Radio broadcast .. (1922-30)

STRAYS FROM THE LARORATORY Regarding Synchronized Stations FROM TIME TO TIME the public press is assailed by someone who has a newly dis- covered scheme for doubling up on the ether by transmitting the same audio-frequency program over several stations which are synchronized to the same r.f. carrier. Such a system is offered as a panacea for those who do not want to pick up the same pro- gram at more than one point on the dial—• as a panacea for those who wish to de- crease the number of stations now on the air, and as a panacea for those who want to increase the number of programs now on the air. Since there is such recurrent interest and speculation on this matter of putting several stations on the same frequency, it seems worth while to review briefly a paper read in March, 1929, before the Wireless Section of the Institution of Electrical Engineers (London) by Captain P. P. Eckersley and A. B. Howe. Until recently Captain Eckersley was chief engi- neer of the British Broadcasting Company, and he is well known among all serious and well-informed radio engineers. In 1926, 1927, and 1928, four British stations operating on low power and transmitting the same program were synchronized to within 100 to 200 parts in a million. Consider first the unmodulated carriers of two stations. If the stations were situ- ated close enough so that either carrier could be heard if the other were turned off, an interference pattern would be set up. Under these conditions a listener in a locality where the carriers come to him in phase will receive signals which may be twice as strong as those whii'h could be produced by either station, and a listener in a locality where the carriers are out of phase may receive nothing at all. Now if the carriers are modulated with the same single audio- frequency tone, listeners near either station will re- ceive the modulation undis- torted, but listeners n.idway between the two stations will receive "mush." In other words, turning on the second of two such stations restricts and reduces the service area of the other. Now suppose the stations send out the same pro- gram consisting of tones situated at various parts of the audio-frequency spec- trum. Because these side frequencies may not arrive at a listener's receiver in phase, even though the car- riers are in phase, the listener will get distorted signals. To test these and other possibilities, two stations, c5BG and G5rr, were tuned to 610 kc. and maintained there by transmitting a syn- chronizing signal of 305 kc. from a third station. This signal was picked up by both stations, doubled, and .used to drive the two transmitters. They were supplied with the same program by a 38- mile wire line which connected them. A portable field strength measuring set and a standard receiver were transported to 0-02 MMFQ VOLTMETER various localities in the field of the two stations. All types of distortion discussed above were found; in addition it was learned that if one station were five times stronger than the other, the program of the first would be received properly and without dis- tortion. If one station differed by five cycles in carrier frequency from the other, it would be necessary that the first station be ten times stronger than the second at a given locality in order to receive an umlistorted signal. If the two stations transmitted different programs but on exactly similar carrier frequencies, the strength of one would have to be from 100 to 200 times stronger than the other in order to receive undistorted programs. The difficulty in using the same carrier frequency for several programs, or for the same program, comes from the fact that the direct ray from the transmitter The apparatus pictured above is the new high-speed facsimile receiver developed by R.C.A. for transat- {antic service. is supplemented by the indirect ray re- flected from the Heaviside layer. At lo- cations remote from the transmitter the latter is the more important; it causes fading and distortion. It seems wise to use antennas which transmit poorly toward the sky and which confine their radiation more nearly to the ground wave. Such radiators are high vertical antennas. Captain Eckersley states that for large distances between stations, it is better to use low frequencies; for short distances it is better to use higher carrier frequencies. This is due to the fact that the direct ray falls off more with distance as the wave- length is decreased, while the indirect radiation seems to be more or less inde- pendent of the distance. Another interest- ing result of the author's experiments leads to the statement that when 6 or 7 stations share the same frequency, their service areas are not affected by the addi- tion of other stations. The problem seems to resolve itself into several phases: (1) to provide accurate synchronism between stations either by transmitting a standard radio-frequency signal from some centrally located station, or by means of land lines; (2) to restrict the radiation as much as possible to the ground wave; and (3) to choose properly the location and power of the stations shar- ing the common frequency. Measuring Screen-Grid Capacity Measurement of the extremely small capacity existing between plate and grid in a screen-grid tube is a difficult problem. Comparative measurements are not diffi- cult to make by putting the tubes across tuned circuits and measuring the change in frequency, but to get the capacity in exact units involves a standard of the order of 0.02 mfd. In Experimental Wireless (England), June, 1929, the following method is put forth as a way out of the difficulty. It in- volves putting another and larger condenser in series with the desired capacity, and measuring the voltage across this large capacity when a known voltage is put across the two capacities in series. Thus in Fig. 1 the voltage is shared by the tube capac- ity and the large known ca- pacity, which is about 10* times as great as the grid- plate capacity whose value is desired. A potential at 1500 kc. of about 600 volts was gener- ated in a tuned circuit. This voltage was measured by means of an electrostatic voltmeter and applied to the two condensers in series. The capacity is found from p Cg.p = C s ^r since C s is about 10 4 times Cg. P . DECEMBER 1929 • 101