the intermediate frequencies, 15- and 11-megacyclebands, for transitionary periods of time; i.e., when thetransmission path is partly in darkness and partly indaylight. Because several variable factors greatly in-fluence the propagation characteristics of short waves,it is necessary to use the frequency best suited for anexisting condition of transmission and upon the properchoice of frequency depends to a large degree thesuccess or failure of a short-wave broadcast or relay.The frequencies selected for daily operation, i.e.,

the station operating schedule, are determined byexhaustive study of (1) the United States NationalBureau of Standards radio wave propagation data,10 (2)field-intensity-measurement data, (3) professional re-ception reports such as those compiled by the BritishBroadcasting Corporation receiving station at Tats-field, England, (4) frequency measurements made bythe Union Internationale de Radiodiffusion ControlCenter, formerly located at Brussels and more recentlylocated at Berne, Switzerland, (5) reports from CBSrepresentatives abroad, and (6) correspondence fromshort-wave-station listeners.The Union Internationale de Radiodiffusion Control

Center frequency measurements are very useful forpredicting sources of interference from other short-wave stations. They indicate, quite accurately, thenumber and identity of stations operating in each ofthe frequency bands. The 6- and 9-megacycle bandsare exceedingly crowded at present resulting in con-siderable chaos and interference. It is hoped that withthe conclusion of present unsettled conditions, worldradio conferences will again convene, and this situa-tion be improved. The recent inter-American radioconference held at Santiago, Chile, made noteworthyprogress in this direction.

10 T. R. Gilliland, S. S. Kirby, N. Smith, and S. E. Reymer,"Characteristics of the ionosphere at Washington, D. C.," monthlyreports published in the PROC. I.R.E., vols. 25-29; 1937-1941.

INTERNATIONAL RECEIVING STATIONIt is necessary that the receiving-station facilities,

the signals from which are used for rebroadcasting, becapable of performance equal to those of the trans-mitting station. Either space- or phase-diversity uni-directional antenna systems should be employed, andthe entire receiving-station facilities properly en-gineered. A great deal of information has been publishedon this subject and will not be detailed here.""2"3

CONCLUSIONThe present and proposed service by international

broadcast stations of North America, including expan-sion of existing facilities and construction of new sta-tions, will undoubtedly accelerate interest in this serv-ice."4 Transmissions from the United States to LatinAmerican countries will soon be equal to or betterthan those now received from other countries. Thenew WCBX and WCRC transmitting stations willincrease the intensity of CBS signals to Latin Americaand Europe, based on a conservative estimate, by atleast 20 decibels. This is equivalent to a hundredfoldincrease in the power of the existing facilities.

ACKNOWLEDGMENTSincere thanks and appreciation are extended to Mr.

Andrew Alford and associates of the Mackay Radioand Telegraph Company, and to my associates in theCBS General Engineering Department, for their co-operation and assistance in obtaining much of thematerial presented.

1 A. A. Oswald, "The Manahawkin musa," Bell Lab. Rec., vol.8, pp. 130-134; January, 1940.

12 J. B. Moore, "Recent developments in diversity receivingequipment," RCA Rev., vol. 2, pp. 94-116; July, 1937.

13 H. T. Friis and C. B. Feldman, "A multiple unit steerableantenna for short-wave reception," PROC. I.R.E., vol. 25, pp. 814-917; July, 1937; Bell Sys. Tech. Jour., vol. 16, pp. 337-419; July,1937.

14 Raymond F. Guy, "NBC's international broadcasting sys-tem," RCA Rev., vol. 6, pp. 12-35; July, 1941.

The Velocity of Radio Waves Over Short Paths*R. C. COLWELLt, MEMBER, I.R.E., H. ATWOODt, ASSOCIATE, I.R.E.,

J. E. BAILEYt, STUDENT, I.R.E., AND C. 0. MARSHt, ASSOCIATE, I.R.E.

Summary-The velocity of radio waves was measured directly inthe following manner. Two radio stations were set up on frequencies of3492.5 and 2398 kilocycles, respectively. One station was fixed whilethe other was portable. The fixed station sent out pulses which were re-ceived at the portable station. A thyratron control set off return pulseswhich came back to the base station. At the base station the two pulsesappeared upon a cathode-ray oscilloscope with a sweep of 22,800 inchesper second. The separation of these pulses gave the time for the pulsesto travel twice the distance between the stations plus the time requiredto pass through the receiving apparatus. By taking the portable stationto two positions one 0.73 kilometerfrom the base and the other 3.67 kilo-meters, it was possible to eliminate the time lag in the receiver and so tofind the exact time of propagation. Each station was in sight of theother. The average of 180 measurements was 2.985 X 101" centimetersper second.

* Decimal classification: R111.1. Original manuscript receivedby the Institute, July 17, 1941.

t West Virginia University, Morgantown, West Virginia.

Tp HE VELOCITY of radio waves in air is alwaysassumed to be the same as the velocity of light.Exact measurements over long distances are sub-

ject to the uncertainty that the real path of the wavescannot be determined. If, however, the path of the waveremains in the line of sight, it is reasonable to assumethat it will follow the straight line connecting the twopoints. The distance between the two points can bemeasured but the time interval becomes so small thatordinary measuring devices cannot be used. However,very short intervals of time (1 microsecond) may bemeasured upon a cathode-ray oscilloscope with a fastsweep.

