Radio Broadcast (May 1928-Apr 1929)

190 RADIO BROADCAST JANUARY, 1929 overlaps that of the other tube. Diagram c of Fig. 2 is an oscillogram of one tube operating for a period of more than one-half cycle. PUSH-PULL FEATURES THE outstanding feature of the push-pull radio-frequency arrangement is less manifest than that of the back-to-back device. European writers consistently inform us that they are able to produce oscillations of a higher frequency and better stability with such circuits than with single tubes. I am quite unable to find similar results in either regard, nor do I see any theoretical basis for the belief. It is, of course, quite possible that the use of series feed via a center-tap connection may result in smoother tuning over a range of frequencies, because one avoids the troublesome natural frequencies and varying reactance of the feed choke. However, this is beside the case since the push-pull (or balanced) circuits have no monopoly over this method of feeding. The pushpull circuits can be shunt fed as suggested by the dotted lines in diagram d of Fig. 1, and, on the other hand, single-tube circuits can be fed without chokes by using a neutral connection of the tuning system as the feed point. For example, in Fig. 3 we have the one-tube Hoffman version of the Colpitts circuit in which one avoids both plate chokes and grid-leak losses. Incidentally, this circuit escapes the tiresome tendency of the push-pull systems to generate two frequencies near each other but unfortunately not quite the same. Short-wave Tuner and Adapter Troubles THERE appears to be considerable confusion and uncertainty among the purchasers of short-wave tuners and "adapters" for short-wave reception. On the one hand perfectly excellent apparatus is being accused unfairly, while on the other hand good results are vainly being sought with equipment that is quite incapable of decent work and which is unfairly giving the short waves a bad name. Perhaps it will be well to go over the possibilities and difficulties. First of all, what has one a right to expect? Assuming that one has a well-made set in good order, there is still no certainty as to the results which will be obtained. While the reception be A One Cycle of Supply Voltage BWhat one would expectin Ci La of Fig. 1C C One operating condition permitting smooth Output tween 200 and 550 meters changes greatly between day and night this change is as nothing compared to the corresponding change on some of the short waves. Very roughly, wavelengths between 13 and 55 meters may be thought of as the "daylight waves" which work better by day than by night, but always over rather long distances. At shorter ranges they are rather unreliable and often very weak. The waves above 55 meters have the more normal ability to go further by night than by day, although between 30 and 60 meters lie waves which not infrequently do good daytime work. On dark days even 80-meter waves come through rather well. Thesummertime shifts the best working wavelengths toward the shorter end of the spectrum, simply because there is more daylight. In all cases one may expect rather rapid changes during sunset or sunrise at either transmitter or receiver. With this rough set of rules as a guide one may begin to listen. However, at this point a very distressing difficulty may develop, namely, a "soupy" or "growly" audio quality that is entirely worthless as entertainment and often quite unintelligible. Such things happen in the 200-550 meter region at times, but not often or on all stations. But on short wavelengths every available station may be affected for an hour, a day, or a week, and in some locations the effect seems permanent! If the latter is the case, and one is sure that the tuner is not at fault, the short-wave idea had best be dropped. Usually, however, the effect is present only a portion of the time, and even then is seldom as prohibitive as the strong static which often mars reception on standard wavelengths. Assuming now that we have managed to find signals and that they are decently free from the "audio-frequency fading" just mentioned, we may next proceed to determine the usefulness of the various available signals. On short waves it will be found that the strength of a signal has even less relation to the station's distance and power than is the case between 200 and 550 meters. Thus, at my own location the English station 5SW is somewhat stronger than wgy's various short-wave transmitters, though the latter are but 100 miles away and materially more powerful. Also, the English station fades far less. Usually it is the fading which determines the usefulness of a short-wave signal; if it is very bad one is subjected to such extreme changes of volume that all the pleasure is eliminated, especially as the effect usually carries with it the so-called "selective fading" which produces weird and unpleasant shifts of quality. All this sounds as if the short-wave game were a most unreliable one and of no real value. This is by no means the case. If one will but accept its vagaries, and avoid them intelligently, when that is possible, one may hear quite an array of things with a short-wave receiver that would otherwise not "come in" at all. No inconsiderable part of this matter will arrive in good order and with steadiness as is shown by the occasional rebroadcasting of material received from another continent on a shortwave channel. Such work is usually done with a powerful initial signal. While it is true that a weak transmitter does occasionally put a strong signal into a remote region one still must not take seriously the claims of a broadcasting station which "broadcasts to the entire civilized world" with a 50 or 250-watt set. Having agreed that the short-wave game is moreof asportand less of a utility than the standard waves, we arrive near the truth. SilverMarshall's phrase, "The thrill bands" is correct. There is no thrill in the routine reception of a reliable station — but there is a possibility of a C-Hartley with Tubes Back-to-back D-Hartley with Tubes in Push-pull FIG. I. FOUR POPULAR OSCILLATOR CIRCUITS FOR SHORT-WAVE TRANSMITTERS