Radio Broadcast (May 1928-Apr 1929)

A Non-Radiating ShortWave Tuner RADIO BROADCAST has described in past months a number of interesting short-wave receiver units of various designs. The unit described here, a product of the National Company, contains no audio system — which makes it applicable to any audio system whether a part of a standard receiver or not. The set is non-radiating, and due to the isolation of the radio-frequency stage from the detector circuits, the tuning points as noted on the dial do not vary with antennas of varying lengths. The antenna circuit is choke-coupled to the screen-grid tube. A "picture diagram" of the set and most of the constructional data have not been included in this article because a blueprint of the hook-up and layout and constructional data are available through Radio Broadcast or direct from the National Company. — The Editor. N( rOW that short-wave broadcasting has passed through its early experimental stages and reached the state where reliable reception of good quality programs is readily obtained by means of easily constructed and inexpensive receivers, a great many readers who in the past have confined their efforts to the construction of radio receivers for use on the regular broadcast band, desire to build a good short-wave receiver. Aside from the mere fun of building a "different" type of radio set, there is that thrill of receiving understandable programs from distant and foreign stations. With a short-wave receiver, distance takes on an entirely new meaning. It is not uncommon to receive broadcasting from ane at Java, 3L0 at Melbourne, Australia, 5SW at London, pcjj in Holland, and many others; and static and fading are frequently entirely absent when reception on the regular broadcast band is exceedingly poor. T THE DESIGN OF THE RECEIVER HE National Screen-Grid Short-Wave receiver comprises several interesting features. One is the single tuning control. Another is the foundation unit design which permits an efficient layout of parts, with but a few connections to be made by the assembler. As a result of the 222 type tube in the first stage, the sensitivity of the receiver in general is materially better than that of the plain regenerative detector type formerly so much in use. Furthermore, the use of the 222 tube ahead of the essential regenerative detector prevents radiation — a problem which would soon become quite serious if all the short-wave receivers were of the radiating variety. Still another important advantage secured by the use of the 222 tube as in this receiver is the elimination of tuning "holes," or dead spots commonly encountered with plain regenerative receivers. Although heretofore rather carefully placed shielding has been SYMMETRY AND SIMPLICITY IN THE FRONT PANEL considered essential to a receiver using the 222 tube, the use of the untuned antenna circuit employed in this screen-grid short-wave receiver makes shielding unnecessary. The elimination of the shielding not only reduces the cost of parts and simplifies the work of construction, but also makes it a simple matter to change coils when going from one band to another. To cover the band of from 15 to 115 meters (20 to 2.65 megacycles) four interchangeable transformers are used. These transformers differ in a number of respects from the conventional "short-wave coils" with which everyone is familiar. In the past it has been the general practice to employ coils of fairly large diameter — usually about 3 inches or so. As a result, all but perhaps the 100-meter coil would have a diameter much greater than its length. It is a well known fact that the most efficient coil is one having what is known technically as "unity form factor," or in other words a length of winding equal to the diameter. By using a coil diameter smaller than customary and at the same time varying the spacing between turns and size of wire, a coil of high efficiency for each band has been developed. FIG. I. UNDERNEATH THE SUB-PANEL 286 In addition, in order to secure a high mutual inductance between the primary and secondary of the r.f. transformer without unnecessarily high capacity coupling, the primary or plate coil is wound of very fine wire located between the turns of the secondary or grid coil. The tickler winding in each instance is located in a slot at the low potential end of the transformer. One of the most essential and most neglected features of a good short-wave inductance is rigidity. Without rigidity any slight vibration or jar in the room where the receiver is being operated will result in unsteady signals. Also, such coils will not stand up under continual handling, with the result that stations are seldom received from time to time at the same dial setting. In the case of these coils, such difficulties are entirely overcome by winding the transformers on threaded micarta tubing, and soldering the ends of each coil directly to the special one-piece contacts located around the bottom of the tube. While some readers may think that, the use of such a micarta tube would increase the losses in the coils by a noticeable amount, such has been found not to be the case, as the dielectric is located in the weakest part of the magnetic field of the coil. Another interesting feature is; the special tuning condenseremployed. This condenser,, while resembling in general appearance the standard National Girder Frame condenser, is one designed especially for use in short-wave receivers. In the first place, it has a straight frequency line characteristic, so as to make tuning equally easy at both ends of the dial. A further arrangement for facilitating tuning is the 270 degree rotation, which spreads the stations over 50 per cent, greater dial' range than if the standard 180-degree rotation had beenemployed. The double plate spacing that will be noted from the illustrations, is employed to "smooth out" any slight irregularities in the characteristic curve of the condenser, which, while not ordinarily detectable at broadcast frequencies withi