Developments Radio Sky-Wave Propagation Research … · Developments in RadioSky-Wave Propagation Research and Applications During the War* J. H. DELLINGERt, FELLOW, I.R.E., AND …

PROCEEDINGS OF THE I.R.E.-Waves and Electrons Section

Developments in Radio Sky-Wave Propagation Researchand Applications During the War*

J. H. DELLINGERt, FELLOW, I.R.E., AND NEWBERN SMITHt, SENIOR MEMBER, I.R.E.

Summary-This paper discusses the work done by the Interserv-ice Radio Propagation Laboratory during World War II. The circum-stances leading to the establishment of IRPL are described and theproblems which are faced are stated. The measures taken in thesolutions of these problems are outlined, and some of the results arepresented. Specific services performed by IRPL during the war forthe armed forces and commercial companies are recounted.

r HE INFLUENCE of the ionized layers of theearth's upper atmosphere, the ionosphere, onradio wave propagation has been recognized ever

since the experiments of Breit and Tuve and of Apple-ton proved its existence. Because of the scarcity of ade-quate ionospheric data, however, and because relativelyfew radio men realized its importance, the use ofionospheric data in radio communications before thewar was relatively small.The important part played by radio during the war

brought to light the necessity for having adequate radiopropagation information. No matter how good theequipment was at the transmitting and receiving ends,satisfactory communication could not be had unless thewaves were propagated with sufficient strength to bereceivable. Variations in propagation conditions provedto be several orders of magnitude greater than varia-tions in transmitter power or receiver sensitivity.Furthermore, the extreme crowding of the radio-fre-quency spectrum made necessary full utilization of allavailable frequencies, and an appropriate selection offrequencies could be made only with the help of radiopropagation data. Also, security considerations dictatedthat the frequencies used should be the best for use andthe least likely to be intercepted by the enemy. Thedesign of equipment, especially of antenna systems,was found to depend critically upon a knowledge ofradio propagation conditions. In addition, other appli-cations of radio, such as radar and direction finding, in-volved considerations of propagation regarding range,accuracy, and receivable intensities.With the widespread use of radio communication by

the armed forces, especially in parts of the world wherebut little experience had been had, the need for im-proved radio propagation information became apparentearly in the war. An aircraft disaster in the EuropeanTheater led to the establishment of the British Inter-Services Ionosphere Bureau (ISIB) in 1941, and the

* Decimal classification: R112.41 XRI13.65. Original manuscriptreceived by the Institute, January 15, 1947; revised manuscript re-ceived, May 2, 1947. Presented, 1946 Winter Technical Meeting, NewYork, N. Y., January 26, 1946.

t Interservice Radio Propagation Laboratory, National Bureauof Standards, Washington, D. C.

exigencies of air force operation in the Southwest Pacificresulted in the formation of the Australian RadioPropagation Committee (ARPC), both instituted tofurnish radio propagation data and predictions to theirrespective armies, navies, and air forces. Correspond-ingly, in 1942, the Interservice Radio PropagationLaboratory (IRPL) was established in the NationalBureau of Standards by order of the U. S Joint Chiefsof Staff, acting through the Wave Propagation Com-mittee of the U. S. Joint Communications Board, withthe functions of (1) centralizing data on radio propaga-tion and related effects, from all available sources, (2)keeping continuous world-wide records of ionospherecharacteristics and related solar, geophysical and cosmicdata, and (3) preparing the resulting information andfurnishing it to the armed forces. This involved main-taining ionospheric observatories, centralizing data fromthese and other ionospheric observatories operated byother agencies and other countries, performing experi-mental and research work as necessary to supplementexisting sources of data, preparing predictions and fore-casts of radio propagation conditions for all parts of theworld, issuing charts, tables, handbooks, and bulletinsfor immediate dissemination to the armed forces,maintaining a "special problem" consulting service togive immediate answers to urgent military problems,and co-operating in this work with other agencies of theUnited Nations.The groundwork for the prediction of radio propaga-

tion conditions and ranges of useful frequencies hadbeen laid by the previous ionosphere researches of theNational Bureau of Standards, some of the results ofwhich were published in the PROCEEDINGS OF THE I.R.E.from 1937 to 1940, under the title, "Characteristics ofthe Ionosphere at Washington, D. C." During 1941 to1943, at the request of the National Defense ResearchCommittee, the National Bureau of Standards made astudy of the correlation of direction-finder errors withionospheric conditions, and prepared a "radio trans-mission handbook" to permit usable frequency calcula-tions.

