Radio Broadcast (May 1928-Apr 1929)

A Receiver for ShortWave Broadcast Reception By BERT E. SMITH VT 2 ■HP VT 1 R2 GROUNO ANT. VT 3 A COMPACT RECEIVER FOR SHORT-WAVE WORK 1 1 HIS short-wave receiver uses a screen-grid -*■ tube preceding a conventional oscillating detector and a two-stage, transformer-coupled audio amplifier. In the Laboratory it was possible to hear code stations which were inaudible without the screen-grid tube. This means that this tube not only acted as a blocking tube to prevent the detector oscillations from getting into the ether, but contributed some amplification as well. The receiver can he operated from a conventional plate supply device provided it is well filtered. On the very short waves — 20 meters and below — any electrical noise, which may be a.c. hum, or the spark-plug system of passing trucks disturbs the receiver. A 45-volt B-battery for the detector plate circuit will give much quieter operation and enable the listener to get down to the bottom oj the signal level. At the present time it is not feasible to use a.c. tubes on a short-wave receiver. — The Editor. IN THE design of a short-wave receiver for broadcast reception, several factors must be considered in order to produce a satisfactory product. 1 . The receiver must be essentially non-radiating. Due to the surprising distances which may be covered by short-wave transmitters with a limited amount of power, it is essential that little or none of the high-frequency oscillations generated locally by the receiver shall reach the antenna, for otherwise, the ether would be filled with an annoying congestion of squeals and howls. 2. Adaptability to phone or c. w. operation. This requirement applies principally to the type of audio amplification employed in the receiver. It has been customary in receivers for c. w. operation to employ transformers having little amplification of the bass notes because c. w. signals are usually heterodyned to a high-pitched whistle and very low grade transformers are adequate for the amplification of the signals, although they amplify but little the low frequencies now transmitted by good broadcasting stations. }. Smooth oscillation control without extraneous noises. This requirement will be discussed more fully and is very important, due to the fact that many "noise producing" features of a design which are completely negligible in the broadcast band, assume astounding proportions in the vicinity of twenty to thirty meters. 4. Ease of operation. It is quite important that a short-wave receiver should be as easily controlled as the average broadcast receiver so that the operator may not be forced to learn new procedure and new methods. 5. Adequate range of wavelengths. Short-wave broadcasting stations have not yet assumed a permanent status because they all have experimental licenses, and it is important that the receiver should be adapted to cover a rather wide band of frequencies. USE OF THE SCREEN-GRID TUBE IN ORDER to limit the radiation of the re* ceiver, the screen-grid is at once considered as a new contribution to the short-wave receiver. 160 150 140 130 120 110 l6o 90 ; so ' 70 60 50 40 30 20 10 0 ""Co il So t 1 O a No s ~~c I No 2 Coil No 1 100 90 80 70 60 50 40 30 20 10 0 SCALE DIVISIONS FIG. I 167 The insertion of this tube between the antenna circuit and the oscillating tuned circuit of the short-wave receiver will limit the transfer of energy from the tuned circuit to the antenna, due to the extremely low grid-to-plate capacity of the 222 type of tube. It was the original intention as the plan of this receiver was made to use the screen-grid tube as a radio-frequency amplifier with a tunedgrid circuit coupled to the antenna, but the idea was abandoned for two reasons; the first being that the tube is not strictly a non-oscillating one, and when connected with tuned circuits in the grid and plate, they must be adequately and carefully shielded; also, plug-in coils must be used in order to cover the necessary band of wavelengths, and to have shielded these circuits would have made necessary the removal of two shield tops and the replacement of two coils for each change of wave band. In addition, there is the fact that the tube possesses not zero, but an appreciable grid-to-plate capacity, which causes a disagreeable "hang-over" effect in regeneration resulting in lack of control when there are two controls to handle. It was found experimentally that a low distributed capacity r.f. choke coil served very well as an aperiodic input circuit to the screen-grid tube. This choke is shown at Li in the diagram, Fig. 2. 1 1 was also found that by the use of this connection a considerable degree of amplification was produced by the screen-grid tube as against coupling the antenna directly to the coil, La, as would be done in the conventional radiating type of receiver. This amplification was "velvet," since all we had hoped from the screen-grid tube was a blocking action, keeping detector oscillations from reaching the antenna circuit. As connected in Fig. 2, the screen-grid tube also contributes to the ease of operation by the elimination of the so-called "holes" in the tuning range of the conventional short-wave receiver. These "holes" are due to the fact that when the set is tuned to the natural wavelength or submultiples of the natural wavelength of the antenna, sufficient energy is subtracted from the tuned circuit to cause the detector tube to cease oscillating, whereupon the antenna coupling must be reduced and again increased as the "hole"