Radio broadcast .. (1922-30)

STRAYS FROM THE LARORATORY A Novel Wavemeler The picture in Fig. 1 illustrates an in- teresting and effective addition to the resonance type of wavemeter. This device, which was first described in the General Radio Experimenter, September, 1929, consists simply in a small fixed capacity which can be thrown across the variable tuning capacity by means of a push but- ton, and is useful in getting an accurate setting of the meter. (See Fig. 2). Suppose for example, as in Fig. 3, the variable capacity is equal to Ci so that the current is indicated at Ii. Then pressing the push button throws the fixed capacity into the circuit and makes the current equal to L .which, in this case, is numeri- cally equal to Ii. In other words, at one point on the variable condenser dial, push- ing the button will not cause a change in current. This point, Ci, is taken as the calibration point. At any other setting of C pushing the button causes a change in current. There is one disadvantage of this type of wavemeter—the tuned circuit is not exactly in tune with the circuit whose frequency is being measured. Therefore, the tuned circuit of the wavemeter has some reactance which will be reflected into the tuned circuit under test. This may change the frequency of the circuit with the result that the reading will not be accur- ate. This difficulty can be obviated by using very loose coupling, and by changing the coupling as the setting of the wave- meter is adjusted to the proper point. An International Broadcaster Down in Heredia, Costa Rica, "the center of America," is a patient amateur, Amando Cespedes Marin. For several years Sr. Marin, who has many medals for proficiency and artistry in photography, has maintained and operated a 7.5-watt amateur radio telephone station on 30.8 meters. Despite such lim- ited power, Sr. Marin's voice and music have been heard all over North, South, and Central Amer- ica and he has not only secured considerable favorable notice for his station but also for his city and his nation. For some time he has been making a plea for funds with which to enlarge his station and to increase its power. Contributions have been re- ceived in small amounts from many of his listeners in the United States and elsewhere and one en- terprising American manufacturer sent him a batch of tubes and a check for $50. Now the Costa Rican city, Heredia, has given him $150 and money for telephone lines to the municipal band stand. He has collected nearly enough to bring his power up to 150 watts which should fling his voice into the short-wave receiver of many hundreds of avid listeners. Con- C —^ Cj Cj Fig. 3 tributions may be mailed to Sr. Marin and will be appreciated. An Interesting Formula An interesting expression in the next column g ves the relation between stability in an r.f. amplifier and the circuit con- O o O o o o o ^-Q- .1 7T PUSH BUTTON Fig. 2 slants, such as plate resistance, coil re- sistance, grid-plate capacity, etc. The low- est resistance the tuning coil can have may be found by substituting the circuit con- stants in this expression at the highest frequency to be used, viz., 1500 kc. for the broadcast band. The circuit will be stable at all other frequencies. The plate of the preceding tube and the grid of the following tube are connected across the entire coil. Perfect shielding is assumed except that due plate-grid capacity. Cgp h) Gm = 2 where Cgp = grid-nlale capacity <•> = 2 x f Gm =•= mutual conductance in mhos r = high-frequency resistance of coil Rp = plate resistance of tube A "New" Recording System In October the public press in England was greatly excited by the report of a new invention, made in Germany, having an application in the talking motion picture industry and in the recording of sound for home entertainment. The method consists simply of impressing audio-frequency cur- rents on a pair of electromagnets between which an iron or steel wire is drawn at a constant rate. The wire is magnetised according to the voice modulation, and when the process is reversed tke sounds may be reproduced. The time interval be- tween recording and reproduction may be as short as desired. Some time after the disclosure of the principle and the general hurrah about it in the newspapers, someone discovered that the idea was not new at all but had been invented some thirty years before by Valdemar Poulsen, the Danish radio en- gineer of world-wide fame. This was then related in the papers and altogether there was quite a hullabaloo. We remember discussing this method of sound recording with Theodore H. Nakken several years ago. He had worked with it and found it a successful method provided the wire could be made to retain the "sound," which, at the time of the discussion, had not been found possible. Newspaper clippings about the affair were sent through the courtesy of Lawrence Corbett, formerly of the editorial staff of RADIO RROADCAST and now of London, England. Fig. 1 — General view of the new wavemeter developed by General Radio. Two New Booklets Research Papers Nos. 77 and 90 of the Bureau of Standards de- scribe, respectively "A Course- Shift Indicator for the Double- Modulation Type of Radiobeacon" and "A Comparison for the Cal- culation of the Inductance of Coils and Spirals Wound With Wires of Large Cross Section." The former is by Harry Diamond, radio engi- neer, and F. W. Dunmore, physi- cist, and the second by Frederick W. Grover, consulting physicist. • JANUARY 1930 151