SOME CONSIDERATIONS ON THE MEASUREMENT OF BEARING

OF THE INCOMING SHORT WAVES (I)

By

Isao KASUYA

ABSTRACT

In this paper, it is discussed that the lateral deviation and its varyjng range of the short waves are closely related with the condition of ionospheric 1propagation, especially the maximum usable frequencies for th巴 circuitand tilting of the ionosphere from the result of the measurement of arrival bearings of the standard radio wave (JJY, 4 Mc/s), observed at several domestic monitoring stations during Feb. 1-Feb. 7. 1954.

1. Introduction

It has been pointed out that the lateral deviation of the arrival direction of

the sky radio wave is mostly due to the change of the ionospheric or geomagnetic condition. <1 )(2）均的 Their experimental investigations have, however, been scarcely

made because of lacking in accuracy of the direction finding apparatus or incom-pleteness of the measuring techniques. Since the cathode ray tube type direction

finders which have been remarkably improved in accuracy, have recently been provided at several radio wave monitoring stations in Japan, we can have opport-

unities of making the experimental survey by means of these apparatus. In this paper, some corelations between the arrival direction of radio waves and the iono-spheric condition are mainly considered under the following measuring method.

2. Plan of Measurement

( 1 ) Period of measurement :

Feb. 1-Feb. 7, 1954 ...・.1stterm,

Mar. 18-Mar. 24, 1954 ..一2ndterm. ( 2 ) Stations making measurements :

一一一一 一一..一 一一

Station Location Distance Tru巴 DirectionAngles From North Direction

Kushiro 43°01/N (2nd Monitor. Station of Hokkaido) 144°17/E 920km 208.3°

Sendai 38°281N 伶llonitor.. Station of Tohoku) 141°15'E 340km 207.3°

Iwaoka 34°471N 伶llonitor.Station of Kinki) 134°561E 420km 76.0°

Miyakonojyo 31°41明 880km (2nd Monitor. Station of Kyushu) 131°03'E 58.3°

29

30 I. Kasuya

Fig. 1. The Location of the Radio Wave Monitoring Stations.

( 3) Apparatus used : Cathode ray tube type direction finder;

Aerial system ...... U-Adcock type antenna.

( 4) Objects to be measured :

a. Measur Standard radio wave (JJY, 4 Mc/s Ao Wave) transmitted from Koganei, Tokyo (35°421N, 139°311£).

b. Elements : Bearings (degrees) and rang巴sof its variation (degrees); Periods of the

variation (times/sec). ( 5 ) Measuring time :

For 2 minutes beginning at ev巴ryhour.

3. Results and Considerations

During the 1st measuring term, the radio wave propagation condition, i.e., the ionosph巴ricor geomagnetic condition, was calm and normal. We may, there-

fore, consider the normal arrival bearings. The 2nd term belonged to the some”

what disturbed period, but the problem of this term is not yet touched upon in this paper.

( 1) Relations between the varying range of bearing and MUF.

At first, the mean values of arrival bearings with their varying ranges mea-

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sured at every station during the former period are shown in Fig. 2, in order to find the general aspects of the measuring results from which we 'can immediately learn that the wave arrived from nearly true directions with small variation with-in about土5°,at the stations of Kushiro and Mfyakonojyo located at longer dis-tances from the transmitting station, but the variations of bearings were of greater value at the nearer stations, i.e., at Sendai they are about±l0° in daytime and more than土35° at night, while at Iwaoka about±5° in daytime and土 15°at night.

In order to examine this effect, we compare the bearing data at each station with E, F-MUF curves derived from the ionospheric data in Japan on the same days. Erom some examples as shown in Fig. 3, it is possible to understand that at any time of the condition of 4Mc/s>E, F-MUF, orρE,F三 4Mc/s/E,F-MUF>l, the varying ranges were greatly increasing. In the case of PE,r->l, for example, before sunrise or after sunset, the wave used to propagate abnormally and arrived variably from di任er官 itdirections in the form of scattering waves, for the receiving station is placed within the skip zone. Sometimes it seems that the waves would arrive from just the opposite direction of the wave source, as experienced in the

Fig. 2.

1. i<asuya

backscattering phenonena, so that direction finding may be made impossible. However, at Kushiro and Miyakonojyo, as mentioned above,・ the varying ranges would scarcely be observed, for the stations are located at longer distances from the transmitter, and MUF for the circuit incr巴ases,or E, F-MUF>4 Mc/s, or ρE,Fく1, all the daytime, especially during the day when the waves propagate mainly through E layer. If Es ionizations were observed, it may also be said that the varying rang巴sof bearings were small, as the result of increasing MUF (as-suming that the secant law is also satisfied with the propagation by Es, too). In addition, the mean valu巴sof the bearings show approximately the true bearings, even when their variable ranges increased at the three other stations than Sendai during the period.

( 2 ) Relations betwe巴nthe measured bearings and the tilted equiモlectrondensity surface in the ionosphere.

It is a notable phenomenon that the bearings observed at Sendai would deviate from the true direction to a certain degree at twilight as shown in Fig. 6. i.e.,

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Some Considerations on the Measurem仰 tof Bearing of the lnco附 ingShort Waves 33

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being in lower degrees in the morning and in higher clegr巴巴sin the evening than the true direction. Now, let us illustrate a simple model such as shown in Fig. 4 to explain exaggeratively these phenomena, the figure representing that the equi-

electron density surface in the ionosphere inclines to the ground at twilight and the waves being refracted toward the highly ionized side. In the case of the cir-cuit r巴ctangularto the inclined line of reflecting layer, the following relation is formed:

1 I 2h' ¥ o == tan-Id sine)

where o: lateral deviation; h' : apparent height at the reflecting point, or apparent distance from

the ground to the refl巴ctingpoint; d: distance between the transmitter and the receiver ; e : inclination angle of reflecting layer to the ground.

