134 BELL SYSTEM TECHNICAL JOURNAL 



antenna (in this case a loop) and, therefore, the measured power of 

 the signal can be translated directly into the field strength of the 

 received waves. 



The measurement tone signal is transmitted from the Rocky Point 

 sending station by substituting for the microphone telephone trans- 

 mitter a source of weak alternating current of about 1/100 watt at 

 a frequency of approximately 1500 cycles. This tone modulates the 

 radio telephone transmitter in the same way that voice currents 

 would and is radiated from the antenna as a single-frequency wave 

 of 5260 meters (57,000 cycles per second). It, therefore, constitutes 

 a means of sending out a single-frequency continuous wave for 

 measurement purposes. At the receiving end this continuous wave 

 is demodulated to the same tone frequency which it originally had. 



For measuring the strength of the received noise, i.e., the radio 

 frequency currents arising from static or other station interference, 

 the method is quite similar. In this case, however, the noise received 

 is so different from that which can be set up artificially in any simple 

 manner that no attempt is made to compare it directly with a local 

 noise standard. Instead the volume of the interfering noise is expressed 

 in terms of its effect in interfering with the audibility of a local tone 

 signal by measuring the local signal which can just be definitely 

 discerned through it. This is a threshold type of measurement 

 which is necessarily difficult to carry out with accuracy. In order to 

 increase the sharpness of definition of the local signal and to make 

 it correspond more closely to speech reception the signal tone is 

 subjected to a continuous frequency fluctuation. The comparison 

 signal has therefore a warbling tone which occupies a frequency band 

 not unlike that of the voice. This method of measuring the inter- 

 ference is discussed in more detail also in the measurement paper 

 referred to above. 



Procedure in Making Transmission Measurements. The three 

 quantities which are included in the transmission measurements, 

 namely, the signal strength, the noise strength, and the percentage 

 of words received correctly, are observed one after another in what 

 might termed a unit test period. Although the duration of this test 

 period and the order of making the measurements has been changed 

 somewhat during the course of the experiments, the following pro- 

 gram is representative of the conditions under which the data pre- 

 sented below were taken. 



A 25-minute test period divided as follows: 



5 minutes of tone telegraph identification signals (for receiving 

 adjustment purposes). 



