Radio broadcast .. (1922-30)

In the R. B. Lab. 909 LOOP UL FILTER 0 PRI A i T PICKUP COIL ^ .00035 MF LOOP fT) 'o ' f / 1 ST I & DET. \ j5^ 1 m FILTER § ^y PRI. 2 o 7 TUBE 0 _| Q *•§•• EXTRA BE TURNS _ r\r\(\ T PICKUP COIL FIG. 2 "A" shows the usual loop connection in most superheterodynes. "B" indicates the additional turns and condenser which cause regeneration in the first detector tube. This arrangement is particularly advantageous on ox reception LOW LOSS COILS AND THE ROBERTS SET FIGURES 3, 4 and 5 illustrate the manner in which standard three-circuit low loss coils can be adapted to the Roberts Knockout circuit. In principle, the adaptation consists merely in supplying a neutralizing winding to the radio frequency output circuit. Either one or two low loss tuning units can be employed. If one set of coils is obtained, the substitution is effected only in the r. f. and tickler circuit, the usual spiderweb or similar antenna coupler being unchanged. With two units the complete system is made low-loss, from the antenna through to the audio output. The units employed in experiments in The R. B. Lab were the " Lopez Low Loss Tuners." The same directions and manner of procedure hold good for other types. USING ONE UNIT N SUBSTITUTING a single set of coils for the usual "N, P, S, and tickler" inductances, the primary of the low loss tuner arrangement is used as the transformer primary (P) and is placed in the plate circuit of the first tube. However, it is first necessary to wind either alongside or on top of the primary coil, the neutralizing winding. This consists of one more turn than the primary, wound in the same direction, with any convenient wire, such as No. 24. The beginning of the neutralizing winding is connected to the end of the primary, thus giving a common tap to the two coils, which is connected to the plus side of the B battery. The remaining terminal of the primary is wired to the plate of the r.f. tube and the end of the auxiliary winding to the neutralizing condenser. The secondary and tickler coils are connected in the usual manner. USING TWO UNITS WHEN a duplicate set of coils is employed, the procedure is slightly different. The antenna coupler is formed by removing the tickler coil from one of the units, thus leaving primary and secondary. These two remaining coils may be remounted in numerous ways that will suggest themselves to the experimenter. The tickler that has been eliminated from the first unit, is now substituted for the primary of the second unit, and a neutralizing winding wound upon it as already described. The number of turns on the tickler are generally more suited to the transformer primary requirements, than the primary designed for the antenna circuit. Using these coils, the primary taps are eliminated, as the antenna primaries are generally of the semi-aperiodic type. The ground, in addition to running to the lower end of the primary, should be connected to the minus side of the A battery. Figs. 3, 4, and 5 show how the coils used in this laboratory were mounted. Low loss coils are generally more bulky than the less efficient inductances, for which reason their disposal on the panel presents more of a problem. The shaft to the transformer primary (the upper coil in Fig. 4) has been removed, as there is no occasion for varying the coupling between the primary and secondary, and it is secured permanently by means of a metal strip. Other forms of winding, such as the diamond weave and spiderweb may be satisfactorily substituted tor the basket weave coil illustrated.