Effect of radiation on the dielectric constant and attenuation of two coaxial cables

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Title:
Effect of radiation on the dielectric constant and attenuation of two coaxial cables
Physical Description:
iii, 13 p. : ill. ; 28 cm.
Language:
English
Creator:
Weeks, R. A
Binder, D
Oak Ridge National Laboratory
U.S. Atomic Energy Commission
Publisher:
U.S. Atomic Energy Commission, Technical Information Service
Place of Publication:
Oak Ridge, Tenn
Publication Date:

Subjects

Subjects / Keywords:
Coaxial cables -- Effect of radiation on   ( lcsh )
Dielectrics   ( lcsh )
Genre:
bibliography   ( marcgt )
non-fiction   ( marcgt )

Notes

Bibliography:
Includes bibliographical references (p. 13).
Statement of Responsibility:
R.A. Weeks and D. Binder.
General Note:
"ORNL-1700."
General Note:
"March 19, 1954."
General Note:
"Contract No. W-7405-eng-26."
General Note:
Work performed at the Oak Ridge National Laboratory.

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University of Florida
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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
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oclc - 727963458
System ID:
AA00012262:00001


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UNCLASSIFIED


















SU: II .-.-~~



US DEPOjSIT~,F


oRNL-17oo

Subject Category: PHYSICS



UNITED STATES ATOMIC ENERGY COMMISSION



EFFECT OF RADIATION ON TIHE:
DIELECTRIC CONSTANT AND ATTENUATION
OF TWO COAXIAL CABLES


By
R. A. Weeks
D. Binder


3ri i 1956


March 19, 1954

Oak Ridge National Laboratory
Oak Ridge, Tennessee


Technical Information Service, Oakr Ridge, Tennessee


UNCLASSIFIED















This report was prepared asa scientific account of Govern-
ment-sponsored work. Neither the Unoited States, nor the Com-
mission, nor any person acting on behalf of the Commission
makes any warranty or representation, express or implied, with
respect to the accuracy, completeness, or usefulness of the in-
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mation, apparatus, method, or process disclosed in this report
may not infringe privately owned rights. The Commission assumes
no liability with respect to the use of,ar from damages resulting
from the use of, any information, apparatus, method, or process
disclosed in this report.


Date De~classified: February 23, 1955.


This report has been reproduced directly flam the best
available copy.

Issuance of this document does not constitute authority
for declassification of classified material of the same or
similar content and title by the same authors.

Printed in USA, Price 20 cents. Available from the
Office of Technical Services, Department of Commerce, Wash-
ington 25, D. C.


OPD B229O 1




DRNL-1'100


Contract N~o. W-7405-eng-26


SOLID STATE DIVISION



EFFECT OF RADIATION ON THiE DIELE;CTRIC CONSTANT AND

ATTENU~ATION OF TWfO COAXLIL CABLES


R. A. Weeks and D. Binder






ABSTRACT

Measurements have been made at radiation induced changes in the phase
constant and attenuation of two coaxial cables while being irradiated. The
measurements were made in the region of four megacycles. At this frequency
the change in dielectric constant was (1.4 +t 0.4) qb for both dielectrics
after roughly 2 x 1018 nvt. The change in attenuation was (9 + 2) P$ for
polyethylene and within the range of error for teflon. The phase constant
and attenuation were found by measuring the input impedance of an opena-ended
length of cable in the neighborhood of its quarter-wave frequency. Assuming
a uniform cable dielectric and no other variables the input impedance has a
minimon at this frequency. Fran the minimum the attenuation and phase con-
stant are found.



March 19, 1954


Oak Ridge National Labor-atory
operated by
Carbide and Carbon Chemicals Company
A Division of Union Carb'ide and Carbon Corporation
Post Office Box P
Dak Ridge, Tennessee


iii



































Digitized by the Internet Archive
in 2012 with funding from
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EFFECT OF RADIATION ON THE DIELECTRIC CONSTANT
AND ATTENIUATION OF TWD COAXIAL CABLES

RI. A. Weeks and D. Binder
Oak R~idge National Laboratory, Oak Ridge, Tennessee


Introduc tion

Plastic dielectries in the form of coaxial cables are often used in and

around radiation sources. Mlany of the physical properties of such plastics have

been studied(1) as a function of irradiation in neutron and gamma ray sources.

It is of somre interest to extend these studies to the effect of radiation on

certain electrical properties, dielectric constant and attenuation, of importance

in high frequency applications.