In meeting the requirement of predicting useful fre-quencies over any paths anywhere in the world, theIRPL was confronted by five major problems: (1) theobtaining of adequate ionospheric data on a world-widebasis, (2) the development of methods for calculatingmaximum usable frequency over long paths, (3) the de-velopment of methods for calculating sky-wave field in-tensities, (4) the determination of minimum requiredfield intensities, and (5) the development of methodsfor forecasting ionosphere storms.

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Dellinger and Smith: Developments in Radio Sky- Wave Propagation Research

At the outset, the IRPL was faced with the necessityof obtaining sufficient ionospheric data to permit predic-tions to be made anywhere in the world. At that timeionospheric observations were being made only at sixlocations in the world: Washington, D. C.; Slough, Eng-land; Huancayo, Peru; Watheroo and Sydney, Australia;and Christchurch, New Zealand. Regular data wereavailable to the IRPL only from the first, third, andfourth of these. Immediate steps were taken to ex-pand the world-wide coverage, with the cooperationof the Carnegie Institution of Washington; the UnitedStates Army and Navy; the Canadian Navy andAir Force; the British Admiralty, ISIB, the NationalPhysical Laboratory and the British BroadcastingCorporation; the Australian organization, the ARPC;and the U.S.S.R., with the result that by the end of thewar 44 stations were regularly reporting ionosphericobservations, as shown in Fig. 1. This network of sta-tions, together with analysis of radio traffic data froma number of communications networks, permitted thecontinual improvement of world charts of predictedionosphere characteristics, from the small beginnings in1941, based on only three stations, to the comprehensivecharts now published monthly in the IRPL-D seriesreports. The knowledge gained from the greatly ex-panded world-wide ionospheric coverage permitted amuch improved delineation of the regular variations ofthe ionosphere with latitude and local time, so that, forexample, the hitherto seemingly anomalous behavior ofnorthern and southern hemisphere stations fell into aconsistent world picture.

In the course of their work it became necessary toplace radio propagation predictions on a regular, world-wide basis, such that the great mass of data could behandled expeditiously and practical predictions issuedregularly.

In order to use the data which were being receivedfrom all over the world for prediction purposes, it wasnecessary not only to understand their geographic,diurnal, and seasonal variations, but also to determinetheir relationship to relative sunspot numbers. A simplecorrelation of values of ionosphere characteristics withrelative sunspot numbers had been previously found,but during the war trends of the variation of these char-acteristics with sunspot number were determined forthe locations on earth of many of the ionosphere sta-tions. A technique of prediction of ionosphere character-istics at any location, using standard statistical methods,was evolved, involving an estimate of the relative sun-spot number for the month of prediction. A nomographicmethod for doing this type of prediction rapidly waslater developed.As another consequence of the improved world-wide

coverage, the so-called "longitude effect" was dis-covered and put into operational use in 1943. This wasthe discovery that ionosphere characteristics were not,as previously supposed, the same, at the same local time,for stations at about the same latitude but different

longitudes. Instead, they depended to a great extent onthe geomagnetic latitudes of the station. Thus thestation at Delhi, India, showed quite different char-acteristics from those observed at Baton Rouge, La.

Following this discovery, the world was divided, forpractical operational purposes, into the three zonesshown in the map of Fig. 2. In each zone the character-istics are independent of longitude, to a good enoughpractical approximation.The second problem faced by the IRPL was the de-

velopment of a simple rapid method of obtaining themaximum usable frequency (m.u.f.) over any paths inany part of the world. The groundwork for this was laidin 1936 when the "transmission curve" method of scal-ing ionospheric records was devised, leading to factorswhich could be applied to critical-frequency data to ob-tain m.u.f. values. These factors were satisfactory fordistances up to 2500 miles, but for greater distances themethod of multiple hops proved clumsy and, indeed,quite inadequate in the light of observed radio propaga-tion data.

