Radio Broadcast (May 1929-Apr 1930)

Design and Construction Data A SUPER-SENSITIVE SHORT-WAVE CIRCUIT By THOMAS A. MARSHALL Office of Fleet Radio Officer, Pacific Fleet, U. S. N. THE short-wave receiver described in this article has several unique points of interest and advantages which clearly distinguish it from the commonplace. The circuit, in its most sensitive form, consists of a single-stage tuned radio-frequency amplifier followed by an autodyne detector and a two-stage transformer-coupled audiofrequency amplifier. In the radio-frequency amplifier and detector modified push-pull circuits are used because of the ease in generating oscillations and because of the low circuit losses on frequencies up to as high as 80,000 kilocycles (3.75 meters). The conventional types of receiver circuits as developed in the past have been incapable of giving amplification in the upper frequency bands due to the relatively low input impedance of the tubes and the relatively low L/C ratio. The low input impedance is due to the relatively high grid-to-filament capacity which, under actual operating conditions, may be several times the geometrical capacity. The capacity between the grid and filament markedly affects the input impedance which fact is of importance in determining the input power and the signal voltage impressed on the tube elements. The input impedance of the tube may be represented by a capacity with a high resistance in series. This resistance causes the absorption of power in the input of the tubes to become very high. In Fig. 1 the value of Rg in ohms is very small compared with the reactance of Cg in ohms. Neglecting the slight effect of Rg we can say tbat the current will divide between the tuning condenser, C, and the parallel circuit, Cg, in direct proportion to the capacities of C and Cg. The current, Ig, flowing to the grid is obtained by the formula: X I C + Cg The power, Pg, dissipated in Rg may be calculated by the formula: Pg = Ig2 Rg The power dissipated in the push-pull circuit, as shown in Fig. 5, is approximately one half the value calculated for the circuit shown in Fig. 2. Features of Push-Pull FIG. 2 shows a three-electrode tube and its associated circuits as used in single-tube circuits. The points G, F, and P represent the three electrodes, the grid, filament, and plate. The capacities between them are represented by Ci, C2, and C3. Fig. 3 shows a similar tube and associated circuits as used in a push rWWWV This article describes a receiver that is more sensitive than the average short-wave set. The author has built a number of such receivers for the Navy where they enable the operators to get down to much weaker signals than with conventional receivers. The model of the set illustrated schematically on these pages was built by Herbert M. Isaacson of the QRV Radio Service. His conclusions, after constructing and operating the receiver, are that it is more sensitive than usual receivers for the high frequencies, and, although not adapted for the usual amateur traffic, it would "prove in" where an operator desired to communicate with a few transmitters whose positions have been located on the tuning dials, and then charted. — The Editor pull receiver, Fig. 5. The inter-electrode capacity, Ci, and the grid resistance, Rg, are in series across the input circuit. Thus, the capacity is halved while the resistance across the input circuit is doubled, giving a much lower conductance for the input grid circuit. The same arrangement of capacities and resistances applies to the output or plate circuit. In ordinary circuits, as shown in Fig. 2, the fixed capacity, Ci, across the coil system Fig. 2 Fig. 1 restricts the tuning range. The circuit as shown in Fig. 5, due to reduced inter-electrode capacity, permits a relatively large L/C ratio, thus giving considerably more inductance than the single-tube circuit. The increased value of inductance with which to couple the tickler feed-back gives stable oscillations over the entire range of frequencies. By the arrangement as shown in Fig. 5, the intra-electrode tube capacities are reduced by using the two split condensers, Ci, which are in series and by connecting the two tubes so that each grid-to-filament capacity is across one of the series sections. The total effective tube capacity upon the tuned circuit is halved. The two tube grid-to-filament circuits are across each half of the tuned circuit input voltages which decreases the grid-to-filament conductance to half value for each tube. Since the two reactances are in series the total conductance across the tuned circuit is one quarter the value of that in the usual circuit. It is, therefore, quite apparent that the input impedance of the new circuit is increased, making it possible to maintain a much higher signal-voltage potential across the tuning condenser terminals. The circuit has been operated successfully in the 10-meter hand. Reception of second harmonic values from distant stations such as hug, wik, wekq, npg, and wgt have often been accomplished during day time. Kwe on approximately 1,600 kilocycles may be heard on 32,000 kilocycles, and wik on 13,930 kilocycles may be heard on 27,860 kilocycles which is very close to the upper limits of the 10-meter band. The circuit does not radiate energy due to the employment of the tuned radio-frequency amplifier and to the minimum antenna coupling. The antenna circuit within the receiver consists of a small series inductance, L, which is coupled to the amplifier tuned input inductance. These inductances are wound with fixed relationship to one another so that the calibration of each coil system will remain fixed. The receiver may be operated from an antenna of any length from a few feet up to several wavelengths, such as a Beverage system. A doublet type of antenna or a directional loop may be used. European stations may be heard by using a single-turn loop about one foot square and connected directly to the coil jacks. The loop coil will act as an antenna and inductance system. Fig. 4 shows the circuit for loop reception. Note that the circuit preserves symmetry which is so essential to efficient and stable operation of a loop system. Due to symmetrical operation of the loop system, no compensating capacities are required to keep the loop balanced capacitively on each side of the earth. The second inductance, Li of Fig. 5, is connected to the tuning condenser, Ci, and feeds to the grids of the two tubes through the coupling condensers, C2 and C4. The grids obtain a negative bias through the onemegohm leaks, Ri and R?, which are connected to the battery end of the resistance, R6, thus giving a bias potential of approximately 1.6 volts which is obtained by utilizing the voltage drop of the series resistance. Special Apparatus Used THE condensers C10 and C12 are shown as having two series sections and a common rotor. This device may be made by cutting the bus bar which holds the stator, thus separating it into two parts. The rotor is connected to the plus filament and is grounded, in this way eliminating hand effect in tuning. It will be observed that the grid coils have no center connection. These coils are permitted to find their own electrical center which may be different from the apparent center due to electrical irregularities existing in the circuits. The choke, Le and L7, isolates the junction • may, 1929 page 43 •