Ex~periraental Method

An accuibate technique of measuring relative changes in the dielectric con-

stant and attenuation in coaxial cables during irradiation has been devised.

With this technique and the theory developed below, it wsas possible to determine

changes in the r-elative value of the attenuation and specific dielectric constant,

e', with an error less than 2% and 0.80 respectively. Two types of coaxial

cable have been inserted ia the ORZNL graphite reactor and measurements of

attenuation and e' have been made during irradiation. One of the cables was

type RG-11/U with a polyethylene dielectric. The other cable was a high

voltage cable with a teflon dielectric. .Its characteristic impedance, Zo, was

appjroximately 90 ohms. The latter cable was irradiated for approximately 700

hours (Fig, II) and the RG-11/U for approximately 900 hours in a flux of 9,2 x

1011 thermal neutrons (Fig. III). The observed e' increased about 15 for both




(1) 0. Sianan, C. D). Bopp, "Phy;,sical Properties of Irradiated Plastics", ORNML-928

Refer to Bibliography for other reports.

1-






-2 -


cables. The observed attenuation of the teflon cable dbowed little change

during the period of irradiation. The observed change in attenuation of the

RSG-11/UT cable w~as approximately 95, The measurements were made in the region

of four megacycles.

Theory of Measurement

The equations pertinent to the technique are developed below, The input

impdance of a transmission line of length R is2))

(1) ZinZ( 1) coshyg io2 sinh VA

where

Zin = input impedance

Z~f) = impedance of load

Zo a characteristic impedance

if = propagation constant


F~or low loss lines



ot: attenuation per unit length



We frequency in radians per second

J// permability

u = 1 (for dielectrics of interest here)
41 = dielectric constant




(2) George C. Southworth, "'Principles and Applications of Waveguide Transmission",

p. 39-72, 195;0, D. Van Nostrand Co.

(3) G. G. Montgomery, et al, uPrinciples of Microwave Circuits"', p. 67, 19L8
McGraw Hill.






-3L


For an open ended line, Z(I ):6 Zo, equation (1) becomes

(2) Zin Zo cothyf
Then if


equation (2) becomes
(3) Zin a Zo tanh o4)
At the qu ~rter-wave frequency, p 1



and ( e' c Y )2
co 2 ta.

where c is the velocity of light, 9tY3 is the angular quarter-wave frequency,
p is the length of the line, eo is the dielectric constant of free space, and
e' is the specific dielectric constant.

Equation (2) can be written

Z6 1% .I e-2< (cos 2ff -i sin 2#./ )
o1 e-2 (cos 2P/ -i sin 2 /P )
When

( 7) 2B~a d // a9, A << g7
and retaining; only first order terms equation (6) becomes

(8) n e~Pf 1!


The absolute magnitude is




and when d ga 0

(10)~ in 0 c




-4-


At the quarter wave frequency the maEgnitude of Z, goes through a minimumm.

By plotting I~lversus frequency, one can find both the attention and the
quarter-wave frequency. A curve drawn from equation (9) is compared with ob-

served values of 1 in Figure IV. The agreement between observed and
calculated values of _Zi is quite good. The value of oC4 used in the equation

~was that found from the minima in the experimental data, J 5 0.016j9.

Apparatus

A block diagram of the circuit used to measure I1D is shown in Figure I-k.
The procedure of the experiment was set up so that c~ would be found from
two voltages that were easily measured, In Fig. I-B the signal generator is
treated as a Thevenin voltage source t). With the terminls open the potential
across than is

(11) V a VI
where V1 a Thevenin voltage. With the cable attached to the generator terminals
the Thevenin voltage is increased until the potential across the terminals is
the same as that in equation (11), Then

(12) V = i Zin
where i I current, and the ne~w Thevenin voltage is

(13) V2 (g & Zin)
where Zg is the generator impedance. Substituting i P = in equation (13)s
wre h~ave for the input impedance

(14) Zi,

At the quarter wave frequency (14) becomes

(15) Zo tanhoff V1 g
V2 "1


(4) George C. Southworth, "Principles and Applications of Wlaveguide Trnsmissionl
p. 26, 1950, D. Van Nostrand Co.









