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The Measurement of Thermal Radiation at Microwave Frequencies R The Measurement of Thermal Radiation at Microwave Frequencies R. H. Dicke Citation: Review of Scientific Instruments 17, 268 (1946); doi: 10.1063/1.1770483 View online: http://dx.doi.org/10.1063/1.1770483 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/17/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Frequency measurements of optical radiation Phys. Today 36, 52 (1983); 10.1063/1.2915445 Microwave Bridge Measurement of Plasma Frequency Rev. Sci. Instrum. 37, 168 (1966); 10.1063/1.1720120 Microwave Measurements of the Radiation Temperature of Plasmas J. Appl. Phys. 32, 25 (1961); 10.1063/1.1735953 Measurement of Dipole Moments at Microwave Frequencies J. Chem. Phys. 32, 315 (1960); 10.1063/1.1700940 Dielectric Measurement of Liquids at Microwave Frequencies J. Chem. Phys. 16, 364 (1948); 10.1063/1.1746891 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.104.46.196 On: Mon, 28 Jul 2014 18:26:30 268 R. H. DICKE The work being carried on at present in this dilute nitric acid gives a more negative value laboratory is a study of passivity of iron and than the passive iron, a result which perhaps other metals. Preliminary results show with iron means the presence of a different oxide or a surface potential change of about 0.2 volt in the possibly a basic nitrate. negative direction upon passivation in concen­ Acknowledgments are due Doctors L. P. trated nitric acid followed by a reverse change Gotsch and Laubscher for suggesting the use of a when made active with hydrochloric acid. This pH meter for this purpose and Mr. Robert Busch is in the direction that would be expected on the for laboratory assistance. This project was sup­ basis of an oxide layer. However, activation by ported in part by the Line Material Company. THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 17, NUMBER 7 JULY, 1946 The Measurement of Thermal Radiation at Microwave Frequencies R. H. DICKE* Radiation Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts** (Received April 15, 1946) The connection between Johnson noise and blackbody radiation is discussed, using a simple thermodynamic model. A microwave radiometer is described together with its theory of opera­ tion. The experimentally measured root mean square fluctuation of the output meter of a microwave radiometer (OA°C) compares favorably with a theoretical value of OA6°C. With an r-f band width of 16 me/sec., the O.4°C corresponds to a minimum detectable power of 10-16 watt. The method of calibrating using a variable temperature resistive load is described. INTRODUCTION Johnson noise can best be shown by considering INCE radio waves may be considered infra­ the system of Fig. 1. S red radiation of long wave-length, a hot body An antenna is connected to a transmission would be expected to radiate microwave energy line which is in turn terminated by a resistor. thermally. In order to be' a good radiator of The radiation impedance of the antenna is as­ microwaves, a body must be a good absorber and sumed to be equal to the characteristic impedance the best thermal radiator is the "blackbody." of the coaxial line, i.e., the antenna is "matched" Although their discoveries were historically to the line. Also the resistor is assumed to unconnected, there is a very close connection "match" the line. When a transmission line is between "Johnson noise" of resistors and ther­ terminated by a "matched" load, the running mal radiation. The thermal fluctuations of elec­ waves in the transmission line incident on this trons in a resistor set up voltages across the load are completely absorbed without reflection. resistor. These "noise voltages" are of such a The antenna is completely surrounded by black magnitude that a noise power per unit frequency of kT can be drawn from the resistor. k is Boltz­ mann's constant; T is the absolute temperature R"is!or of the resistor. T.m".rolu" The comJ.ection between thermal radiation and ~~~~~r Ene/Dlur. * Now at Palmer Physical Laboratory, Princeton Uni­ WalnS/ack versity, Princeton, New Jersey. .m".rtlfur. T ** This paper is based on work done for the Office of Scientific Research and Development under contract OEMsr-262 with the Massachusetts Institute of Tech­ nology. FIG. 1. Antenna system in black enclosure. