Pub Date : 1938-08-01DOI: 10.1109/JRPROC.1938.228494
M. Strutt, A. van der Ziel
By recent measurements of input loss, output loss, and feed-back capacitance of modern high-frequency amplifier tubes (pentodes) up to 300 megacycles, a considerable increase of these values in the short-wave range has become manifest. Contrary to the opinion, expressed in several recent publications, for many types of valves the main cause of this increase must not be sought in electron-transit-time effects, but in the action of capacitances, mutual inductances, and self-inductances of the tube electrodes and of their leads within and without the tubes. A general theory of the effect of these quantities on input admittance, output admittance, feedback admittance, and mutual admittance is put forward for tetrodes, pentodes, hexodes, etc., used as high-frequency amplifiers. By three series of measurements it is shown, that about one to two thirds of the input damping of modern European high-frequency valves of normal dimensions on short waves must be ascribed to inductive effects and not to transit times. Several measurements on transit-time effects are described, showing that the transit time between the input grid and the screen grid may not be neglected as compared with the transit time between the cathode and the input grid. Theoretical formulas for inductive effects are well checked and those for transit-time effects are not so well checked by measurements. Causes for the latter deviations are given. Output admittance and feed-back admittance are almost wholly due to inductive effects in the short-wave region, as shown by several measurements described.
{"title":"The Causes for the Increase of the Admittances of Modern High-Frequency Amplifier Tubes on Short Waves","authors":"M. Strutt, A. van der Ziel","doi":"10.1109/JRPROC.1938.228494","DOIUrl":"https://doi.org/10.1109/JRPROC.1938.228494","url":null,"abstract":"By recent measurements of input loss, output loss, and feed-back capacitance of modern high-frequency amplifier tubes (pentodes) up to 300 megacycles, a considerable increase of these values in the short-wave range has become manifest. Contrary to the opinion, expressed in several recent publications, for many types of valves the main cause of this increase must not be sought in electron-transit-time effects, but in the action of capacitances, mutual inductances, and self-inductances of the tube electrodes and of their leads within and without the tubes. A general theory of the effect of these quantities on input admittance, output admittance, feedback admittance, and mutual admittance is put forward for tetrodes, pentodes, hexodes, etc., used as high-frequency amplifiers. By three series of measurements it is shown, that about one to two thirds of the input damping of modern European high-frequency valves of normal dimensions on short waves must be ascribed to inductive effects and not to transit times. Several measurements on transit-time effects are described, showing that the transit time between the input grid and the screen grid may not be neglected as compared with the transit time between the cathode and the input grid. Theoretical formulas for inductive effects are well checked and those for transit-time effects are not so well checked by measurements. Causes for the latter deviations are given. Output admittance and feed-back admittance are almost wholly due to inductive effects in the short-wave region, as shown by several measurements described.","PeriodicalId":54574,"journal":{"name":"Proceedings of the Institute of Radio Engineers","volume":"26 1","pages":"1011-1032"},"PeriodicalIF":0.0,"publicationDate":"1938-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1109/JRPROC.1938.228494","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"62307904","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1938-08-01DOI: 10.1109/JRPROC.1938.228548
T. R. Gilliland, S. S. Kirby, N. Smith
Data on the ordinary-wave critical frequencies and virtual heights of the ionospheric layers are presented for the period indicated in the title. The monthly average values of the maximum usable frequencies for undisturbed days, for radio transmission by way of the regular layers is also provided.
