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Vol. Vol. 18 No. 97 four η al of the Research Laboratories May 1971 Printed Printed in Tokyo, Japan pp. 183-190

UDC 621. 812. 391. 63. 029. 52 MEASUREMENT OF THE FIELD INTENSITY OF HF RADIO WAVES BY MEANS OF THE NARROW BAND RECEIVER

By

Noboru WAKAI, Shu FUJII and Yoshiyuki MIYAMOTO

(Received (Received Aug. 28, 1970)

ABSTRACT The narrow band receiver developed for receiving exclusively the JJY signals in in the presence of interference by other standard frequency waves with an identical carrier carrier frequency has been used since 1967 for the observation of HF radio wave absorption absorption in the nighttime on board the ship for the Japanese Antarctic Research Expedition Expedition (戸田町. This This paper describes the technical summary of the receiver, and presents the results results of measurement obtained by the receiver on the 10th (Nov., 1968 ~ Apr., 1969) 1969) JARE. Dependence Dependence of the ionospheric absorption in the nighttime on the solar activity was confirmed quantitatively by the same analysis as in the preceding paper.

1. 1. Introduction

This paper forms substantially the second chapter of the preceding paper [Wakai et al., 1971] in which the results of observation of the nighttime ionospheric absorption absorption made on board ship for the Japanese Antarctic Research Expedition since 1958 1958 are presented. A portable HF 負eld intensity meter relying wholly on manual operation was in use use before 1966. In the second series of observation of the nighttime absorption after after 1967, automatic recording of the upper sideband of JJY signals was done by the the use of newly developed receivers capable of filtering out unwanted interference due to other standard frequency waves. This paper describes the technical summary of the receiver -as well as the results results of measurement obtained on the 10th (Nov., 1968 ~ Apr., 1969) JARE by the use use of the receiver, in order to confirm the dependence of the nighttime absorption on the solar activity by the same analysis as in the preceding paper.

2. 2. HF Narrow Band Receiver for Recording the Standard Frequency JJY

The standard frequency waves in the HF range emitted on the same series of frequency frequency from various countries have several advantages in the practical usage. Nevertheless, Nevertheless, it brings trouble in receiving separately wanted signals, arising from

1邸 184 184 N. Wakai, S. Fujii and Y. Miyamoto

the the mutual interference among the standard frequency waves with an identical carrier carrier frequency. These di 伍culuties were avoided by developing the narrow-band receiver receiver capable of picking up just the sideband of WWV and WWVH standard frequency frequency waves modulated by 440 Hz, for the purpose of the radio propagation warning service at Hiraiso Branch, Radio Research Laboratories [Isozaki et al., 1970]. 1970]. This type of receiver for recording automatically the field intensity of the upper sideband of JJY 2. 5 MHz waves (modulation frequency: 1000 Hz) was con- structed structed and installed on the ice-breaker "Fuji ” for the 9th JARE that left Tokyo in in November, 1967. The measurement of nighttime absorption with the changing distance distance of transmission was reinforced by adding year by year the receivers for JJY 5 MHz in 1968 (10th JARE) and then for JJY 10 MHz in 1969 (11th JARE~. Table 1 shows the summary of observation. The block diagram of receivers

Table Table 1. Outline of measurement of the HF field intensity on board between between Tokyo and Antarctica

Frequencies Frequencies Course v~~~ge Month, Year 豆罰 Observer J .A.R.E. (MHz) To Via

9th 9th Nov.-Dec., 1967 2.5 Tokyo Antarctica Australia 100 K. lsbizawa 10th 10th Nov., 1968-Jan., 1969 2.5, 5 Tokyo Antarctica Australia 110 Y. Ota 10th 10th Feb.-Apr., 1969 2.5, 5 Antarctica Tokyo Indian 108 K. lsbizawa Ocean Ocean R. Saruwatari 11th 11th Nov., 1969-Jan., 1970 5, 2. 5, 10 Tokyo Antarctica Australia 106 I. Shiro T. T. Sakamoto 11th 11th Feb.-May, 1970 2.5, 5, 10 Antarctica Tokyo Indian lOfi Y. Ota Ocean Ocean T. Hirasawa