If the generator impedance is matched to the characteristic impedance of the

cable, Zo. "g, tanhoc/ Molf, then

(16) rl/ 1L

All that is necessary is to measure Vl1 and V2 in the region of W/ In the

signal generator used in these measurements, a General Riadio 605-B3, a calibrated

attemuator preceding the output terminal was used to measure the relative mgani-
tuesofVlan V.The signal generator imnpedance was matched to the cable by

a series resistor of appropriate value. The incidental shunting capacitance was

negligible at the frequencies used. The value of the series impedance matching

resistor was found by measuring the impedance of the generator and the character-

istic. impedance of the cable. Since the cable impedance was larger than the

generator impedance for both cables, the matching resistance was in series wc~ith

the generator impedance. The cathode follower, receiver, and VTVM shown in Fig.

LA were used to set the voltage V at a constant value. This was done by modulat-

ing the output of the signal generator; a small fraction of the r-f signal being

passed by the cathode follower to the receiver, a National HRO0-60, the modula-

tion component then going to the VTVEM. A cathode follower was used because of

its high input impedance, approxuimatelyr 100 megohms. The deflection of the

VT~M. was kept constant for the conditions of cable connected and disconnected

by varying the calibrated at-tenuator of the signal generator. The frequency

was measured by the receiver tuning condenser of the HRO-60 which ha~d been

calibrated.

The advantages of this method of measuring relative changes in e' and

attenuation are: a) with one meas~urement the relative magnitudes of e' and

-attenuation can be determined with an accuracy not achievable by other methods,

b) instruments and personnel can be shielded from the radiation source dur~ir






- 6-


irradiation of cables c) measurements can be made during irradiation, and d)

standard instruments can be used. The matjor disadvantages are: a) the cable is

not uniformly irradiated, and b) there is a temperature gradient along the length

of the cable. However, the conditions of irradiation are similar to those that

would exist if the cables were used in and around a radiation source.

Cable Measuranents

Using the above method changes in e' andolfin two coaxial cables have

been measured during irradiation in the ORNL graphite reactor. The cables were

inserted in Hole B of this reactor and the measurements made over a period of

approximately 700 hours (approx. 1.6 x 1018 nvt) for one cable and 900 hours

(approx. 2.1 x 1018 nvt) for the other. The one irradiated for the 700

hour period had teflon as the dielectric, the other had polyethylene. The

results for the twro cables are shown in Figs. II and III. It can be seen from

these figures that the effect of radiation on the "measured er is small, the

total observed change for both cables is approximately 1%k of the initial value.

The change in e' is proportional to twice the change in W6 i.e.,





The "measureddsf a for the teflon cable exhibited changes that were not outside

the range of error for theperiod or irradiation. The polyethylene cable had

a total change in the "measuredog n of approximately 95%.

La order to determine whether there were transient effects resulting from

variations in the flux around the cable measurements were made durng the weekly

shutdowns of the reactor. These were then compared to measurements made just

prior to shutdown and just after start-up. In the teflon cable no significant

change in e' and ocd oould be detected between the reactor-on and reactor-off






S7 -


conditions. In the RG-11/U cable there weJre observable differences in the value

of e' but none in or4 between the two conditions, These differences were much

smaller than the total change in e' for the period of irradiation. The change

ia e' for the two conditions seems to be correlated with the change in the

temperature of the irradiated section of the cable. This temperature effect

can be seenl by comparing adjacent points on the f i graph, Fig. III, for

which they was a change in the average temperature of the irradiated section

of the cable. The changes in e' are of the same sign as the changes in average

temperature ,

The average temperature is the average of the temperature of the pile inlet

air and outlet air. The air coolant flowed through the channel in which the

cables lay, the inlet air being at one end of the cable and the outlet air at the

other. Hence the average of the inlet and outlet air temperatures is an

approximate measure of the average temperature of the cable. Ait full pile power

the difference between inlet and outlet air temperature was approximately 7000.