{"title":"Characteristics of the Ionosphere at Washington, D.C., June, 1938","authors":"T. R. Gilliland, S. S. Kirby, N. Smith","doi":"10.1109/JRPROC.1938.228548","DOIUrl":"https://doi.org/10.1109/JRPROC.1938.228548","url":null,"abstract":"Data on the ordinary-wave critical frequencies and virtual heights of the ionospheric layers are presented for the period indicated in the title. The monthly average values of the maximum usable frequencies for undisturbed days, for radio transmission by way of the regular layers is also provided.","PeriodicalId":54574,"journal":{"name":"Proceedings of the Institute of Radio Engineers","volume":"20 1","pages":"1033-1036"},"PeriodicalIF":0.0,"publicationDate":"1938-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1109/JRPROC.1938.228548","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"62308543","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1938-08-01DOI: 10.1109/JRPROC.1938.228489
F. Offner
{"title":"Notes on the Impedance of a Carbon Microphone","authors":"F. Offner","doi":"10.1109/JRPROC.1938.228489","DOIUrl":"https://doi.org/10.1109/JRPROC.1938.228489","url":null,"abstract":"","PeriodicalId":54574,"journal":{"name":"Proceedings of the Institute of Radio Engineers","volume":"26 1","pages":"1009-1010"},"PeriodicalIF":0.0,"publicationDate":"1938-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1109/JRPROC.1938.228489","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"62307764","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1938-08-01DOI: 10.1109/JRPROC.1938.228495
R. Dome
A descriptive and mathematical treatment is given of a means for obtaining modulated radio-frequency power with good conversion efficiency from the usual direct-current source. The method employed is to modify the load line on a saturated radio-frequency amplifier to take care of positive peaks and to grid modulate the amplifier for negative peaks. Load modification is accomplished by absorption and in this system the absorbed power is not dissipated in heat but is returned to the direct-current source for the power-amplifier tube and thereby reduces the drain from the supply. This results in direct-current-to-carrier conversion efficiencics in the order of 50 to 60 per cent when average tubes are used.
{"title":"High-Efficiency Modulation System","authors":"R. Dome","doi":"10.1109/JRPROC.1938.228495","DOIUrl":"https://doi.org/10.1109/JRPROC.1938.228495","url":null,"abstract":"A descriptive and mathematical treatment is given of a means for obtaining modulated radio-frequency power with good conversion efficiency from the usual direct-current source. The method employed is to modify the load line on a saturated radio-frequency amplifier to take care of positive peaks and to grid modulate the amplifier for negative peaks. Load modification is accomplished by absorption and in this system the absorbed power is not dissipated in heat but is returned to the direct-current source for the power-amplifier tube and thereby reduces the drain from the supply. This results in direct-current-to-carrier conversion efficiencics in the order of 50 to 60 per cent when average tubes are used.","PeriodicalId":54574,"journal":{"name":"Proceedings of the Institute of Radio Engineers","volume":"26 1","pages":"963-982"},"PeriodicalIF":0.0,"publicationDate":"1938-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1109/JRPROC.1938.228495","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"62308474","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1938-08-01DOI: 10.1109/JRPROC.1938.228496
L.F. Gaudernack
The increasing economic importance of power efficiency for high-power broadcast stations is illustrated in Fig. 1 showing that some 50 per cent saving is possible. Different known modulation schemes are compared as regards necessary input power, including the recent Chireix and Doherty systems. The influence of rates for electric power is stressed. A so-called "phase-opposition" system is discussed starting with simple circuit theory, first for seriesand then for parallel-tuned circuits. Principally the system is based on the co-operation of two generators; i.e., one "carrier" generator constantly excited, and one "side-band" generator whose excitation is changed 180 degrees in phase according to the phase of modulation. Mean input power is calculated for this system, giving about the same input as good systems for about 30 per cent modulation, but 10 to 25 per cent more power at 100 per cent modulation. The paper is concluded with a consideration of tube characteristics and possible means for obtaining linearity are shown.
