(PDF) Characteristics of the Ionosphere at Washington, D.C., February, 1940, with Predictions for May, 1940 - DOKUMEN.TIPS (2024)

SIMILITUDEThe principle of similitude, whereby a change in the

linear dimensions of a resonator effects a proportionatechange in its resonant wavelength, has been proved inthese experiments to apply to all shapes investigated.The proof we submit is based on the measured curvesof resonant frequencies for the several lowest-ordermodes for the resonator of Fig. 1, and for an exact one-half-scale model of it; in every instance, the resonantfrequency in the model was exactly twice that of theoriginal resonator. An investigation of (7) will in factshow that if all linear dimensions are changed by ascale factor k, the resonant frequency for perfect-coax-ial and perfect-cylindrical resonators is altered by thefactor 1/k, thus

1ft ,m,n= kfl,m,n (12)


where f'j,m,o denotes the resonant frequency of themodel.

It is believed that the principle of similitude holds

also for resonators of any shape and for all modes of os-cillation.

Special cases arise in which the frequency is inde-pendent of all but one of the linear dimensions, where-upon the principle of similitude may be expressed interms of that one dimension only. For example, forthose modes in which there is no variation along theaxial co-ordinate (n = 0), the resonant frequency variesonly with the transverse dimensions a and b. Similarly,for the conventional transmission-line mode in the per-fect-coaxial resonator, the frequency varies only withthe longitudinal dimension L. Other special cases, too,have been found.The principle of similitude permits the results from

measurements or calculations on a model of one size tobe extended to proportionately scaled models of othersizes, and therefore provides an important engineeringtool in resonator design. Thus, the numerical data pre-sented in the curves of this paper possess a general sig.nificance for all resonators whose relative dimensionsare the same as those of our experimental one.

Characteristics of the Ionosphere at Washington, D.C.,February, 1940, with Predictions for May, 1940*


D9vATA on the ordinary-wave critical frequenciesand virtual heights of the ionospheric layersduring February are given in Fig. 1. Fig. 2

gives the monthly average values of the maximumusable frequencies for undisturbed days for radiotransmission by way of the regular layers. The F2and F layers ordinarily determined the maximumusable frequencies during the day and night, respec-tively. Fig. 3 gives the distribution of hourly valuesof F and F2 critical-frequency data about the undis-turbed average for the month. Fig. 4 gives the ex-pected values of the maximum usable frequencies forradio transmission by way of the regular layers, aver-age for undisturbed days, for May, 1940.

Attention is called to the fact that Figs. 1, 2, and 4also implicitly give maximum ionization densities ofthe layers of the ionosphere. The equivalent electrondensity in electrons per cubic centimeter is 0.0124times the square of the critical frequency (or zero-distance maximum usable frequency minus 800 kilo-cycles).

*Decimal classification: R113.61. Original manuscript re-ceived by the Institute, March 11, 1940. These reports have ap-peared monthly in the PROCEEDINGS starting in vol. 25, September,(1937). See also vol. 25, pp. 823-840; July, (1937). Publicationapproved by the Director of the National Bureau of Standards ofthe U. S. Department of Commerce.

t National Bureau of Standards, Washington, D. C.


Minimum ~~Magnetic Io-Day and hF before Minimumor Noon character1 spono-hour surie'sbefrie fF5 sphericE.ST. sunrise

c (ke) 00-12 12 24 achar-(ki.T.sn)ise sunborse G.M.T. G.M.T. acter

February23 (after 1700) - - -_ - - 0.324 300 2600 7400 0.4 0.3 0.825 376 1900 5100 1.0 1.1 1.626 (until 0600) 376 1800 - 0.4 0.3 0.3

For comparison:Average for un-disturbed days 298 3000 9585 0.2 0.3 0.0

1 American magnetic character figure, based on observations of seven ob-servatories.

2 An estimate of the severity of the ionospheric storm at Washington on anarbitrary scale of 0 to 2, the character 2 representing the most severe disturbance.


G.M.T. RelativeDay Locations of intensity Other

Begin- End transmitters at mini- phenomenaning mum1

Feb.7 2000 2020 Ohio, Mass. 0.08 1346 1400 Ohio Cuba 0.1

15 1919 1930 Ohio, England, Cuba 0.0116 1313 1319 Ohio, England, Cuba, 0.0 Ter. mag. pulse,2

Mass. 1313 to 132016 1850 1930 Ohio, Cuba 0.217 1622 1640 Ohio, Cuba 0.117 1835 1910 Ohio, England, Cuba 0.0 Ter. mag. pulse,2

1835 to 185024 1540 1550 Ohio, Cuba 0.0

I Ratio of received field intensity during fade-out to average field intensitybefore and after, for station WLWO, 6060 kilocycles, 650 kilometers distant.

2 As observed on the Cheltenham magnetogram of the United States Coastand Geodetic Survey.

Proceedings of the I.R.E.April, 1940 191

Proceedings of the I.R.E.

Ionospheric storms and sudden ionospheric dis-turbances are listed in Tables I and II, respectively.Strong vertical-incidence sporadic-E reflections wereobserved above 8 megacycles on only 2 hours duringthe month, and above 6 megacycles on only 3 hours.







0 2 4 6 8 10 12 14 16 18 20 22E ST

Fig. 1 Virtual heights and critical frequencies of theionospheric layers, February, 1940.

10 20 30 40 50 60 7PERCENTAGE OF TIME

Fig. 3-Distribution of F- and F2-layer ordinary-wave criticalfrequencies (and also approximately of maximum usable fre-quencies) about monthly average. Abscissas show percent-ages of time for which the ratio of the critical frequency to theundisturbed average exceeded the values given by the ordinates.The solid-line graph is for 325 undisturbed night hours of ob-servation; the dashed graph is for 289 undisturbed day hoursof observation; the dotted graph is for 60 disturbed hours ofobservation listed in Table I.


; 24

U 20zhI

w 16

IIJ ,2w 120

enID 82

x2 42

0 2 4 6 8 10 12 14 16 18 20 22 0LOCAL TIME AT PLACE OF REFLECTION

Fig. 2-Maximum usable frequencies for dependable radio trans-mission via the regular layers, average for undisturbed daysfor February, 1940. For information on use in practical radiotransmission problems, see Letter Circular 575 obtainable fromthe National Bureau of 'Standards, Washington, D. C., onrequest.


0 kmI

0 2 4 6 8 10 12 14 16 18 20 22 0LOCAL TIME AT PLACE OF REFLECTION

Fig. 4-Predicted maximum usable frequencies for dependableradio transmission via the regular layers, average for undis-turbed days, for May, 1940. For information on use in practicalradio transmission problems, see Letter Circular 575 obtainablefrom the National Bureau of Standards, Washington, D. C.,on request.
















40EB. 140


20~ ~~_0_km__r77m-32 _ -[_ m,

28124 _ _ Ao Y- It s 5


41__2 ____ -_-0 X _m


-410009r7-TQ0 kmr




I1 11H.


(PDF) Characteristics of the Ionosphere at Washington, D.C., February, 1940, with Predictions for May, 1940 - DOKUMEN.TIPS (2024)
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