Fig. Fig. 1. Block diagram of a narrow band receiver for JJY 2. 5 and 5 MHz. HF Radio Wav e Int ensity by Narrow Band R e ceiver 185 fo r JJ Y 2. 5 and 5 MHz is shown in Fig . 1. Frequencies indi cate d outside bo xes in in the figure m ean th e operating frequenc i es in receiving 2. 5 (upper) and 5 (lower) MHz waves (a common operating frequency in the case of non-separation). The receiver is di stinctive in the fact that the output of I. F. A. (4 55 kHz ) is introduced introduced into C. M . F. I. F . A. ( Crystal Mechanical Filter I. F. A. ) w ith a bandwidt h of about ± 100 Hz in order to extract the respective upp er sideband com ponents (2. 501 and 5. 00 1 MHz ) of 2. 5 and 5 MHz waves . For this purpose, the the frequency stability of eac h l oca l osc illa tor (1st, 2nd and 3r d L. 0. ) is required to be mo re than about 1 ×10 -1. This req uir em ent was met by s uppl yi ng th 巴 output of external external osc illator s, sin ce the oscillators contained inside the co mmer cia l receiver proper (NRD -llE 〕 as enci r cled by a dashed line in Fig. 1 are unsatisfac tory in th 巴ir stability. Then the outp ut of C. M. F. I. F . A. is compressed logarithmically in C omp . wit h the dynamic ra nge of about 60 dB an d fina ll y recor ded on the strip chart recorder . The calibration of the receiver is mad 巴 by compari ng the vo ltag e from the the w ith the signa l generator (S.G. ) in an ad 巴qua te int erv al of tim e. Th 巴 in st rum entation of th 巴 receiver for JJ Y 10 MHz is almost th e same as th at for 2. 5 or 5 MHz just mentioned, apart from the remova l of B. P. F . (14.5 ~ 15. 15. 5 MHz) and the 2nd L. 0. (17 .5 MHz ), and from the change of frequency of the the 1st L. 0. ( 13. 0 MHz ).

'• 1 rt r r r r

10 同 iz OCT . 13, 1969

Fig. Fig. 2. E xa m p les of 1eco rd :obta in 巴d by the narrow ba 吋印ceivers (upp er, 10 M H z; lower , 5 MHz ). 1前 N. Wakai, S. Fujii and Y. Miyamoto

Whip antennas 6m long are connected to receivers independently. In order to obtain obtain the absolute field intensity relative to 1 μV/m, measurement by means of the the portable field intensity meter as described in the previous paper [Wakai et al., 1971] 1971] was made once a day. Two examples of the record obtained by the narrow band receiver are shown in in Fig. 2. Letters ], W and B in the figure mean the signals from the transmit- ting ting stations JJY, WWV and BPV, respectively. Other standard fre~uencies such as as ZUO and LOL could also be received during the observational period, although they they are not recorded in the examples. It is necessary to know the schedule of emission emission and modulation on 1 kHz of the standard frequency waves from various countries, countries, in order to identify the transmitting station with the pattern of received signals signals on the record, unless otherwise the direct identification with the . The modulation schedule pertaining directly to the measurement is illustrated in Fig. Fig. 3 [CCIR, 1966]. The upper record in Fig. 2 was obtained at Hiraiso Branch on October 13, 1969. 1969. On this record are seen intense signals with a drop in intensity due to the minute signal during O~ 10 minutes, 20 ~ 25 minutes, 30 seconds from 34 minutes, 40 ~ 50 minutes, and 30 seconds from 59 minutes. They are identified as JJY in reference reference to Fig. 3. Similarly, relatively weak signals received for 1 minute at 12,

ト蜘『 ly modulatlon 。 倒閣ule

45

30

図凶:畑 ti 開国second 仰 l回困問 II sign

Fig. Fig. 3. Emission and modulation schedules of various standard frequ ・ ency ency waves with a modulation frequency of 1 kHz. HF Radio Wave lntensi か by Nanow Band Receiver 187

