PROCEEDINGS OF THE IRE

A Method of Calibrating Centimetric RadiometersUsing a Standard Noise Source*

J. S. HEYt AND V. A. HUGHESt

Summary-In order to compare radio-astronomical measure-ments of power flux observed on different occasions at differentwavelengths, special attention must be paid to methods of calibra-tion. A method is described which is suitable for a centimetricradiometer measuring antenna temperature over a range of 0 to1000cK or more with an accuracy of about 1VK.

The power received by the radiometer is compared with thatfrom a CV1881 argon discharge tube fed through a precision at-tenuator. An additional fixed amount of power is added to that of theradiometer to ensure that the effective temperature is above that ofthe room.

The intensity of solar radiation at X = 10.5 cm was measured bythis method during the eclipse of June 30, 1954. The scale of thechart recording the radiometer output was effectively increased byadjusting the attenuator in steps each time maximum deflection wasattained corresponding to about 200K change of antenna tempera-ture. Finally, to measure the brightness temperature of the un-eclipsed sun, the received power from the sun was compared withthat from a known external noise source consisting of a horn radiatorfed by a CV1881 argon discharge tube.

INTRODUCTION

ACCURATE methods of calibration of radiome-ters used in radio astronomy are essential so thatreliable comparisons can be made between ob-

servations on different occasions and at different wave-lengths. The received power is usually expressed interms of the antenna temperature which can be deter-mined by comparing the received noise with that from athermal noise generator. The radiometer may be re-quired to measure antenna temperatures between 0°Kand 103°K, and to vary the temperature of a referencesource, such as a resistor, over this range would be in-convenient for general use. This note indicates how aparticular discharge tube, in conjunction with a preci-sion attenuator, has been used as a standard variablenoise source. The conversion of antenna temperatureinto brightness temperature of the radio astronomicalsource, such as the sun, involves a knowledge of thesensitivity pattern of the antenna. A more direct de-termination of brightness temperature can be achievedby using a standard source as an external radiator. Theapplication of these techniques to a radiometer at X =10.5 cm which was used for observation of the sun dur-ing the eclipse of June 30, 1954, is described.

THE NOISE SOURCEMumford' first suggested that the gaseous discharge* Original manuscript received, October 14, 1957.t Ministry of Supply, Royal Radar Est., Malvern, Worcs., Eng.1 W. W. Mumford, "A broad-band microwave noise source," Bell

Sys. Tech. J., vol. 28, pp. 608-618; October, 1949.

in an ordinary fluorescent lamp might be a suitablesource of broad-band noise for measuring the noisefactors of receivers. However, mercury-filled tubes arenot very stable, and the noise power available dependson the ambient temperature of the tube. Argon dis-charge tubes were investigated by Johnson and De-Remer2 and found to be more stable. Measurements at10.5-cm wavelength by Hughes3 on the argon tube,CV1881, matched into a waveguide, have shown thatfor a discharge current of 180 ma, the effective tem-perature is 11,140°K with a maximum error of 260°K(0.10 db), and that there is a high degree of stabilityand consistency between tubes. The effective temper-ature varies slightly with discharge current with the co-efficient of -0.004 db per ma. That the noise output ofthe tube is not critically dependent on frequency is indi-cated by comparison with measurements made by Sut-cliffe4 at a wavelength of 3 cm, where a similar effectivetemperature was obtained.The effective temperature of the noise source may be

varied by connecting in series a calibrated attenuator,which both attenuates the noise from the tube and con-tributes its own noise since it is at room temperature.If the coefficient of attenuation is a, the effective tem-perature of the attenuator To, and that of the dischargetube TD, then the effective temperature of the variablenoise source is given by

TD -TT=1D 1+ O.

a

Such a source, using an attenuator calibrated to an ac-curacy of 0.01 db, provided a variable standard tem-perature noise source for the radiometer now to be de-scribed.

THE RADIOMETER USED FOR ECLIPSE MEASUREMENTSRadiometers at centimeter wavelengths can meas-

ure, by means of wide rf bandwidths and long outputtime constants, changes of less than 1°K.To reduce errors due to changes in gain or receiver

noise factor, it is necessary not only to switch continually

2 H. Johnson and K. R. DeRemer, "Gaseous discharge super-high-frequency noise sources," PROC. IRE, vol. 39, pp. 908-914;August, 1951.

3 V. A. Hughes, "Absolute calibration of a standard temperaturenoise source for use with S-band radiometers," Proc. IEE, pt. B, vol.103, pp. 669-672; September, 1956.

4 H. SkitcIiffe, "Noise measurements in the 3-cm waveband usinga hot source," Proc. IEE, pt. B, vol. 103, pp. 673-677; September,1956.

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PROCEEDINGS OF TIE IRE

between the received noise from the antenna and a

standard source,' but also to maintain the output of thestandard source not greatly different from that to bemeasured. During a solar eclipse, a wide variation ofantenna temperature occcurs.

To fulfill the above requirements, the radiometer cir-cuit, shown schematically in Fig. 1, was constructed.The received power was compared 30 times a secondwith that from a calibrated argon discharge tube source

S by means of a waveguide switch. The antenna tem-perature to be measured is often below room tempera-ture. In order to bring the two signals near the same

level so as to improve the accuracy of measurement, a

second discharge tube S' was arranged to add, by means

of a directional coupler, a constant signal to that receivedby the antenna. The fine adjustment of the two signalsto near equality could then be made by the precisionattenuator connected to the reference source S.

