NATIONAL RADIO ASTRONOMY OBSERVATORY Green Bank, West Virginia Electronics Division Internal Report No, 23 UNDERGROUND TEMPERATURE VARIATIONS V. R. VellugoPal October 196$ (Originally submitted January 1963) NUMBER OF COPIES; 100
UNDERGROUND TEMPERATURE VARIATIONS V. R. Venugopal Introduction In an array of antennas one of the ways of transmitting signals either from a local oscillator to the various antenna locations or from the various antennas to a central point is by means of either coaxial cables or wavepides [lb An important requirement in such transmissions is phase stability to better than X/10. One of the factors contributing to the alteration in the electrical length of the coaxial cable or waveguide is variation in temperature. In the best ease the change in the electrical length is 10 PPI n Per C [119 This means that for a distance of 20,000 X the change in electrical length is V5 per C. This gives an idea of the order of temperature variations that can be accepted in order to maintain the phase of the signals within a few electrical degrees. The problem, therefore, is to keep the variations in temperature of the cable to a very small fracti n of a degree Centigrade. The best way to achieve this seems to be to bury the cables underground. It is known that periodic changes of the temperature decrease about exponentially with depth underground depending upon the range of temperature at the surface and the thermal diffusivity of soil [21. The experiment was performed near the Beard house to determine the diurnal variation in temperature at various depths below the surface. W. sperth9. 1tal. Set alp_ The set-up comprises a weather-proof thermistor probe in one leg of a Wheatstone bridge (of the YSI Tele-Thermometer) which is followed 'v an opposition battery, a DC Null Voltmeter and a recorder. With this set-up a variation in temperature of.02 C could be recorded. M. Results The thermistor probe was placed at various depths ranging from 1 foot to 3 feet 9 inches and the diurnal variations were recorded. Typical records obtained for three depths (-I foot, 2.5 feet, and 3.75 feet) are reproduced in Figures 1 through
30 The observed diurnal variation of temperature with respect to depth is shown in Figure 5, The diurnal variations were found to be smooth. The maximum variations at the three depths are found to be 1 C at 1 foot, 0,28 at 25 feet, and 0.05 C at 3.75 feet. The values agree fairly well with those obtained at Davis, California ( Figure 4) reproduced from reference in Hence, cables buried at a depth of 3.75 feet will have their temperature stable to within a small fraction of a degree Centigrade. This seems to meet the requirements mentioned in El), Acknowledgment acknowledged. Dr, Marc Vinoltue s guidance in the course of this experiment is gratefully References M, Vin,okur and N. J. Keen, Internal Memorandum on "Means of Transmission of Local Oscillator Signal" (December 1962), [2] B. Gutenberg, "Physics of the Earth' s Interior", Ed. 1959, p, 121. [3] F. A. Brooks, "An Introduction to Physical Meteorology" 1959-60? p. 81
.7== ^ 717=.7==. Fig.1 Record Of Diurnal Variation Of Temperature At A Depth Of I Foot Below Surface. =M. Em I F 1 PA MI=EAMM ========M =MM. M -Ws===- na-ggamá==`,==,-mm---mm-mfa=== Mr : 1== = ===== = stimmummatmom=== 11 1 E : - 1 l " 9...1m=aseasiMEN -16== "". 1 EMWMIMIMMa ====MNA - ========-M---W-MW- I MIIMENI MM.= NIMINEL MIMIran==nalWIMIM=M1==." WM-M=1MM m g NO Jai Mgarinni =OMMEMINIII WIMM "MMES= M-1-1 EMMMMMWE =MMW11====MIEELMMMEMEMMM I MMWEAM=Mni=== MLEMI-..-M -MM-= ---====a====== Fig.2 Record_Of Diurnal Variation Of Temperature At A Depth Of LS Feet Below Surface. '2".3=11= ===...=.76= ==imep =MIMI II MM ImM == MIIM111111 111111=.= ML; WILIEMWMEMEINNIIIrM wo imme =15=-=== = ====a-m WIMMEMMOMMOIN_ MMIM MENEM in.=... W....I INIIIMINW INOMN iiimumussinnu miasmomomoss ME-ROMMĪNMUMW= euii fli " MMMUM""ElEMM... 111R1=MMENE211/ MENEM= MIIMENEM Fig.3 Record Of Mutual Variation Of Temperature At A Depth Of 3.75 Feet Below Surface.
. Ạ. 90 a m 80 F scg w1 70 I- w.. 2 6O. - c4 90 w 80 c f ; cr Dia 1.1 0 2 w,.. t 60-5,,,', ttuf ta tr 90 p 80. irwa. x 70 tai - 5 60,i) F.A.SROOKS UNIV. ealic t7 JAN Se --- - 1 -j-- - 1 - inch 4.. a. -.,--- - Air I (5ft.),. isivitak 3-inch riklapla.9.10 11 Milik lia I i. 5 cc 0 " 1* 111111 J.. I MU II C ""111 111111"111 ll -,.., AM. ' 'Ir' H 10 ---- 2.17 111111111 mmin alillillailftglik umi lli i t 411 ' 111Earm aiiirze-dinwtdmllril MN A AMIE, ' I Mina miwill 1 I114. EFALVIIIIII IMIIIMWMITIMIIIIIV IIIIIMPI AMPV IIIIIMMI AINIM illk IIIMI '111MI N I, Alb, W l i l''10-1r1,14111 il h MI 1.111111111111 --im sr.11 III 1A1111111111111111111111.1""11111. NMI 6days toter INIM a wcc 90 ạ cr ta a. 80 3 '- asi 70 g 111111.1 60 al Extended using 1933 differences applied to 935 air temperatures. JULY 1934 AUG SEPT OCT NOV DEC JAN FEB MAR APR MAY 1935 JUNE e 0 i 11- w to 70 i-4 z W ta a. 60 2 ṗ- V-, (0 70 L C rx a t- 4 60 5.ẇ t- :g cu 4t Fig. 4 Ten-day Running Averages of Soil Temperatures to 36-inch Depth at Davis, California..,,,,...
2 3 Depth in feet Fig. 5 Diurnal Variation in Temperature with Res p ect to Depth Underground.