IT. s. department of commerce

NATIONAL BUREAU OF STANDARDS

WASHINGTON

JHDiANK1-6

LetterCircularLC615

( Super se clc-s

LC317)

D ISTANpE_RANGES OF RADIO WAVES

October 25 ,19^0*

U. S. DEPARTMENT OF COMMERCENATIONAL BUREAU OF STANDARDS

Wash ins:ton LC615( Supersedes

LC317)

CircularLetter

October 25 , 19^0

DISTANCE RANGES OF RADIO WAVES

The distances over which practical radio transmission is pos-sible are very different at different times of day, seasons, etc.,and for different frequencies of the radio waves. This LetterCircular presents the results of experience in radio communica-tion, by .means of the four graph sheets attached.

Radio wave transmission takes place principally by the propa-gation of a "ground wave" along the ground and a "sky wave" re-flected from che ionosphere. The ionosphere is the electricallyconducting (ionized) region in the upper atmosphere, more than 30miles above the earth's surface. As the radio waves travel outalong the ground or in the atmosphere, their energy is reducedbelow what it would be if no causes of energy absorption existed.The absorption is due to the electrical resistance of materials inthe earth and to ionized particles in the atmosphere. The amountof the absorption determines the maximum distances at which wavesof various frequencies can be received, for given reception condi-tions at the receiver.

The distance range of the ground wave is In general great atlow frequencies (below about pOO kilocycles per second)

,and. de-

creases as the frequency is increased, because the ground-waveabsorption increases with frequency. The distance range of theground wave is different for earth of different conductivitiesand dielectric constants, but is fairly constant with time overa given transmission path at a given frequency.

The distance range of the sky wave is not constant with time,frequency, or path. As the graphs show, it is a minimum in ap-proximately the broadcast band of frequencies (5^0 to loOO kc)

,

increasing with change of frequency in either direction. Inthe daytime the absorption of the sky wave Is so great thatthere is almost no sky wave at frequencies from somewhat belowto somewhat above the broadcast frequency band, particularly inthe summer. Hence sky-wave propagation in the daytime (particu-larly in the summer)

,is only appreciable in the lower and higher

frequency ranges. During the night, however, sky-wave propagationtakes place, throughout the entire range of frequencies.

( over)

LG 615 — 10/25/40

The large variations of sky-wave propagation result fromconditions and changes in the ionization of the ionosphere. Be-sides daily variation of daylight and darkness, factors such aslatitude, season, magnetic storms, and solar disturbances, havebeen found to have effects upon this ionization. These changesin ionization result in variations in the distance range of radiowaves from hour to hour, day to day, season to season, and yearto year.

While the distance ranges of ground waves are calculable,there are no generally applicable formulas for sky waves. Thuswe can not determine .sky-wave, distance ranges by any process ofcalculation but must use the accumulated results of experience.The attached graphs summarize experience and give average distanceranges as determined by numerous experimenters. There are consid-erable variations from the average for particular paths and times;the widths of • the shaded boundaries on the graphs indicate roughlythe variations found in common practice.

Detailed Information about sky waves and 'the ionosphere isgiven in another Letter Circular of the Bureau, "Radio transmissionand the ionosphere-

.

11.

Above a certain frequency (which this year is about 4000 kc/sat night and higher in the daytime; see attached graphs), there isfor each frequency a distance within which none of the regular skywave is reflected back to the earth by the ionosphere. There is azone, with an inner and outer boundary, in which there is no regu-lar radio reception, This is called the skip zone and its outerboundary is called the skip distance.

Thus, in the right-hand portion of each of the attachedgraphs, for a specified frequency (e.g.

,20 Mc/s, Fig. 3 ) the

waves are receivable at distances from 0 up to the ground-waverange (different for land and ocean), are not receivable fromthere up to the distance given by the line marked "skip distance 11

,

and arc receivable from there up to the "distance range 11 line.

In Figs. 1 and 2, part of the right-hand portion is cross-hatched and marked "irregular Sporadic". This means that at thedistances and frequencies indicated there is sporadic radio trans-mission at irregular times, even though in the skip zone. Thetimes at which such transmission occurs are not predictable; itis most prevalent May to August, and occurs particularly in thelate afternoon, the evening, and the forenoon, but may occur atany time of day or night. It is due to reflection from peculiarlyionized patches In the E layer of the ionosphere, and not the regu-lar reflection (from the extended layers of the ionosphere) whichaccounts for the regular transmission. Scattered reflections from

3lc 6ir — 10/25/40

the ionosphere, which ore fluttering ond blurred and usually weak,are frequently receivable in the skip zone.