Consequently, the empirical "two-control point"method was dev"ised (independently at the IRPL andISIB) for paths longer than 2500 miles, whereby them.u.f. over such a path is limited by the lower of the2500-mile m.u.f. at two control points, 1250 miles fromeach station along the great-circle path connecting thetwo stations. This procedure gave much better results.As the volume of data increased, it became more andmore apparent that normal F2- or E-layer propagationwas completely inadequate to account for a consider-able part of the observed transmissions, particularly attimes when E. (sporadic E) was present. Consequentlyan extended analysis of E. occurrence was made, andsufficient regularity was found to make E. predictionspossible, subject to the much wider day-to-day vari-ability than in the case of the normal layers. Consider-able further improvement in m.u.f. calculations wasthen made by including the effects of "sporadic-E" (E.)propagation, also on a two-control-point basis.World charts were prepared giving predictions of

maximum usable frequencies, three months in advance.These, are continued in the monthly publication nowissued through the Government Printing Office.The urgent need for knowing distance ranges and

lowest useful high frequency (l.u.h.f.) led to the nextmajor problem undertaken by the IRPL-the calcula-tion of sky-wave field intensities. To meet this, thefield-intensity-recording program, begun by the Na-tional Bureau of Standards early in the last decade,was greatly expanded by installation of recorders atthe new United States ionospheric stations. Fig. 3 showsa sample automatic field-intensity record. At the sametime, theoretical studies of ionospheric absorption atoblique and vertical incidence were undertaken, in anattempt to obtain a simplified solution to the problemas rapid in operation as the method of calculating m.u.f.An "equivalence theorem," similar to that used in

1948 261



Field Intensity Record of StotionWWV Beltsville, Md

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calculating m.u.f., was used, employing the observedfield-intensity data to supply numerical values to themany uncertain factors in the equation. It was foundthat the diurnal variation of ionospheric absorptionvaried to a good approximation, on the average, linearlywith the cosine of the zenith angle of the sun, a factwhich simplified greatly the integration of ionosphericabsorption over a given transmission path; the absorp-tion could then be determined as a function of fre-quency, distance, and average solar zenith angle overthe path.The determination of sky-wave field intensity did not

of itself give the whole story, however, unless informa-tion also could be available as to the minimum fieldsrequired for radio communication. Thus, the fourthmajor problem confronting the IRPL was the de-termination of minimum field intensities necessary toovercome atmospheric radio noise, which was the prin-cipal type of noise encountered at sky-wave frequencies.This is still the subject about which least is known inthe field of sky-wave communication. Some frag-mentary measurements were available, and a beginningon the problem had been made at the ISIB in England.All available data on atmospheric radio noise and re-quired fields were collected, as well as data on thunder-storms, which are the source of atmospheric noise. Theresult of the analysis was to divide the world into zonescorresponding to different grades of noise intensity, tak-ing into account both the generation and the propaga-tion of the noise. Fig. 4 shows one such chart, forNovember through March. The principal noise-gen-erating centers are in the East Indies, Central andSouth America, and Africa, with secondary centers inthe tropical oceans-the "doldrum belts." For eachnoise grade, a set of curves of required intensities wasconstructed, similar to the one shown in Fig. 5. Thesewere for good 95 per cent intelligible radiotelephone

communications, and empirical factors were deducedfor other types of service; for example, manual c.w.telegraphy required only 1/7 as great intensities, whilefour-tone single-side-band six-channel radio teletypemight required only 1/14 as great intensities. Much ofthis work was done with the close collaboration andassistance of the Radio Propagation Unit of the Officeof the Chief Signal Officer of the U. S. Army.

For convenience in use, the required field graphswere plotted on nomograms involving frequency andabsorption index, so that the l.u.h.f. could be read offdirectly.The fifth major problem of the IRPL was the fore-

casting of ionosphere storms-those abnormalities of-ten associated with geomagnetic storms-which disruptradio communications, especially in the Arctic. Themilitary importance of the North Atlantic, whichreaches into the auroral zone, or zone of maximum dis-turbance, made it indeed urgent to know when com-munications were likely to be interrupted. The urgencyis apparent when it is realized that aircraft dependedlargely on radio aids for navigation over the NorthAtlantic.