The reason for speaking of measuredd e' and"measured o(Q n arises from

the conditions of the experiment. The entire length of the cables was not

irradiated. Approximately 6058 of the cable length was in the active lattice of

the reactor, the remaining LOf passed through the reflector and shield of the

reactor and to the instruments. In addition the flux distribution across the

active lattice is not uniform. It has a maximum flux of 9.2 x 1011 thermal

neutrons at the center, falling off to zero at the reflector. As a result of

these conditions the cables should be considered as having, to a first approxi-

mation, two dielec~trics after the irradiation began. Hence the "measured et n

and measuredd O(f are not those of the irradiated section but of the combination

of the unirradiated and the irradiated section. The "measured d&/ is simply

the arthnmetic smu of the. cl as- of the two sections. The "measured e' is a






- 8-


somewhat involved combination of the e's of the two sections. In view of the

small change in the "measured ec it seems reasonable to assume that the ob-

served change is approximately that of the irradiated section. Since 60;g of the

cable was .irradiated thre "measured oG0 is approximately 60% of the oL4 of the

irradiated section. Hence the change of 9;g in the measuredd oth" of the poly-

ethyene cable would be approximately lIf; change incK/ for the irradiated

section. These changes in e' and oC observed in the region of four megacycles

should not be extended to other frequencies. Somne preliminary measurrements on

teflon and polyethylene in the microwave region before and after irradiation in-

dicate that changes in e' may be of the same order of magnitude as at four mega-

cycles. -The increases in attenuation are much greater for shorter periods of

irradiation at microwave frequencies than those observed at the lower frequency.

Teflon(6) exhibits quite radical changes in its mechanical properties for

an integrated flux of 1016 nrt whereas the effects of radiation damage begin to

show up in polyethylene 7l) between 1017 and 1018nyt. In the above meaurmlements

of er and odithe cables were inserted in a horizontal hole in the pile and were

not moved until the end of the tests. Thus they were subjected to no mechanical

strains. In view of the small changes observed in er and oC (in the region of

four megacycles) in these two materials as a result of irradiation the criteria

of their usefulness in a radiation field would be the effect of this radiation

on their mechanical properties.

Acknowledgments

Th~e method used in these experiments was suggested by Mlr. B. R. Gossick.

We gratefully acknowledge his suggestion. Discussions with R. L. Sproull were

very helpful. We wish to thank hin for his interest.


(6) 0. Sisman and C. D. Bopp, "Phygsical Properties of Irradiated Plastiesns

ORINL-928, p. 88-92.

(7) op. cit., p. 78-82.









MODULATED R-F SIGNAL


MIODULATION COMPON


Z'=E IMdPEDANCE OF SIGNAL GENERATOR
R_ '=SERIES RESISTANCE ADDED TO
OUTPUT OF GENERATOR TO MATCH
CABLE IMPEDANCE, Z.


FIG. 1


CIRCUIT DIAGRAMS FOR MEASURING / AND a/








O0000
0000
N---


O
W


O



o

a








o
z

ad


O OO O OO O O OO
O OO O OO r- (9InW


(~~, )3tlnltlt13dYU31 ~


(salJ3obaut) ~3N3n~3tlJ 3~tlM-t131~VnD ,t~Y/




-ll-


coII'cNlll o/
nO O O OO
OOOOOOcc


O

O











OL

O



OI





-r


OOOOOOOOO
000000000
OOOO OOO


(30 ) 3tln1~~3dVU31 '1




-12-


0.0205


0.0195


O
















S0.0 1651
4.30408 .78 .904I8

fFEUN Y( eayls
FG IV










CABL INUT MPEANC vs FREQUENCY ABOUT fles






s13-


BIBLIOGRAPHY~


(1) 0. Sisman, C. D., Bopp, Physical Properties of Irradiated Plastircs, ORNL-928

(2) J. G. Burr, W. M. Garrison, The Effect of Radiation on Physical Prop3erties
of Plastic (1963) AECD-2078, (decl. 1948)

(3) A. 0. Allen, D, M. Richardson, Effect of Clinton Reactor Radiation on
Plastics, CNL-16 (1968)

(6) M. Burton, Effects of High-Energy R~adiatilon on Organic Compounds", J. of

Phys. Col. Chem., 51,86(9)

(5) J.'Gi W. yan, Effects of Riadiation on Organic M~aterials II, G-EL-97 (1952)

J, W. Ryan, Effect of Gamma Radiation on Certain Rubbers and Plastics,

Nucleonics, 71, No,? 8, 13-15 (1953)

(6) M~emo: s. Gandon, et, al, "Gamnma Irradiation of Insulating MaLterials",

ANL-0CS-7S (19L67)

(7) J. S. Horsman, Summry of NRII Pile Irradiations, NP-1922, 1950

(8) F. E. Faris, A Compendium of Radiation Effects on Solids, Vol. II, NA4-5R-26J1,

1953















"This bibliography is not, by any neans, complete. It is given only to indicate

some' of the reports available.




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