{"title":"A Phase-Opposition System of Amplitude Modulation","authors":"L.F. Gaudernack","doi":"10.1109/JRPROC.1938.228496","DOIUrl":"https://doi.org/10.1109/JRPROC.1938.228496","url":null,"abstract":"The increasing economic importance of power efficiency for high-power broadcast stations is illustrated in Fig. 1 showing that some 50 per cent saving is possible. Different known modulation schemes are compared as regards necessary input power, including the recent Chireix and Doherty systems. The influence of rates for electric power is stressed. A so-called \"phase-opposition\" system is discussed starting with simple circuit theory, first for seriesand then for parallel-tuned circuits. Principally the system is based on the co-operation of two generators; i.e., one \"carrier\" generator constantly excited, and one \"side-band\" generator whose excitation is changed 180 degrees in phase according to the phase of modulation. Mean input power is calculated for this system, giving about the same input as good systems for about 30 per cent modulation, but 10 to 25 per cent more power at 100 per cent modulation. The paper is concluded with a consideration of tube characteristics and possible means for obtaining linearity are shown.","PeriodicalId":54574,"journal":{"name":"Proceedings of the Institute of Radio Engineers","volume":"26 1","pages":"983-1008"},"PeriodicalIF":0.0,"publicationDate":"1938-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1109/JRPROC.1938.228496","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"62308485","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1938-07-01DOI: 10.1109/JRPROC.1938.228425
W. A. Edson
In communication work it is often necessary to design a simple lumped network, the impedance of which approximates at all frequencies that of a certain distributed circuit, such as a transmission line. The present treatment includes a clarified method of making such a design. Former design methods have the common fault that negative circuit constants may be called for. This paper presents a simple method by which the difficulty is avoided. Moreover the best design under the particular circumstances in question may be arrived at quickly and directly. The impedance functions of two common forms of simulative networks are developed and design equations for the establishment of the parameters are presented. Since four elements are to be established it is possible to specify four arbitrary conditions. This is most conveniently handled by specification of the vectorial impedance at two frequencies. The major portion of this paper is concerned with the relation which must exist between these impedances in order that the calculated circuit parameters will all be positive. For the sake of completeness conditions leading to negative parameters are also considered.
{"title":"A Method of Designing Simulative Networks","authors":"W. A. Edson","doi":"10.1109/JRPROC.1938.228425","DOIUrl":"https://doi.org/10.1109/JRPROC.1938.228425","url":null,"abstract":"In communication work it is often necessary to design a simple lumped network, the impedance of which approximates at all frequencies that of a certain distributed circuit, such as a transmission line. The present treatment includes a clarified method of making such a design. Former design methods have the common fault that negative circuit constants may be called for. This paper presents a simple method by which the difficulty is avoided. Moreover the best design under the particular circumstances in question may be arrived at quickly and directly. The impedance functions of two common forms of simulative networks are developed and design equations for the establishment of the parameters are presented. Since four elements are to be established it is possible to specify four arbitrary conditions. This is most conveniently handled by specification of the vectorial impedance at two frequencies. The major portion of this paper is concerned with the relation which must exist between these impedances in order that the calculated circuit parameters will all be positive. For the sake of completeness conditions leading to negative parameters are also considered.","PeriodicalId":54574,"journal":{"name":"Proceedings of the Institute of Radio Engineers","volume":"26 1","pages":"877-891"},"PeriodicalIF":0.0,"publicationDate":"1938-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1109/JRPROC.1938.228425","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"62308159","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1938-07-01DOI: 10.1109/JRPROC.1938.228426
J. Pierce
Records of the field strength of 10-megacycle signals have been made at a distance of 30 kilometers from the transmitter. "Bursts" of received energy, of a few minutes' duration, were noted at times when no reflection from the F layer was present. When the density of ionization in the F layer was above the critical value for this frequency, occasional short periods of strong absorption at a low level have greatly reduced the strength of the reflected signal. Both of these phenomena are apparently due to small volumes of dense ionization in the E region, and they are consistent with a theory that such local ionization is caused by the transit of a single large meteor. Examination of astronomical data on the number and size of meteors shows that this is a possible explanation of the abnormally intense ionization frequently observed in the E layer. The same recently accessible data indicate that the constant meteoric bombardment of the atmosphere can maintain a continuous background level of ionization in the E region of about the magnitude which is observed during the night.