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.104.46.196 On: Mon, 28 Jul 2014 18:26:30 THE R MAL R A D I A T ION AT M I C ROW A V E F R E QUE N C·I·E S 269 walls of an enclosure. The black enclosure and MEASUREMENT OF THERMAL RADIATION the resistor are . assumed to be at the same The detection and measurement of thermal T. temperature radiation at radiofrequencies requires techniques Thermal radiation emitted by the walls of the differing somewhat from those commonly em­ blackbody is picked up by the antenna and trans­ ployed for the monitoring of c.w. signals. A c.w. mitted down the line where it is absorbed by the signal, for instance aC.w. carrier, is characterized resistor. J ohnson- noise in the resistor causes a by a spectrum of small band width, and a re­ noise power to be transmitted down the line ceiver for monitoring such a signal is designed which,. passing out the antenna, is absorbed by with a narrow band. The narrow band rejects the black walls. If these two powers were un­ noise from frequencies for which there are no equal the resistor would either lose or gain en­ ergy, resulting in a violation of the second law . of thermodynamics. Thus, the available Johnson "~""' ' ' '".~ A",,", ~ (lclrk~ noise power from a resistor must be equal to the - - - - power picked up by an antenna pointed at a "., := '11111"'.... blackbody at the same temperature. It is evi­ dent that an antenna pointed at surroundings in FIG. 2. Wide band receiver used as radiometer. thermal equilibrium at a temperature T behaves like a resistor at this temperature terminating signal components, and a good signal to noise the coaxial line. It is convenient to describe the ratio is obtained. radiation intercepted by an antenna in terms 0 Thermal radiation, on the other hand, is usu­ an "antenna temperature." An antenna is said ally characterized by a spectrum which is very to have a certain temperature when it is receiv­ wide. The amplitudes, and phases of different ing an amount of radiation equal to that from a frequency components, are completely inde­ resistor at the temperature in question. pendent and are characterized by a complete The above arguments were made for a coaxial randomness in the phases. A receiver for the transmission line, but it is easily seen that they measurement of r-f noise should employ a wide are valid for an antenna fed by any type of intermediate frequency (i-f) band. single mode transmission line. Figure 2 is a block diagram of a wide band The frequency dependence of blackbody radia­ receiver used as a radiometer. A microwave re­ ceiver somewhat similar to Fig. 2 has been used tion differs from that of electrical noise, and. this by Southworthl in an interesting series of meas­ difference may at first glance seem paradoxical. urements on microwave radiation from the sun. It will be remembered that Planck's blackbody I t will be assumed for purposes of definiteness radiation formula reduces to that of Rayleigh­ that the radiometer of Fig. 2 has an i-f band Jeans at the long wave-length end, and in this width of 10 7 cycles/sec., that the noise figure2 of region the radiation per unit area, per unit solid the receiver is 20, and that the response time of angle, and per unit frequency of a blackbody is the d.c. output meter is 1 second. The operation proportional to frequency squared. However, the of the radiometer is roughly as follows: available noise power from a resistor per unit frequency is frequency independent. The para­ 1 G. C. Southworth, J. Frank. Inst. 239, 285 (1945). 2 The noise figure of a receiver is defined by the following dox is resolved as follows: in general, the lobes operation: A resistor whose absolute temperature is 3000 K of an antenna pattern become more directive is connected to the input terminals of the receiver. This resistor is heated until the available noise power from the at a higher frequency. It accepts noise from a i-f amplifier is doubled. The noise figure is this change in smaller solid angle; in fact, it can be shown that temperature expressed in units of 300°K. It is implicitly assumed that only a single band of r-f frequencies are the average absorption cross section of an an­ converted to the intermediate frequency. If two r-f bands tenna is ;\2/471". The factor ;\2 from the antenna are converted to the intermediate frequency, the noise figure is the change in temperature expressed in units of pattern and 1/>.2 from the blackbody radiation 150°K. It is evident that the noise figure is a function of formula just cancel each other, and the total the magnitude of the resistance. What is commonly quoted as the noise figure of a receiver is the measured value for absorbed power is frequency independent. the optimum value of resistance.
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