Although the complicated expression would be obtained in the case of the direction of the circuit making any angle to the inclined reflecting layer, we may suppose that o tends to zero as they run parallecl with each other. From the above expression, we can learn that the lateral deviation will be large in value, when the apparent height and the inclination of reflecting layer have large values and the distance covered is short. Furthermore it can be expected that o will be maximum, when the direction of the circuit is rectangular to the inclined lime. The ionos-phere is going down toward the east in the morning and toward the west in the evenmg.

Such tendency was well supported by the result of the experiment. The effect

34 λKasuya

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of the lateral deviation at twilight appeared remarkably in the Koganei-Sendai circuit satisfying the condition, that is the north-south circuit in a short distance. Several examples as shown in Figs. 6 to 8 observed at Sendai on Feb. 1, 2 and 3 suggested the fact. In these figtぜes,h’F (4 Mc/s-340 km) and h' E (4 Mc/s-340 km) were the apparent heights of F and E regions, respectively, which were derived from the h’デ ionosphericrecords observed at Akita (39°43.5'N, 140°08.2'E) and Kokubunji (35°42.41N, 139°29.31£) Radio Wave Observatories on the same days. If

it happens to have both “Fernstralung”or high ray and “Nahstralung”or low

ray, the apparent height of the latter which would be less absorbed in the iono・sphere is taken as h' F, E (4 Mc/s-340 km). The values of o were changing in ac-cordance with h' F (4 Mc/s-340 km) and h' E (4 Mc/s-340 Jan), and were small in the daytime when the wave propagated mainly by reflection on the E region, but they suddenly became large when the mode of propagation changed into that of F layer

Some Considerations on the Measurement of Bearing of the Incoming S/lorl Woves 35

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with large apparent heights. The good corr巴spondanceof o to h’proves this fact, especially on the evening of Feb. 1.

As the values of o, h' and d are known from the data, the values of inclina-tions at the reflecting point of the layer can be approximately estimated, but the circuit is not directed exactly to the northsouth direction. And they are around _50～－6° in the morning and + 7°～十12°in the evening for the F「layer(BP) and around±0-2.5° in the daytime for the E-layer (IJ;1) (minus means that the layer inclines to the east and plus to the west). The phenomenon that the deviated radio wave returned rapidly to the normal path in the evening both on the 2nd and 3rd of February might be due to the e任ectof the appearance of Es ionization at the reflecting point.

It a任ordsproof of the fact that /Es were rather greater in vqlue in the even-ings both on the 2nd and 3rd of February, while they were smaller in value (less than 2 Mc/s) or their abs巴nceas observed in the evening on the 1st of February both at Kokubunji and Akita.

It is interesting to remark that the time when the sudden increasing of about 20 d. b. in the signal strength of the same wave was observed at Akita at 19 hours of the 1st and 18 hours of the 2nd, February, coincided with the time of sudden increase of o at Sendai as shown in Figs. 6, 7 and 9. The author has not yet

tried to analyse this e任巴ct,but it can be supposed that some sudden changes in inclination of the ionosphere might occur and a任ectthe signal str・engthand the deviation of direction.

I. Kasuya 36

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Some Consideralio押son the Measurement of Bearing of the Incoming Short Waves 39

Feb. I, 1954

． ．－・・・・・・・・・．．．Feb. 2, 1954

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4. Conclusion

According to the above study, the deta from the measurement of arrival bear-ings will not only develop the possibilities of studying the inclination, inegularities, fine structure, etc., of the ionosphere, but also contribute e任ectivelyto the increase of accuracy in the radio monitoring s巴rvice.

Taking the above considerations into account the following should be applied: 1. When the receiving station is placed within the skip zone, the direction finding service will be beset with di伍cultiesas the result of the scattering waves being received. Moreover, the varing ranges have fairly positive corcelation with ρ三f/MUF.2. With the increasing commumication distance in one hop ionospheric propaga-tion, the lateral deviations as well as th巴irvariation will decrease in value, as MUF for the circuit increases. 3. On the sky wave propagation for the north-south circuit of a short distance, the twilight effect of the lateral deviation was observed, as the inclination of re-flecting layer and remarkable change in height were observed at twilight in accord-ance with the formation and disappearance of the ionosphere.

The experiments should undergo further development in a the following : 1. Measm百 nentof the waves on other frequenci巴s; 2. Measmement in the case of the long distince circuit with over 2-hop ionospheric propagation-the case of vari-ous reflecting conditions of the ground; 3. Measurement of the waves passing

40 I. Kosuya

through the polar zone, or measur巴mentin the case of disturbance period of ionos-phere or geomagnetism; 4. Investigation on the relation with polarization of the radio waves.

In conclusion, the author wishes to express his sincere thanks to Mr. R. Shida of Radio Regulatory Bureau and Mr. T. Yoshikawa of this Radio Research Labora-tories for their assistance, and to the members of the various monitor担gstations engaged in the laborious measurements, especially to Dr. H. Uyeda for his warm encouragement and discussion given to the author. His cordial thanks are also due

to Mr. K. Sawada who helped much to prepare the illustrations.

References

( 1 ) R. Keen, Wireless Direction Finding (1938) 229. ( 2) T. L. Eckersley, Radio Rev. 2 (1921) 60. ( 3) R. A. Wattson Watt, Jour. I. E. E. 78 (1936) 10. ( 4) R. A. Wattson Watt, Wireless Eng. 13 (1936) 3.