Proceedings of the I.R.E.March, 1942 129

Proceedings of the I.R.E.

In the actual experiment two radio stations were setup on frequencies of 3492.5 and 2398 kilocycles, re-spectively. The fixed station sent out 60 pulses per sec-ond, each pulse lasting for 8 microseconds. Thesepulses were received at certain distances on a portablereceiver so arranged that a thyratron control set offreturn pulses from a transmitter in the portable sta-tion. Thus the portable station was made to send outpulses in exact synchronism with the base transmitter.This relayed pulse was received at the base stationupon the screen of the oscilloscope. The original pulseappeared upon the same screen so that the distance be-tween the two pulses, as seen upon the screen, gave thetime taken for the radio waves to travel twice the dis-tance between the stations as well as the time consumedin passing through the equipment. For example, if T1represents the time for the pulse to make the roundtrip over base line b1, and T2 is the time to make theround trip over base line b2, then

T1 = Tb, + T.T2= Tb2 + T.

where T. is the time for the pulse to pass through theequipment and Tb,, Tb2 are the times for the doubletransit of the two base lines b1 and ba. From these val-ues, the velocity becomes

(2)

in which D2 = 2b2 and D1 = 2bi.The alternating-current supply for the base-station

pulser comes from a phase-shifting mechanism whichplaces the pulse upon any portion of the oscilloscopesweep. In this way both the initial and received pulsemay be placed side by side on the screen of the oscil-loscope. The base-station receiver is of the video re-sponse type; it will pass a band of frequencies 2megacycles wide without attenuation. Such a fre-quency response is necessary in order to preserve thewaveform of very short pulses.The portable station was built into a panel delivery

truck which served as the operating room. A power ex-tension cord could be plugged into a receptacle in theside of the truck and any house outlet. A ground rodwas always used to provide a good electrical connectionto the earth. The portable receiver is also of the videotype with television pentode amplifier tubes and avacuum-tube peak voltmeter to measure the outputvoltage of the receiver. Voltage-regulator tubes are em-ployed to eliminate the effects of line-voltage fluctua-tions.The voltage necessary to fire the thyratron must be

measured accurately each time an observation is madebecause the corresponding voltage values along thewave fronts will depend upon the amplification levels.The voltage peak will appear earlier on the wave frontwhich has the higher value. For every voltage value on

one base line, there is a corresponding voltage value forany other base line. Only by the use of correspondingvalues of voltages can accurate results be obtained.The observations consisted of the measurement of

the separation of the direct and relayed pulses on theoscilloscope for various values of voltage injected tothe thyratron. This was done over several base lines ofdifferent lengths. The velocity of the sweep on the os-cilloscope was determined by putting a signal of knownfrequency on the vertical deflecting plates while thesweep voltage was on the horizontal plates. The pat-tern on the oscilloscope screen gives the number ofcycles of known frequency per centimeter and thisgives the velocity of the sweep. The velocity was found

TABLE I

Base Line bt =0.73 kilometer

Volts Separation of Pulses in Centimeters

5 4.9 5.0 4.9 4.8 4.8 4.9 5.0 4.8 4.8 4.87 4.6 4.7 4.7 4.6 4.6 4.6 4.7 4.6 4.6 4.69 4.4 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.4 4.4

11 4.3 4.3 4.4 4.3 4.4 4.3 4.4 4.4 4.3 4.313 4.2 4.2 4.2 4.2 4.3 4.2 4.2 4.2 4.2 4.215 4.0 4.0 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.117 3.8 3.9 3.9 3.9 3.9 4.0 4.0 3.8 3.9 3.9

Base Line b2=3.67 kilometers

5 7.5 7.3 7.6 7.5 7.0 6.8 6.7 6.8 7.2 8.27 6.6 6.5 6.6 6.1 5.8 6.1 5.7 5.9 6.0 6.39 5.8 5.7 5.7 5.6 5.6 5.6 5.5 5.6 5.6 5.6

11 5.6 5.5 5.4 5.4 5.5 5.5 5.5 5.4 5.5 5.513 5.4 5.4 5.4 5.4 5.3 5.3 5.3 5.3 5.4 5.415 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.3 5.217 5.0 4.9 5.0 5.0 5.0 5.0 5.1 5.0 5.0 5.0

to be 22,800 inches per second or 1 centimeter equiva-lent to 17.25 microseconds. The latter number multi-plied by the separation on the screen will give the valuefor Ti on the base line b1. The value T2 is obtained byusing the separation of the pulses for the identical volt-age value on the second base line b2. Five to seven dif-ferent voltages were used on each base line. The sepa-ration for each voltage was the average of ten trials.The distance between the two pulses is measured byplacing a caliper on the screen of the oscilloscope andopening the jaws until they are coincident with thevertical wave fronts of the two pulses.