IG5dUDIREOTIONALi

COUPLER /'~~~~~+X

46,. /TM. DIAMETERRADIO-TELESCI!DISCHARGE !PRECISION|.

TUBE S IATTENUATOR

WAVE-

SWITCH REFERENCE-!

MOTOR VOLTAGE

.sWSI ~~~~~~~~~~30C1/5

REF. VOLTS

ULOCAL r B`ALANCED.LOSCILLATOR NXER j

45 MA/s

F . F. 105SM43A/, I30Y HRANE D.C. OUTPUT STAGSENSITIVE AND

MPLIFIE R PLIFER DETECTOR RECORE.DE

Fig. 1-Block schematic of radiometer.

Fig. 2. The successive readjustment 1) effectively al-lows the ordinate of the recording scale to be increasedand so improves reading accuracy, 2) reduces the error

due to gain fluctuations since the error is proportionalto the difference in the two signals which is kept small,and 3) provides repeated calibrations of the recorderscale. By this method and with a time constant of about10 seconds, the antenna temperature was measuredthroughout the eclipse to an accuracy better than 1°Kcorresponding to less than 0.5 per cent of the antennatemperature of the uneclipsed sun. The results, whichwere obtained at a site where the eclipse was partial,have been discussed elsewhere.6

DOPE

The radiometer was used to measure the intensity ofthe radiation from the sun during the eclipse of June 30,1954. The radiometer antenna which had a beamwidthof about 40 was automatically driven to follow the sun

continuously. Prior to the eclipse, the power from thevariable noise source was adjusted to equal that re-

ceived by the antenna plus supplementary fixed noisesource, so as to produce zero output from the radiom-eter. The gain was set so that a full-scale deflection on

the recorder corresponded to approximately 20°Kor 10 per cent of the total received power from the sun.

As the eclipse progressed and the recorded output ap-

proached maximum deflection, the precision attenuatorwas readjusted to bring the reference signal back to near

equality and the output at the recorder near zero again.This sequence was repeated as necessary throughout theeclipse, and a tracing of the recorded output is shown in

6 R. H. Dicke, "The measurement of thermal radiation at micro-wave frequencies," Rev. Sci. Instr., vol. 17, pp. 268-275; July, 1946.

6

4

2

1120 1140 1200 1220

(a)

IIII

I ~~~~II

1240 1300 1320 1340

(b)

Fig. 2-Recorded output of radiometer during eclipse of June 30,1954; (a) ordinates-relative power received, (b) abscissas-uni-versal time.

BRIGHTNESS TEMPERATURE OF THE SUN

To interpret the power measured by the radiometerin terms of apparent brightness temperature of thesolar disk7 requires an accurate knowledge of the param-

eters of the antenna and of the losses in the waveguidetransmission line between the antenna and waveguideswitch. A more convenient method is to produce an

external noise radiator of standard brightness temper-ature to make a direct comparison with the radiationfrom the sun. Such a comparison source is provided bya waveguide horn fed by an argon discharge tube. Thisnoise radiator then has a brightness temperature equalto the effective temperature of the discharge tube andarea equal to the effective area of the horn aperture.The effective area of the horn is directly proportional

6 J. S. Hey and V. A. Hughes, "Centimetre wave observations ofthe solar eclipse of 1954 June 30," Observatory, vol. 76, pp. 226-228;December, 1956.

7The apparent disk temperature is defined as the brightnesstemperature of a uniformly bright disk equal in size to the observedoptical disk.

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8

6

1.

2

Hey and Hughes: Calibrating Centimetric Radiometers

to gain and it is possible to design a horn with a calcu-lable gain.8 Hence, if the radiometer antenna is directedtowards the noise radiator and the increase in effectivetemperature noted, then from a knowledge of the dis-tance between radiometer antenna and noise radiatoran over-all calibration of the equipment is obtained.The radiator must obviously be sufficiently far away tobe outside the Fresnel diffraction zone of the radiometerantenna, and hence limitations are placed on the size ofantenna that may be calibrated. (Even with the largestantennas the radiator may be used in the near zone inorder to provide a very simple method of comparing theoperation of the equipment from day to day.)The above method was used to obtain the apparent

brightness temperature of the sun about the time of theeclipse of June 30, 1954. The radiometer antenna had adiameter of approximately 1.5 meters; and by spacingthe noise radiator at a distance of 66 meters, the maxi-mum path difference across the antenna was reduced to0.04X. The apparent disk temperature of the sun willbe given by

TAS GHX2 1TS= *TD -

TAD 4rR2 QS8 W. T. Slayton, "Design and Calibration of Microwave Antenna

Gain Standards," Naval Res. Labs., Washington, D. C., Rep. 4433;November, 1954.

where, under the conditions existing at the time of thecalibration,

TAs=antenna temperature of the antenna directedtowards the sun.

TAD= change in antenna temperature due to the noiseradiator.

TD=effective temperature of discharge tube.GH= gain of horn.R =distance between radiator and antenna.Qs=solid angle subtended by optical disk of sun.X = wavelength.

Substituting the experimental values, TAS= 187.50K,TAD = 10.00K, TD = 11,140°K, GH = 65.3, R = 66 meters,Os = 6.58.10-5 steradians, X = 10.5 cm, the apparentbrightness temperature of the solar disk at this wave-length is 4.2.104°K. The main inaccuracy involved is inthe gain of the horn which was calculated by Slayton'smethod. It appears unlikely that the error will exceed5 per cent.

ACKNOWLEDGMENTAcknowledgment is made to the Controller, H.M.B.

Stationery Office, England, for permission to publishthis paper.

c7)A~5D

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