{"title":"Abnormal Ionization in the E Region of the Ionosphere","authors":"J. Pierce","doi":"10.1109/JRPROC.1938.228426","DOIUrl":"https://doi.org/10.1109/JRPROC.1938.228426","url":null,"abstract":"Records of the field strength of 10-megacycle signals have been made at a distance of 30 kilometers from the transmitter. \"Bursts\" of received energy, of a few minutes' duration, were noted at times when no reflection from the F layer was present. When the density of ionization in the F layer was above the critical value for this frequency, occasional short periods of strong absorption at a low level have greatly reduced the strength of the reflected signal. Both of these phenomena are apparently due to small volumes of dense ionization in the E region, and they are consistent with a theory that such local ionization is caused by the transit of a single large meteor. Examination of astronomical data on the number and size of meteors shows that this is a possible explanation of the abnormally intense ionization frequently observed in the E layer. The same recently accessible data indicate that the constant meteoric bombardment of the atmosphere can maintain a continuous background level of ionization in the E region of about the magnitude which is observed during the night.","PeriodicalId":54574,"journal":{"name":"Proceedings of the Institute of Radio Engineers","volume":"69 1","pages":"892-908"},"PeriodicalIF":0.0,"publicationDate":"1938-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1109/JRPROC.1938.228426","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"62308167","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1938-07-01DOI: 10.1109/JRPROC.1938.228428
T. R. Gilliland, S. S. Kirby, N. Smith
Data on the ordinary-wave critical frequencies and virtual heights of the ionospheric layers are presented for the period indicated in the title. The monthly average values of the maximum usable frequencies for undisturbed days, for radio transmission by way of the regular layers is also provided.
{"title":"Characteristics of the Ionosphere at Washington, D.C., May, 1938","authors":"T. R. Gilliland, S. S. Kirby, N. Smith","doi":"10.1109/JRPROC.1938.228428","DOIUrl":"https://doi.org/10.1109/JRPROC.1938.228428","url":null,"abstract":"Data on the ordinary-wave critical frequencies and virtual heights of the ionospheric layers are presented for the period indicated in the title. The monthly average values of the maximum usable frequencies for undisturbed days, for radio transmission by way of the regular layers is also provided.","PeriodicalId":54574,"journal":{"name":"Proceedings of the Institute of Radio Engineers","volume":"26 1","pages":"909-913"},"PeriodicalIF":0.0,"publicationDate":"1938-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1109/JRPROC.1938.228428","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"62308222","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1938-07-01DOI: 10.1109/JRPROC.1938.228387
S. Matsuo
Various theoretical and experimental researches have been carried out to indicate the absolute altitude of aircraft with respect to the ground by radio-wave-reflection methods. By using a new principle, frequency modulation (FM), the following results have been obtained: 1. Completely continuous indication of altitude by a steady pointer and dial is easily accomplished and altitude variations occurring during time intervals of a few milliseconds are clearly detected. 2. Very low altitudes, less than four meters, are indicated quite easily. 3. The indication is quite linearly proportional to altitude. 4. The absolute error is zero and the relative error is less than a few per cent. 5. Altitudes in excess of 160 meters can be measured with a power consumption of only 3.9 watts. The above results show that this sounding gear is suitable for blind-landing use in aeronautics. Moreover, this gear may be applied to satisfy navigational requirements in foggy weather, to avoid the collision of ships, or to lead them into a port.