The scales of abscissas and ordinates on the attached graphsare cubical (i.e., numbers shown are proportional to cube of dis-tance along scale, or, distance along scale is proportional tocube root of numbers). This was chosen because it spaces thedata

.

satisfactorily, A linear scale would crowd the low valuestoo much and a logarithmic scale would cro ,i7d the high values toomuch.

The attached graphs show the limits of distance over whichpractical radio-telegraph communication is possible. They arebased on the lowest field intensity which permits practical re-ception in the presence of average background interference ornoise* For the broadcast freciuencics this does not mean satis-factory program, reception. The limiting field intensity is dif-ferent at different frequencies and times. The following tablegives limiting field intensity values typical of those used' indetermining the distance ranges, based on data in a number ofpapers listed in References at end hereof. This assumes the useof a good receiving set.

0,1 Me 1.0 He o\0A 10.0 Me

Summer day 60 uv/m 10 uv/m 10 uv/m 3 pv/mSummer night 100 50 15 1Winter day 2^ 1 2 1Winter night 35 5 1 1

When atmospherics (

11 static") or other sources of interferenceare great, e.g., in the tropics, larger received field intensitiesare reouired and the distance ranges are less. The graphs assumethe use of one kilowatt radiated power, and non-directional an-tennas. For greater power the distance ranges will be somewhatgreater. For transmission over a given path, received intensityis proportional to the square root of radiated power, but thereis no simple relation between distance range and either radiatedpower or received' field intensity.

The day graphs arc based on noon conditions and the nightgraphs a.re based on midnight conditions. In a general way,there is progressive change from one to the other, but with sometendency for day conditions to persist through dusk, and- nightconditions to persist through dawn. The conditions of spring andautumn are intermediate between those of summer and winter,autumn resembling winter somewhat more than summer. Information

LC 615 ~~ 10 /25/40 4.

is given for each month and for all times of day in the summariesregularly published by the Bureau in the Proceedings of theInstitute of Radio Engineers and Q3T.

The attached graphs are based principally upon data for thelatitude of Washington, but serve as a guide for transmissionanywhere in the temperate zones « They are not as accurate forpolar or equatorial latitudes.

In general, the distance ranges for paths which lie partlyin day and. partly in night portions of the globe are intermediatebetween those shown in the day and night graphs, for the rangeof frequencies which can be used both day and night. For pathswhich cross the sunset line in summer, the usable frequencieswill be about the same as the usable summer day frequencies.For paths across the sunset line in winter, the usable frequencieswill be a little higher than the night frequencies shown in graphsFor transmissions across the sunrise line, both summer and winterthe usable freouencies will be a little lower than the nightfrequencies shown in graphs. Frequently the conditions of theionosphere on the light and dark sides of sunrise are widelydifferent. Under such conditions it is often so difficult totransmit across the sunrise line that it is almost a barrierto high-frequency radio communication.

The attached graphs give distance ranges for the currentyear only. They change from year to year because of changes ofionization in the ionosphere. These changes arc caused by thechanging ultraviolet radiation from the sun in an approximateeleven-year cycle. The graphs will therefore be revised eachyear.

The distance ranges given in the graphs are the distancesfor good intelligible reception; they are not the limits of dis-tance at which interference can be caused. A. field intensitysufficient to cause- troublesome interference may be produced ata much greater distance than the maximum distance of reliablereception

.

Ref r - rene e s

A few selected references, for further information, are-

given here. See also the Bureau's Letter Circular, referredto above, "Radio transmission and the ionosphere."

Radio extension of the telephone system to ships at sea.H. W. Nichols and L. Espenschied. Froc.I.R.E. 11 , 193 (1923)

c6i5 — 10/25/40 5 .

Report on measurements made on >slgnal strength atdistances during 1922 and 1923 by an expedition sentAustralia. H. J. Round, T. L. Eckersley, K. TremellF. C. Lunnon. J.I.E.E. £5 , 933 ( 1925 ),

greatto

en and

Transatlantic radio telephone transmission, Lloyd Espen-schied, C.N. Anderson, and Austin Bailey, Proc.I.R.E. 14, 7 ,

(1926). • •

Some measurements of short wave transmission.J.C.Schelleng, and 0 . G. Southmorth. Proc.I.R.E.( 1926 ).

R, A. He i sing,14

, 613

Short ^ rave commercial long distanceHa 1 1bo rg , L. A. Briggs, and C .

’.V . Han sell,(1927).'

communication. H.E.Proc.I.R.E. 15, 467

The diurnal and seasonal performance of high freouencyradio transmission over various long distance circuits.M. L. Prescott . Proc.I.R.E. IS, 1797 (1930).

Field strength measurements of short wave .transmission.T. L. Eckersley. The Marconi Review, p.l of May-June (1932).

