884 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



I. INTRODUCTION 



A recent theoretical i)aper' by A. M. Clogston presents the very 

 interesting disc(j\ery that luider certain conditions skin effect losses in 

 the conductors of a transmission line at elevated frequencies can be 

 much reduced by laminating the conducting surfaces, parallel to the 

 direction of current flow, with alternate thin layers of conducting and 

 insulating material. The requirements are that the thickness of each 

 conducting layer must be considerably smaller than the skin depth in 

 the conductor, and the phase ^•elocity of waves on the transmission line 

 must be held very close to a certain critical value, which depends on the 

 relative thicknesses and the electrical properties of the conducting and 

 insulating layers. Under these conditions the "effective skin depth" of 

 the laminated surface is greatly increased; in other words, the eddy cur- 

 rents induced by a high-frequency alternating field will penetrate much 

 farther into such a laminated structure than into a solid conductor, with 

 consequent marked reduction of ohmic losses in the metal. The metal 

 losses can also be made to vary much less with fretiuency, over a fixed 

 band, than the ordinary skin effect losses, which are known to be very 

 nearly proportional to the square root of frequency. 



Clogston goes on to show that a laminated material composed of 

 alternate thin conducting and insulating layers may itself be regarded 

 as a transmission medium. For example, if the space in a coaxial cable 

 which is ordinarily occupied by air or other dielectric be filled with a 

 large number of coaxial cylindrical tubes which are alternately conduct- 

 ing and insulating, the cable will propagate various transmission modes, 

 and under the proper circumstances some of these modes will exhibit 

 lower attenuation constants than the transmission mode in a conven- 

 tional coaxial cable of the same size at the same frequency. 



Experimental verification of Clogston 's theory of laminated conductors 

 has been obtained- at the Bell Telephone Laboratories, and the trans- 

 mission properties of a line filled with laminated material have also been 

 measured at these Laboratories and found in reasonable agreement with 

 theory. However experiments with structures as complex as those pro- 

 posed by Clogston are by no means simple, and the experimental work 

 on laminated conductors is still in an early, exploratory stage. Inasmuch 

 as the experiments are necessarily time-consuming, it has been thought 



1 A. M. Clogston, Proc. Inst. Radio Enqrs., 39, 767 (1951), and Bell System Tech. 

 J., 30, 491 (1951). References will be to the Bell System Technical Journal article, 

 although except for equation numbers the two papers are identical. 



2H. S. Black, C. O. Mallinckrodt, and S. P. Morgan, Jr., Proc. Inst. Radio 

 Engrs., 40, p. 902 (1952). 



