Radio Broadcast (May 1928-Apr 1929)

362 RADIO BROADCAST OCTOBER, 1928 + 180 ORDINARY COIL (C) also actuating a very sensitive vacuum tube voltmeter. By this means the input signal could be kept constant regardless of frequency variation. The output of the coil under test was led to a second vacuum tube voltmeter whose input characteristics were similar to those of a typical detector circuit. A second transformer, having the same flat amplification-against-frequency characteristic already mentioned, was used as a standard of comparison for plotting figures of merit on all couplers used. This test was given to coils of sixteen types, as many of each type being tried as were deemed necessary to determine their worth. Inasmuch as the reproduction of all these curves here would only lead to confusion, due to their number, and would serve no particularly useful purpose, we will show the results obtained in those most usual types which had a bearing on the final result. However, as a matter of information, it may perhaps be advisable to outline roughly the types involved and the major reason for their abandonment. All coils were of the general type illustrated in the photograph on page 361, having 77 turns of wire in the secondary circuit, air-spaced to conform to an approach to the ideal shape factor and supported by a skeleton bakelite frame, so that the insulation losses are kept at a minimum figure. The self-inductance of the secondary alone was 167.4 microhenries and the radio-frequency resistance of the coil in series with a Cardwell condenser varied from 3.85 ohms at 550 meters to 9.6 ohms at 200 meters. These figures are meaningless in direct relation to almost all of the actually tested coils, as the introduction of a primary coil, or the use of a portion of the secondary coil for coupling, have a decided effect on both the inductance and the highfrequency resistance of the secondary. Among the types tested were: 1. A tuned impedance, directly from plate and grid to ground. Fig. 2-A. 2. An auto transformer, in which a portion of the secondary is used as primary, the low potential ends being common. Fig. 2-D. + 90 +180 1:1 TRANSFORMER <B> + 90 + 180 AUT0F0RMEU (D) FIG. 2 3. A transformer in which the primary and secondary are coupled by a bypass condenser at the low potential ends, the direction of the winding being continuous from plate to grid, and the coil being tuned from plate to grid, as in the R. B. Lab. circuit and Betts circuit adaptations. 4. A primary wound to take up a length of if", inside the secondary. Fig. 2-B. 5. A primary wound to take up \" , inside and in the center of the secondary. 6. A primary wound to take up \" , inside and opposite the low potential end of the secondary. Fig. 2-C. 7. A primary wound with a length of \" , placed in both positions above described. 8. A primary wound in a 3V slot, coupled adjustably to the secondary. 9. A primary wound on the same diameter as, and at an adjustable distance from, the secondary. 10. A tuned primary with adjustable coupling to the secondary. Each was tested with a varying number of placed placed 18 ~18~ 36^ J_6 y •* s y 9 '9 --.Avtoformer — '/i" Primary — l'/t Primary » 200 250 300 350 400 450 WAVELENGTH IN METERS 500 550 FIG. 3 The numbers on the curves refer to the number of turns used in the primaries of the coils that were tested primary turns, and where possible, with varying degrees of coupling, as well as with primary windings in both directions in the first few tested. Empirical analysis previous to the test had led us to believe that where capacitative coupling existed between the plate circuit of the preceding tube and the grid circuit of the following tube, the voltage in the secondary due to this coupling would be in quadrature with that generated by the inductive coupling when the winding was continuous in direction from plate to grid, and hence would reduce the amplification obtainable, although probably flattening the curves somewhat due to the change in relative energy transfer by capacity and inductance at varying frequency. This was borne out in the first few curves, and since the object of the test was to secure the highest possible amplification throughout, the balance of the tests was made entirely with the windings in opposite directions. RESULTS IN FIG. 3 curves of amplification are shown on * three windings in types 2, 4 and 6, and will be discussed at more length later. Type 1, illustrated in Fig. 2-A, generally advocated for use with the screen-grid tube, showed no greater amplification than several other types, and was pronouncedly poor in selectivity. Type 3 gave beautifully flat curves, but the amplification was low, as only a portion of the built up voltage was impressed across the grid and filament of the following tube, and the selectivity was rather poor. Types 7 and 8 appeared desirable from some angles for particular purposes, but in general were not considered as useful as the standard types. Due probably to the large distributed capacity of this type of winding, decided resonance peaks were obtained which varied in their amplitude with the degree of coupling and number of turns used. When a coupling was adjusted to the optimum degree where only three peaks were observed in the broadcast spectrum and a merit figure of 47 to 5^ was obtained, the selectivity was very poor, and when either coupling or self-inductance was so adjusted as to allow appreciable selectivity, variations in amplification as high as 50 per cent, wereunavoidable. It seems therefore evident that this type of winding is not as a rule desirable. Type 10 gave extremely good results, but was impractical for use in receivers because each stage would require three controls, two for tuning and one for coupling, each of which required adjustment for every frequency change. Types 4 and 6 are very commonly used, and hence we have selected them for detailed presentation in connection with type 2, which was finally adopted as best. Type 4, illustrated in Fig. 2-B and generally advocated for use with the screen-grid tube when the tuned impedance arrangement is not employed, is shown on the chart of Fig. 3 in dot-dash lines, and it will be noted that after the number of turns increases to a certain point, no further increase in amplification is obtained, and hence the i:i ratio which has been recommended is not only unnecessary, but undesirable, since the selectivity, poor at all times with this type of construction, is very bad when more than eighteen turns are used. Type 6, illustrated in Fig. 2-C, is the type most commonly used at the present time and the results indicate that if conventional circuits are to be employed, it is considerably superior to any of the others tried. It is at least as good in amplification as the widespread primary, with 600