17, 17, 32, 52 and 59 minutes are identified as WWV. The WWV signals to be record- ed ed at 7, 22 and 42 minutes could not be found owing to the masking by intense ]JY signals. BPV signals not registered formally by CCIR are emitted from Shanghai (China) or vicinity at three frequencies of 5, 10 and 15 MHz. The modulation schedule is also indicated in Fig. 3 as inferred from the survey by the Radio Regulatory Bereau in Japan. The BPV signals can be seen during 12h 15m ~25m and 12h 30m ~40m on the upper record in Fig. 2. It is to be noted that during 20 ~ 25 minutes the BPV signals can be found in preventing the minute signals signals of ]JY dropping down to the noise level. This record revealed that the c.s. (call (call sign) of BPV was modulated by 1 kHz, regardless of non-modulation accord- ing ing to Fig. 3. The lower record in Fig. 2 was obtained when the ship was sailing near the Philippines Philippines (15° N, 130° E) on December 4, 1968. The interference with BPV is seen seen about 1 hour before and after 20h. For the rest of hours, it is clearly reco ・ gnized gnized on record that 刀Y and WWV waves are received regularly. No example of record at 2. 5 MHz is introduced in this paper, since the inter- ference ference is not so serious as in higher HF ranges.

3. 3. Results of Measurement and Analysis

The measurement of field intensity on board the ship for the 9th JARE (1967 ~

60 JJY 5.0MHz ・oufw ロrd (Nov.1968-Jon.1969) homeward • homeward (Feb.-Apr. 1969) 40 .. ・.岬.’・.. . . . 宅20 • JIC1'. x •• --- 〉 • x x ·~ •• 主 .【 x . za - 訟提 。 x x dF-- 二。 - 〉

:§-1 喝 2αJO 4000 6000 8000 10000 12000 14000 .』 宅 E 句 JJY 2.5 MHz 〉、 』 .。 ul 附 rd (Nov.-Dec.1968) .c x homeward (Apr. 1969) :!= :!= 40

τ-- ョ

IL 20 - . 。 ・・-2唱.ー .

ー10 占 2000 4000 6000 8000 IOC おO ロ000 14000 Ground Ground Range in km Fig. Fig. 4. Median field intensity of ]JY standard frequency signals measured measured on the 10th (Nov., 1968 ~ Apr., 1969) voyage against the the ground range (curve in the figure; unabsorbed field intensity). 188 188 N. W akai, S. Fujii and Y. Miyamoto

1968) 1968) was made over the whole course. The data obtained, however, have not quality quality good enough to be analyzed, because :fluctuations in the receiver gain caused by the direct stroke from an adjacent transmitter just after the departure a妊ected the the data for the whole period of observation. The data at the 11th JARE are being scaled scaled at . Thus, this paper gives only the results of measurement at the 10th 10th JARE, and describes the characteristics of the nighttime absorption obtained. In In scaling the field intensity on the recording paper, attention is paid to keep the the uniformity of the value in comparison with the observational data before the 9th 9th JARE. The median (50%) value of fading signals is read out of the record every 30 minutes. They are converted into the absolute field intensity in reference to to the simultaneous reading of the field intensity meter metioned above. The nighttime absorption was derived by the use of the same equations and procedures procedures as given in the previous paper [Wakai et al., 1971]. The correction to the the transmitting power has to be necessarily made, since the 負eld intensity dealt with in this paper is for just the upper sideband received by the naηow band receiver. receiver. This brings about the decrease of unabsorbed field intensity (the modula- tion tion degree of JJY is 8096). Fig. Fig. 4 shows variations in the field intensity measured and the unabsorbed intensity intensity thus obtained against the ground range. It is evident that the rate of

60 JJY 5.0 MHz • outward (N 。v.1968-Jan.1969 l x homeward ( Feb.-Apr. 1969) 40

20

国 て3 L い二戸ど出子 .: .: 0 」 c 0 0.-20.!- g 2000 4000 6000 8000 10000 12000 14000 .0

20 一 ・>-伊三!?

J エユ主___,__・

。ド x x ” X

-20 占 2000 40α コ 6000 8000 I0000 12000 14000 Ground Range. D, in km

Fig. Fig. 5. Variations in the ionosp 】1eric absorption loss for JJY 2. 5 and 5 MHz waves against the ground range. HF Radio Wave Intensity by Narrow Band Receiver 189

Table Table 2. Coe 伍dents a and b in the expression L1(dB)=a+b ・10-a・D(km)

2.5 2.5 MHz 5 MHz 10 MHz 11.8 MHz Voyage Voyage 5罰 (JJY) (JJY) (JKI-4) a b ~JJY)o ~zuo6 a b a b

