TRANSMISSION LINES FOR RADIO SYSTEMS 447 



An experimental attenuation curve for a 600-ohm line comprising 

 0.162-inch iron wire conductors is given in Fig. 24. The current enter- 

 ing the line was approximately one ampere. The measured resistivity 

 for the iron is 12,300 e.m.u. The attenuation could be explained upon 

 the basis that the permeability is 92.2o This is not an unreasonable 

 value. In fact it may be very desirable to obtain the permeability of 

 iron at radio frequencies by forming the material into a transmission 

 line and observing the line attenuation and direct-current resistance. 



An iron wire line 1600 feet long has been in use at the Deal Labor- 

 atories for several years for the purpose of testing transmitters. This 

 line successfully dissipates 15 kw. at 20 megacycles. 



MIL Conclusion 



In conclusion a few remarks on the relative utility of open-wire and 

 concentric-tube lines may assist in selecting the most desirable con- 

 struction for a particular service. A definite discrimination between 

 the two is not readily made because the economics of the entire radio 

 plant are involved. 



Concentric lines are more costly than open-wire lines. On the other 

 hand, concentric lines permit the installation of a number of radio 

 units within a single structure without incurring difficulties from cross- 

 talk. The first cost and annual charges upon a compact installation 

 may more than offset the cost of the lines when compared with an in- 

 stallation comprising several widely separated structures. Also, con- 

 centric lines may be constructed so as to be weatherproof. 



There is little choice between the losses in open and concentric lines 

 provided that a reasonable degree of current balance in the open-wire 

 lines is maintained. In order to obtain balances the open-wire line 

 terminal equipment both at the antenna and at the radio unit ends of 

 the line must be carefully designed. The chief source of current un- 

 balance difficulties resides in couplings between the antenna and an 

 open-wire line. These may be materially reduced but cannot be com- 

 pletely eliminated. Another source may be unbalances with respect to 

 neutral at the radio unit. 



Complete isolation of the antenna from the line can only be obtained 



with shielded lines. Similarly, complete isolation from static and other 



noise sources for which discrimination by the antenna is obtained can 



only be effected by shielded lines. This is particularly important in 



reception. In this case small concentric-tube lines with losses as much 



as 2 db per 1,000 feet may be used provided that the noise level is 



reduced by a corresponding amount. 



-" P. P. Cioffi, Bell Telephone Laboratories, New York City, found an initial 

 permeability of 95 for a sample of the above wire. 



