164 BELL SYSTEM TECHNICAL JOURNAL 



This makes a total loss between subscribers of 27 units which is 

 satisfactory for a good talk under fairly quiet conditions. This 

 equivalent was usually realizable under the conditions of the test 

 and the majority of the calls put thru from local stations in New York 

 with the ship 100-200 miles (160-320 km.) out were successful despite 

 occasional spark interference in the radio circuit. 



The transmission loss is, however, too high in such a circuit to 

 enable it to be extended inland over long distance wire circuits. If 

 this is attempted, two limitations come into play. In the first place, 

 the volume of the talk becomes too weak. If the call were extended 

 over a toll circuit having a 10-unit equivalent, for example, the over- 

 all equivalent would become something like 37 units, which is ex- 

 cessive. This could be overcome to some extent by a cord circuit 

 repeater at New York. A second limitation which existed in the 

 experimental set-up resided in the unbalance between the line and the 

 balancing network at the radio station. This unbalance permitted 

 currents received over the radio link to be fed back thru the radio 

 transmitter of the land station, as described above. These fed-back 

 currents overload the radio transmitter if they are large compared 

 with the currents being supplied to the radio transmitter from the 

 shore subscriber. In other words, if there is sufficient amplification in 

 the shore transmitter to enable very weak voice currents arriving 

 over a line of high equivalent to load the transmitter fully, then the 

 transmitter is likely to be overloaded by currents which get through 

 the hybrid coil from the associated radio receiver. For these reasons, 

 the two-wire-four-wire circuit of Fig. 12 is not good enough for 

 extension over long distance circuits. 



The four-wire type of circuit which is suitable for long distance 

 land line connections is shown in the second diagram with repre- 

 sentative transmission equivalents. A brief comparison of the two- 

 wire and the four-wire circuits will make it evident why the four-wire 

 circuit gives the better equivalent. It enables the land line loss 

 between the radio station and the toll center to be more or less wiped 

 out, thus in effect placing the radio station electrically at the toll 

 center. Another way to express the situation is this: regard the 

 hybrid coil unbalance as the limiting factor, then assume that, while 

 holding to a given unbalance, the four-wire circuit (the loss in which 

 can be largely wiped out by one-way amplifiers) is extended inland. 

 The length of the remaining two-wire line back to the land sub- 

 scriber is thereby decreased and the ratio of the current received at 

 the radio station over the line as compared with that transmitted 

 across the hybrid coil thru unbalance is increased. It will be observed 



