Radio Broadcast (Nov. 1925-Apr 1926)

How a Portable B Battery Transmitter Works Details of Actual Operation of a Forty' and Eighty -Meter Transmitter Using B Batteries for Plate Supply — How the Record of Twenty' Six Thousand Miles Per Watt Was Attained By THE LABORATORY STAFF STATION 9ECC AT MINNEAPOLIS Note the chemical rectifier on the floor and the businesslike transmitter arrangement W; 'ITH the approach of the International Tests and receiving conditions in Garden City nothing to brag about, it became necessary to make arrangements for an expedition to the wilds of some "blooperless" land. The problem of communication with Headquarters was a serious one indeed, for there was no telling where the expedition might lead. There seemed but one solution — amateur radio. What the Test committee needed was a light, easily portable transmitter and receiver that could be transported with its power to points unknown with the assurance that pressing the key in the wilds would awake activity in Garden City. For some time the Experimental Station 2 gy operated by Radio Broadcast Laboratory and the National Carbon Company had been the scene of much activity on short waves and low power. A circuit described in the January magazine by Niklaus Hageman had proved quite successful but secured a perfectly steady note at the expense of greater plate currents, and for hauling a transmitter to distant points, a foolproof, and especially a very economical circuit was essential. The transmitter shown in Fig. i is the result. The circuit is the simplest possible, the Hartley, and is shown at Fig. 2. In construction or operation there is nothing easier. A coil made of No. 8 soft copper wound on a dry cell tube and allowed to expand until the required diameter of four inches is reached constitutes the closed circuit inductance the ends of which are attached to the grid and plate of the oscillator tube. For the 40 and 80-meter bands, 10 turns are required when a .00025-mfd. condenser is used for tuning. Another coil of four turns of the same construction couples the antenna to this circuit. So much for the construction for the present. Let's imagine for a few minutes that you are sitting out in the radio shack at 2 gy with the operators, about to try the new transmitter. It is to get its power from batteries; storage battery to light the filament of the tube, B batteries for plate supply, for it is obviously impossible to carry generators to the country and who can predict the kind of power available from the lighting mains of Podunk? It is 8:00 p. m. on the night of December nth and Operator Mann (Mn) is at the key with Bob Blanchard standing by to throw switches and be generally helpful. The air is filled with 40-meter stations and among them is 9 dde, John Wilcox, Chicago whom we call without result. Then at 8:15 we send out a " cq " which is answered by 9 ecc, Floyd E. Wilkins, at Minneapolis. At that time the transmitter tube, a ux-210, was drawing 22 milliamperes at 400 volts. For an hour 2 gy conversed with 9 ecc, reducing the power with the following results: CURRENT 22 16 6 3.8 400 300 130 90 4.8 .78 .342 SIGNAL STRENGTH R4 R3.5 R3.3 R1-R3 Not bad for a start! 1000 miles on .342 watts — less power than is taken by the average receiving set. On the 15th at 9:32 p. m. E. S. T., the same stunt is repeated. 2 gy connects with 8 bzk, Paul Roth, Cleveland, using 40 mils, at 500 volts and power is reduced as follows: adcast Photograph FIG. I Transmitter receiver and B battery compartment complete. Space is provided for ten standard receiving B batteries totalling 450 volts. The throw-over switch is unnecessary if an additional antenna is provided for receiving. The same batteries can be used on both transmitter and receiver if proper precautions are taken