grid, andthe third, a plate.The circuit is shown in Fig. I. For purposesof illustration the circuit is eliminated of all theunimportant parts. Upon examination we find aninductance, a triode tube, a plate blocking con-denser, and power supply leads. The inductanceis tapped with a lead to the filament which dividesthe inductance into two parts P and G (signify-ing the plate and grid sections of the inductance).The filament supply and connections are elimi-nated, as they would add unnecessary confusionto the diagram. We will suppose that the fila-ment is heated and is emitting electrons. Gridleak, grid condenser, and inductance tuning con-denser were all omitted from the circuit diagramas they are unimportant in explaining the theoryof operation. The circuit is technically knownas the Hartley, but the theory of all radio fre-quency generators is the same. The heavy linesin the circuit diagram are used to denote the partsof the circuit in which radio frequency current isflowing.Let us suppose that the circuit is adjusted toproper operating conditions. The electron emis-sion from the filament completes the circuitthrough the portion of the inductance known asP. At the present state there is current flowingfrom the minus lead of the power supply throughthe tube elements, then out the plate element tothe positive lead, and back to the power supply.At the same time a current flows through the in-ductance P and back to the plate through theplate-blocking condenser. This instantaneouscurrent in coil P causes a current to flow in in-ductance G. The grid element of the tube be-comes negatively charged. The grid gets thenegative charge in a simple manner. The minussupply lead (that comprises one end of induct-ance G) causes the inductance end nearest it tobe positive; therefore the other end must assumea negative charge. The negative end of the in-ductance is connected to the grid element and thusthe grid gets the negative charge. Now let usFEBRUARY, 1929THE OHIO STATE E N G I N E E R 11Fig. 3—Left: Cross-section of wire carrying direct current.Right: Cross-section of wire carrying radio frequency current.see what happens inside the tube. The negativeend of the inductance is connected to the gridelement and thus the grid gets the negativecharge. Now let us see what happens inside thetube. The negative electrons carrying currentto the plate are repelled by the negatively chargedgrid. This stops the current from reaching theplate and breaks the circuit. There is now nocurrent flowing in the circuit, and the motion of{he electricity comes to a rest. The grid now as-sumes a neutral charge as there is no currentflowing in the inductance to give the grid a nega-tive charge. The above described action repeatsitself. There has been described but one cycle ofoperation; actually the oscillations are very high(in the neighborhood of many thousands persecond, depending on the characteristics of thecircuit).When this article speaks of radio frequencycurrent it considers only those produced by un-damped oscillations, as those explained above.This article will now attempt to characterize andcompare radio frequency current with alternatingor direct currents.CHARACTERIZATION OF RADIO FREQUENCY CURRENTWe must first summarize alternating currentso as to have a basis for showing the characteris-tics of radio frequency current. It is believedthat when a comparison of radio frequency cur-rent is made to alternating current none will haveto be made to direct current as the difference istoo apparent to warrant discussion. Radio fre-quency current is generally classified as alternat-ing current, since the currents have a great dealin common.Briefly, alternating current is an electrical cur-rent whose current flow alternates so many timesper second giving it a definite frequency. Sowhen we say a 60-cycle current, we mean that thecurrent undergoes 60 alternations per second. Onconsulting Fig. 2, we find the current flow plottedagainst time. The current flow is reversed everytime it crosses the neutral line marked 0. Thefrequency would be the number of wave crestspassing a fixed point such as A per second. Com-mon industrial frequencies are 25 and 60 cycles.Special alternators have been built having fre-quencies of one hundred thousand cycles per min-ute. Direct current undergoes no oscillations asdoes alternating current. The current flow of di-rect current is continuous in one direction. Thusno curves are possible for direct current (a dia-gram would be a straight line). Direct currentmay be generated by chemical reaction as obtainedfrom batteries that generate their current by dif-FEBRUARY, 1929ferent rates of decomposition of their elements. Itmay also be obtained from a mechanical sourcesuch as a generator.It is a well-known fact that conductors carry-ing direct or alternating current utilize the en-tire mass of the conductor. Tests show thatradio frequency current utilizes only part of theconductor, that nearest the surface. On referringto A in Fig. 3 we find a cross section of a con-ductor through which alternating or direct cur-rent is passing. The dots On the cross sectionrepresent the current distribution. It is easilyseen that the current (shown by dots) is evenlydistributed through the conductor. Diagram Bin Fig. 3 represents a similar conductor cross-section through which is passing radio frequencycurrent. The dots show that nearly all of thecurrent flows along the surface of the conductor.From the above it may be deducted that a hollowconductor will pass nearly as much current as asolid one. It will be also easily seen that a hol-low conductor will pass much more current, pro-vided the mass of the solid conductor is equal tothe mass of the hollow conductor. No law isfeasible for the above because some of the radiofrequency current passes through the center ofthe conductor. Also the larger the hollow con-ductor becomes the higher the capacity of the in-ductance becomes and capacity cuts down theradio frequency current.The above fact brings up a most perplexingquestion. "If the radio frequency currenttravels along the surface of a conductor, then willa conductor whose surface is oxidized pass lessradio frequency current than a conductor whosesurface is a conductor such as silver?" Numeroustests and theories have been presented, the re-sults of which are conflicting. If an oxidizedsurface does not hinder the passage of radio fre-quency current, then will this disprove the theorythat radio frequency current does not passthrough the center of a conductor? Should theabove be so, where does the radio frequency cur-rent flow? One newr theory has been advanced—that the current does not pass through the con-ductor, but through the ether just outside the con-ductor! This question is still open for the re-search engineer.Now let us consider what happens in an in-ductance through which radio frequency currentis passing. In Fig. 4 there is represented an in-ductance with the radio frequency flow repre-sented by dots. Examination shows that theradio frequency seems to flow where the capacityof the circuit is greatest.(Continued on Page 26)Fig. 4—Inductance Showing Radio Frequency Flow26 T H E O H I O S T A T E E N G I N E E RRADIO FREQUENCY CURRENT(Continued from Page 11)Alternating- current can easily be transformedto any desired intensity or potential if the coreand primary winding are designed to work withthe potential and frequency of the current. Radiofrequency may be transformed by means ofresonated circuits. These are circuits that areboth tuned to the same frequency. No core isneeded as in the case of alternating current.Another interesting phenomenon is the factthat when a metallic circuit is brought in thefield of two radio frequency circuits in resonance,a current is generated in the metallic circuit al-though it may not be tuned to the resonant fre-quency.It is a common thing to have radio frequencycurrents generated in powers up to 50 KW.With the use of properly designed equipment itis possible to generate radio frequency currentwith a power loss of only 25 per cent. The totalloss is due to heating losses in the tube and re-sistance of the circuit. Present circuits will notgive a better energy transfer.This article has attempted by a few illustrationsto show that a large and interesting field in radiofrequency research is open to the investigator andengineer. The radio field is rapidly growing tobe one of our great commercial enterprises andthere is certainly a most interesting future await-ing anyone who is interested in its developmentand gifted with an analytical mind.

Radio Frequency Current

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Radio Frequency Current

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