Therefore a program was undertaken, in collaborationwith the Department of Terrestrial Magnetism, Car-negie Institution of Washington, to study the relationsbetween ionosphere storms and the sun, whose radia-tions produce the storms. Improved observational tech-niques, like the Harvard University coronagraph, a de-vice for photographing the extremely active solarcorona, contributed to the study. As a result of theanalysis, a weekly forecast was issued, which proved tobe of some value to the armed services. The world wasdivided into zones of varying ionospheric disturbance,as shown in Fig. 6, and forecasts were made for eachzone.A different approach, however, led to a considerably

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more accurate service of forecasting disturbances ashorter time in advance. In this, studies of the behaviorof radio-d.f. bearings over the North Atlantic pathshowed that it was possible to issue warnings of radiodisturbance a few hours to a half day or more in ad-vance. Consequently a short-time warning service was

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inaugurated whereby such warnings were telephonedand telegraphed daily to interested agencies. With thelifting of wartime restrictions, this warning service isnow being broadcast regularly over WWV, the Na-tional Bureau of Standards station at Beltsville, Md., at20 and 50 minutes past each hour. A group of N's or

W's is transmitted, the former meaning "no warning"(quiet conditions expected) and the latter "warning"(disturbed conditions over the North Atlantic expectedor in progress).

During the war the IRPL performed many specificservices for the armed forces and commercial com-

panies doing war work, involving consultation and ad-vice, on their special problems involving radio wave

propagation. Types of problems included the determina-tion of best usable frequencies for specified services,such as point-to-point, short-distance tactical opera-

tions, plane-to-ground, high-frequency broadcast, theprediction of ground-wave and sky-wave distanceranges under different conditions, advice as to typesof antennas and lowest required radiated power forspecified purposes, and frequency allocation. As thetechniques promulgated by IRPL became more widelydisseminated, many types of problems, especially thosein frequency allocation, were eventually solved by theArmy and Navy groups in which they originated.

In January, 1944, a two-weeks training course in ra-

dio wave propagation was given by IRPL. It was for of-ficers who were to be taught the principles of radio wavepropagation and methods of problem solution and thenassigned to overseas communication groups, where theycould put on a scientific basis the assignment of radiooperating frequencies in the field. Others were then to

be sent to training units within the United States to or-ganize courses in which additional officers could be in-structed in this work. The student body consisted oftwo groups, the first group consisting of eleven Army AirForces officers, four officers from the Signal Corps, andthree Navy officers, and the second group consisting offifteen enlisted men and one officer from the SignalCorps, who formed the nucleus of the Radio Propaga-tion Unit of the Signal Corps. The course comprisedtwenty-five lectures by scientists and others working di-rectly in radio wave propagation, interspersed withproblem sessions in which the students were coached inthe solution of practical radio wave propagation prob-lems.As a further aid in determining the proper usage of

radio frequencies, three handbooks were issued. Thefirst handbook, "Radio Transmission Handbook Fre-quencies 1000 to 30,000 kc.," was issued in January,1942, giving the basic principles of radio sky-wave prop-agation, and such computational procedures as wereextant at that time, together with preliminary versions ofprediction charts and predictions for the winter. A sup-plement to this handbook was issued June 1, 1942,which gave summer predictions.On November 15, 1943, the "IRPL Radio Propaga-

tion Handbook, Part 1 " was issued. This handbook, is-sued as an IRPL publication, and also as an Army train-ing manual (TM 11-499) and a Navy publication (DNC-13), gave a descriptive discussion of the behavior of theionosphere and of the theory behind maximum usablefrequencies and lowest useful high frequencies. Predic-tion charts of maximum usable frequencies and absorp-tion constants were given. Techniques for the determi-nation of m.u.f. and l.u.b.f. over any path at any timewere given to the extent that they had been developed atthe time. It is impossible to express fully the valuableaid and support received by the IRPL from other agen-cies. Close and continuous liaison was maintained withthe Department of Naval Communications (CNO) andthe Radio Propagation Unit of the Army Signal Corps(SPSOL); valuable assistance and suggestions were in-terchanged with personnel in those departments work-ing with IRPL in the solution of basic problems and ap-plications. Close co-operation was maintained with otherbranches of the Army and Navy having need of radiopropagation data, including the Army Security Agency,the ArmyAir Forces, other branches of the Signal Corps,the Navy Bureau of Ships, Bureau of Aeronautics,Coast Guard, and other branches of Naval Operations.Acknowledgment is made to the Department of Ter-

restrial Magnetism, Carnegie Institution of Washing-ton, for its extremely valuable assistance in operatingionosphere stations and collecting much of the geophysi-cal and solar data upon which the services of the IRPLwere based, and to the untiring work of its staff in co-

operating in the entire radio propagation program. Ac-knowledgment is also wholeheartedly given to the fullco-operation, from the very beginning, of the radio prop-