In this method, the assumption is made that the Teof (1) is constant over a wide range of signal levels anddistances. This assumption was substantiated by vary-ing the length of the receiving antenna at the portablestation. In this way a wide range of input signalstrengths was obtained. Provided the gain of the re-ceiver was adjusted so that the output voltage re-mained the same, no change in the separation of thepulses occurred.The characteristics of the thyratron tubes used also

influence the results. There is a time delay of severalmicroseconds between the application of the ionizingpotential to the grid and the actual breakdown of thetube. This delay depends upon the type of gas used inthe tube, the bulb temperature, and the increase involtage after the ionizing potential has been reached.

130

D2- Div =

T2 -Tl

Tubes filled with argon were found to be most satisfac-tory for this investigation. The characteristics of twotubes of the same type are not identical and data ob-tained with one tube should not be compared with datafrom another.The entire apparatus contains almost 60 vacuum

tubes and a large change in the characteristics of anyone of them will affect the accuracy of the measure-ment. However, modern tubes retain their characteris-tics for a long time and no trouble has been experiencedin this respect. The general method is not particularly

accurate at present but it is subject to many refine-ments.A typical set of data is given in Table I. From this

table the values for 15 volts give2(3.67 - 0.73) X 105

v =

17.25 X 10-6 (5.21 - 4.07)-2.985 X 1010 centimeters per second.

Eighteen similar sets of measurements were made or180 for each station. The most probable average wasvery close to the velocity of light.

Directional Characteristics of Tropical Storm Static*STEPHAN P. SASHOFFt, MEMBER, I.R.E., AND WILLMAR K. ROBERTSt, STUDENT, I.R.E.

Summary-This paper discusses tabulated data of static re-corded during the hurricane seasons of 1938 and 1939. It shows thatstatic arriving at three recording stations totalized over long periods oftime seems to come from certain well-defined points on the compass andindicates that the directional distribution of static, for the summermonths at least, may be associated with areas very active in producingatmospherics. The paper discusses records obtained on the tropical dis-turbance of August, 1939, which, although mild in intensity, was ofconsiderable interest since its center passed only 100 miles from therecording station at Gainesville, Florida. The results indicate that(a) only certain portions of the storm may be regarded as importantsources of static, (b) the relative position of each static-producing arearemains fairly well fixed with respect to the storm center, and (c) as faras can be determined, no static emanates from the eye of the storm.

INTRODUCTIONA METHOD OF recording the direction of arrival

of atmospherics and of triangulating for the ap-parent position of the source based on data ob-

tained during the summer of 1937 has been describedpreviously.' During the summers of 1938 and 1939similar observations were made on 10 kilocycles, andadditional information was secured on the characteris-tics of the incoming static, particularly on static whichappeared to have its origin in tropical disturbances.While tabulating and statistically analyzing the dataobtained during these periods, two significant factsstood out: First, that the angular distribution of theincoming static totalized over long periods of timeseems to favor certain points on the compass; and sec-ond, that static reasonably identified as coming froma tropical disturbance reaches a peak at points relatedto the reported positions of the storm.

DISTRIBUTION OF STATIC OVER LONG PERIODS OF TIMETabulation of static arriving at each recording sta-* Decimal classification: RI 14. Original manuscript received by

the Institute, May 13, 1941; revised manuscript received, Novem-ber 5, 1941. Presented, U.R.S.I.-I.R.E. Meeting, Washington,D. C., April 26, 1940.

t University of Florida, Gainesville, Fla.1 S. P. Sashoff and Joseph Weil, "Static emanating from six

tropical storms and its use in locating the position of the disturb-ance," PROC. I.R.E., vol. 27,. pp. 696-700; November, 1939.

tion was made by segregating the crashes into 10-degree angles in eighteen groups. Each group wasdesignated by the direction of the center of the groupangle. The crashes falling in each group angle for asingle observation period of three minutes durationwere first added, and then the totals for all periods andall angle groups for the corresponding summer wereobtained.The distribution of static for the three recording sta-

tions at Gainesville and Pensacola, Florida, and RioPiedras, Puerto Rico, from data obtained during thesummer of 1938 is shown on Fig. 1. This figure showsthe static bidirectional. It should be noted, however,that past experience has shown that for the threerecording stations used in making the records prac-tically all of the static originates in the third and fourthquadrants. Keeping this fact in mind, it can be seenthat:

1. The Gainesville station shows two main peaks; oneat 140 degrees and the other at 240 degrees fromthe true north. The minimum points are at 100and 190 degrees.

2. The distribution for the Pensacola station also ex-hibits two maxima at 135 and 225 degrees. Theminima are at 90 and 175 degrees.

3. The Rio Piedras station on the other hand showstwo main peaks without a definite minimum be-tween these but with a small additional peak at230 degrees. There is a wide angle of static-freearea extending from 90 to 155 degrees and from115 to 270 degrees.

4. The distribution of static for all three stationsshows that the directions from which a consider-able number of crashes arrive are invariablypointing to areas of high mean annual days withthunderstorms as indicated on the climatic mapof North America, published by the Blue HillObservatory of Harvard University.

Proceedings of the I,R,E.March, 1942 131