The propagation of short waves over the North Atlantic.C. R. Burrows. Proc.I.R.E, 10, 1634 ( 1931 ).

High frequency atmospheric noise. R. K. Potter. Proc.I.R.E. IQ, I73 I (1931).

Contribution a 1' etude de-s lots de propagation des ondesHertziennes dans la gamme de 150 k 1500 kc/s. ('Contributionto study of laws of Hertzian wave propagation from 150 to1500 kilocycles). International Broadcasting Union.Document No. 196 of the 1932 meeting of the C.C.I.R. atCopenhagen.

Some studies of radio transmission over long paths, madeon the Byrd Antarctic Expedition. L, V. Berhner. Bureauof Standards Journal of Research g, 265 (1932) J

ResearchPaper 412.

An estimate of the frequency distribution of atmosphericnoise. R. K, Potter. Proc.I.R.E. 20, 1512 (1932).

Radio field intensity measurements at frequencies . from 265to 5400 kilocycles per second.. S. 3. Kirby and K. A. Norton.Bureau of Standards Journal of Research 6

, 463 (1932); Re-search Paper 429.

LG 615 — 10/25/40 6

North Atlantic ship-chore radiotelephone transmissionduring 1930 nnd 193^» 0. N. Anderson. Proc.I.R.E. 21,si (1933).

'Short-wave transmission to South America. G. R. Burrowsand E. J. Howard. Proc.I.R.E. 21, 102 (1933).

North Atlantic ship-shore radiotelephone transmissionduring: 1932-1933. C. N. Anderson. Proc.I.R.E. 22, 1215( 193 *0 .

An analysis of continuous records of field intensity atbroadcast frequencies. K. A. Norton, S. S. Kirby, andG. H. Lrster. J. Research N3S 11, 297 (1934); ResearchPaper 752 . Proc.I.R.E. 2_5, IIS 3 (1935).

• Terrestrial magnetism and its relation to worldwideshort-^ave communications. H. E. Hallborg. Proc. I.R.E.24, 455 ( 1356 )

.

The propagation of radio waves' over the surface of theearth and in the upper atmosphere. K. A. Norton. Froc.I.R.E. 24, 1367 ( 1936 ).

Characteristics of the ionosphere and their applicationto radio transmission. T. R. Gilliland, S. S. Kirby,N, Smith, and S. E. Reymer. J. Research NIBS 12, 645(1937); Research Parer 1001. Proc.I.R.E. 25, Ҥ23 ( 1937 ).

The ionosphere and radio transmission, . NationalBureau of Standards. Published each month in Proc.I.R.E.,starting with the Sept. 1937 issue.

Trends of characteristics of the ionosphere for half asunspot cycle. N. Smith, T. R. Gilliland, and S. S. Kirby.J. Research NBS 21, 235-245 (1932) (RPII 59 ).

Report of Committee on Radio Wave Propagation. Proc.I.R.E. 26, 1193 (1932).

Application of graphs of maximum usable frequency tocommunication problems. N. Smith, S. S. Kirby, and T. R.Gilliland, J. Research NBS 22!, 21 (1939) ResearchPaper 11 67 .

The role ofJ.H. Dellinger.

the ionosphere in radio wave propagation.AIEE Trans. 52, 203-222 (1939).

LC615 — 10/25 Ac (

Predictions of useful distances for amateur radio com-munication, . National Bureau of Standards. Pub-lished quarterly in QST, starting with the Sept. 19^0issue o

Attached

i

Fig. 1, Summer 10 4-1, Day, (l kw)

.

Fig. 2, 11 11 Night (1 kw)

,

Fig. S. Winter 1 940-4-1, Day, (l kw) *

Fig. h-„11 11 11 Night, (1 kw) *

o

MILES

MEGACYCLES0.01 004 01 0.2 0.4 0.6 I 2. 3. 4. 6. 8 10 12 16 20 24 30

MEGACYCLES

194i DAY

Fi9 t

SUMMER

KiLCMETFRS

MILES

MEGACYCLES001 004 01 0.2 0.4 0.6 I 2. 3 4. 6. 8 10 12 16 20 24 30

1941Fig. 2

SUMMER NIGHT

KILOMETERS

MILES

MEGACYCLES001 004 01 02 0.4 06 I 2. 3 4 6 8. 10 IP. 16 20 24 30 36.

MEGACYCLES

1940 “ 4 !

Fig. 3

WINTER DAY

KILOMETERS

-

MILES

MEGACYCLES0.01 004 01 02 0.4 06 I. 2. 3 4 6 8 10 12 16. 20 24 30

WINTER 1940 - 4 ! NIGHT

1L0METERS