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agation organizations of other countries-Canada,Great Britain, Australia, and New Zealand withoutwhich no world-wide radio propagation program wouldhave been possible.Many commercial organizations and other agencies

gave valuable assistance to the work of the IRPL. Ananalysis of radio traffic data was regularly provided byUnited States communication companies, such as RCACommunications, Mackay, Press Wireless, and A. T.& T., from Army and Navy networks, and from other

Government agencies such as the C.A.A. and theF.C.C.; this analysis assisted greatly in predicting andchecking predictions of usable frequencies, and in cor-recting and corroborating theoretical processes ofanalysis.With the conversion to the postwar era, the work of

the IRPL is carried on and extended by the Central Ra-dio Propagation Laboratory of the National Bureau ofStandards. Particular emphasis is laid on research inradio propagation at all frequencies.

Alternating-Current Measurements of

Magnetic Properties*HORATIO W. LAMSONt, FELLOW, I.R.E.

Summary-Herein is presented a critical analysis of various pro-cedures for determining the permeability and core loss of ferromag-netic materials, together with a discussion of the limitations underwhich such observations are made and the interpretations whichshould be applied to the data obtained.

INTRODUCTION

T IS THE purpose of this paper to discuss variousmethods for the a.c. measurement of the magneticproperties of a specimen of ferromagnetic material

and, it is hoped, to clarify certain phases of these tech-niques about which some misunderstanding has existed.The system of magnetic nomenclature and definitionsrecently adopted by the American Society for TestingMaterials' will be used.

MAGNETIC CONDITION OF THE SPECIMEN

It is a well-known, but sometimes ignored, fact thatall ferromagnetic materials exhibit the phenomenon ofhysteresis and have, in effect, a "memory," so that anypresent (instantaneous) condition is more or less influ-enced by past events. To obtain significant and repro-ducible data it is first essential to erase all memory ofprevious conditions. This preliminary demagnetizationmay be accomplished by subjecting the specimen to asubstantial a.c. magnetization which is then graduallyreduced to zero. Thereafter, any applied a.c. magneti-zation must be removed by a gradual reduction to zero,rather than by interrupting the circuit at some arbi-trary time in the cycle.

* Decimal classification: R282.3. Original manuscript received bythe Institute, March 11, 1947; revised manuscript received, July 10,1947. Presented, Rochester Fall Meeting, November 11, 1946,Rochester, N. Y.

t General Radio Company, Cambridge, Mass.1 A.S.T.M. Specification A127-44T, 1944 Book for Metals, pp.

1437-1443.

If any subsequent magnetization is then due to asymmetrically alternating current (having no d.c. com-ponent), the specimen will be in a symmetrically cy-clically magnetized (SCM) condition, wherein the meanvalues of both induction and magnetizing force are zero.For either polarity of magnetization, the peak values ofeach of these parameters will be equal, and are desig-nated as their normal values.

In addition to hysteresis, it is less generally knownthat some materials exhibit a definite magnetization lag.If, having acquired in them a stabilized variation in Band H, a change is made in the amplitude of the cyclicvariation of H, some time (representing a considerablenumber of cycles) may elapse before a completely sta-bilized variation and a new maximum value of inductionis attained. This magnetization lag appears to be morepronounced for a given increase than for a correspondingdecrease in magnetizing force, doubtless due to the ef-fect of retentivity.When measuring a specimen at different values of

cyclic H, it would thus appear desirable to start with themaximum contemplated value and successively reducethis parameter. For incremental measurements, how-ever, the biasing component ofH must be increased pro-gressively from an initially demagnetized condition toavoid any retentivity in the biasing H.

It should be remembered that the magnetic proper-ties of some materials may, to a certain degree, be modi-fied by mechanical strains in punching operations onflat laminations, or in the rolling of flat stock into to-roidal cores. Subsequent metallurgical treatment maythen be necessary to restore the natural magnetic con-dition of the material. The author is of the opinion,however, that a limited amount of easy and carefulshearing may not disturb the specimen as much as issometimes anticipated (see Appendix E).

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Documents

Developments Radio Sky-Wave Propagation Research … · Developments in RadioSky-Wave Propagation Research and Applications During the War* J. H. DELLINGERt, FELLOW, I.R.E., AND …