2-3rd 200-180 -3 臼 3. 9 I-5 24 2 15 -4. 47 2. 45 0.001 0.26 1. 93 0. 45 6th 6th 50- 40 -1. 51 2. 44 3.79 0.28 0. 75 1. 80 7-Sth 7-Sth 23- 72 -2. 72 1. 31 7.14 0.006 10th 10th 110-108 0.80 2.16 -1.26 0.96

Table Table 3. Equivalent absorption index I fitting in the measured values

2.5 2.5 MHz 5 MHz 10 MHz 11.8 MHz Voyage Voyage 蕊 N (JJY) (JJY) (ZUO) (JJY) (JKI-4)

2-3rd 200-180 0.09 0.11 0.12 0.04 0.08 6th 6th 50- 40 0.05 0.02 0.1 7-Sth 7-Sth 23- 72 0.03 <0.01 10th 10th 100-108 0.05 0.05

decrease decrease in field intensity at 2. 5 MHz is much larger than that at 5 MHz with respect respect to the unabsorbed intensity. In In the same presentation, the nighttime absorption derived finally is shown in Fig. Fig. 5. The straight line in the figure represents a line best fitted in the measured absorption. absorption. Coefficients a and b in the expression Li (dB) =a+b ・10-a.n (km), as as well as the equivalent absorption index ,よ are indicated in Tables 2 and 3 respectively. respectively. In both tables, those values obtained in the preceding measurement are are reproduced for reference. Conclusions Conclusions given in the previous paper are summarized again as follows: (1) The nighttime E layer [Wakai, 1968] is responsible for the absorption as detected by measurement over the period of almost half cycle of the solar activity; (2) The nighttime nighttime absorption varies depending upon the solar activity; and (3) The equiva- lent lent absorption index extended to the nighttime was given tentatively to be useful for for the practical estimation of sky-wave field intensity in the HF wave range. From this point of view, the results obtained in this paper are examined as follows: follows: The sunspot number S互市( 12 month running average of relative sunspot number) on the outward voyage (Nov. ~ Dec., 1968) was about 110, while that on the home- ward voyage (Feb. ~ Apr., 1969) was about 108 in the declining of the present solar . In any case, the solar activity on the 10th JARE may be comparable with that on the 6th (S ま万= 50 ~40), in the sense of medium solar activity. Both absorption indexes for 2. 5 and 5 MHz obtained on the 10th JARE are 190 190 N. Wakai, S. Fujii and Y. Miyamoto consistent consistent each other, suggesting that the absorption measured satisfies the frequ- ency dependence resulting from the non-deviative treatment. If, If, however, we assume the linear dependence of the index on S豆N based on the data data before the 8th observation, the absorption index for 2. 5 MHz on the 10th in Table 3 shows value rather smaller than expected from the difference in ぷ・::; N. On the the other hand, the index for 5 MHz shows a reasonable value improving much the too too low value obtained in the 6th observation. From these considerations, it can be said that the present result confirmed the dependence dependence of the nighttime absorption upon the solar activity, supporting conclu- sions sions in the previous paper. It It is expected that the relation between the absorption index and solar activity should should be established after the accumulation of data by experiments.

4. 4. Conclusions

Conclusions Conclusions derived from the analysis of data obtained in the 10th measurement of of HF field intensities are given as follows: (1) (1) The narrow band receiver developed for recording exclusively JJY standard frequency frequency waves proved to be effective enough to eliminate interference due to to other standard frequency waves with an identical carrier frequency. (2) (2) The present analysis supported the conclusions in the previous paper that the nighttime nighttime absorption is controlled by the solar sctivity, as expected from the dependence dependence of the ionization in the nighttime E region upon the solar activity.

Acknowledgement

The authors wish to express their thanks to Messrs. Y. Ota, K. Ishizawa and R. R. Saruwatari for their efforts given to the measurement on board for several months. months.

References References

C. C. C. I. R., Report 267-1, Xlth Plenary Assembly, Oslo, Vol. ][, 285-294, 1966. Isοzaki, Isοzaki, S., Takenoshita, Y. and Ohbu, K., Internal Research Note (in Japanese), Rad. Rad. Res. Labs., 1970. Wakai, Wakai, N., Mean variations of the nighttime ionospheric E layer, J. Rad. Res. Labs., Labs., 15, 109-132, 1968. Wakai, Wakai, N., Ose, M. and Tanohata, K., Solar control of HF radio wave absorption in in the nighttime, J. Rad. Res. Labs., 18, 1-17, 1971.