{"title":"A Direct-Reading Radio-Wave-Reflection-Type Absolute Altimeter for Aeronautics","authors":"S. Matsuo","doi":"10.1109/JRPROC.1938.228387","DOIUrl":"https://doi.org/10.1109/JRPROC.1938.228387","url":null,"abstract":"Various theoretical and experimental researches have been carried out to indicate the absolute altitude of aircraft with respect to the ground by radio-wave-reflection methods. By using a new principle, frequency modulation (FM), the following results have been obtained: 1. Completely continuous indication of altitude by a steady pointer and dial is easily accomplished and altitude variations occurring during time intervals of a few milliseconds are clearly detected. 2. Very low altitudes, less than four meters, are indicated quite easily. 3. The indication is quite linearly proportional to altitude. 4. The absolute error is zero and the relative error is less than a few per cent. 5. Altitudes in excess of 160 meters can be measured with a power consumption of only 3.9 watts. The above results show that this sounding gear is suitable for blind-landing use in aeronautics. Moreover, this gear may be applied to satisfy navigational requirements in foggy weather, to avoid the collision of ships, or to lead them into a port.","PeriodicalId":54574,"journal":{"name":"Proceedings of the Institute of Radio Engineers","volume":"26 1","pages":"848-858"},"PeriodicalIF":0.0,"publicationDate":"1938-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1109/JRPROC.1938.228387","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"62308078","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1938-07-01DOI: 10.1109/JRPROC.1938.228386
G. W. Kenrick, A. Braaten, J. General
This paper presents the results of a quantitative study of the relationship between the proximity of great-circle transmission paths to the magnetic pole and of signal stability during terrestrial magnetic disturbances. Reception from Europe as observed at Riverhead, Long Island, and San Juan, Puerto Rico, is compared during normal and disturbed periods. The great-circle path from Europe to San Juan is about 1000 miles farther from the north magnetic pole than a similar path to Riverhead. A brief description of the duplicate equipment and antenna systems employed at the two locations is included. It has been known for some time that the more southerly transmission paths are apparently less susceptible to effects accompanying magnetic disturbances. The results of observations covering a period of years confirm the anticipated relationship between signal stability and proximity of the transmission path to the magnetic pole. The average advantage of San Juan over Riverhead for European signals during disturbed periods is found to be approximately 8 decibels. Evidence of higher ionospheric ionization over the more southerly path is noted in a number of cases by a comparison of nighttime cutoff effects and allied phenomena.
本文对地磁扰动时大圆传输路径与磁极的接近程度与信号稳定性之间的关系进行了定量研究。在Riverhead, Long Island和San Juan, Puerto Rico观察到的来自欧洲的接收，在正常和干扰时期进行了比较。从欧洲到圣胡安的大圆路径比到河头的类似路径距离北极约1000英里。扼要说明在这两个地点使用的重复设备和天线系统。一段时间以来，人们已经知道，更靠南的传输路径显然不太容易受到伴随磁干扰的影响。覆盖数年的观测结果证实了预期的信号稳定性与传输路径到磁极的距离之间的关系。在干扰期间，圣胡安的欧洲信号平均优势约为8分贝。通过对夜间截断效应和相关现象的比较，在许多情况下都注意到在较南的路径上电离层电离程度较高的证据。
{"title":"The Relation between Radio-Transmission Path and Magnetic-Storm Effects","authors":"G. W. Kenrick, A. Braaten, J. General","doi":"10.1109/JRPROC.1938.228386","DOIUrl":"https://doi.org/10.1109/JRPROC.1938.228386","url":null,"abstract":"This paper presents the results of a quantitative study of the relationship between the proximity of great-circle transmission paths to the magnetic pole and of signal stability during terrestrial magnetic disturbances. Reception from Europe as observed at Riverhead, Long Island, and San Juan, Puerto Rico, is compared during normal and disturbed periods. The great-circle path from Europe to San Juan is about 1000 miles farther from the north magnetic pole than a similar path to Riverhead. A brief description of the duplicate equipment and antenna systems employed at the two locations is included. It has been known for some time that the more southerly transmission paths are apparently less susceptible to effects accompanying magnetic disturbances. The results of observations covering a period of years confirm the anticipated relationship between signal stability and proximity of the transmission path to the magnetic pole. The average advantage of San Juan over Riverhead for European signals during disturbed periods is found to be approximately 8 decibels. Evidence of higher ionospheric ionization over the more southerly path is noted in a number of cases by a comparison of nighttime cutoff effects and allied phenomena.","PeriodicalId":54574,"journal":{"name":"Proceedings of the Institute of Radio Engineers","volume":"26 1","pages":"831-847"},"PeriodicalIF":0.0,"publicationDate":"1938-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1109/JRPROC.1938.228386","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"62308062","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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