Radio Broadcast (May 1928-Apr 1929)

FIG. 5. FROJiT VIEW OF RECEIVER A Two-Tube T*R»F» Short-wave Receiver and W, T. A RECEIVER to operate satisfactorily on short waves must fulfill several important requirements, and in explaining the design of the short-wave receiverdescribed in this article, it will be well to consider these one by one, so that the reader may have a clear idea of the various phases of the problem at hand. In the first place the receiver must be completely free from body capacity, and arranged so that its coils pick up no energy themselves. This means complete housing in good metallic shields. The receiver should be equipped with a regenerative detector the control of which must be smooth and easy. Capacity control of regeneration has been selected as best meeting these conditions. Any receiver placed before the public for construction, should, in the opinion of the authors, be a non-radiating device. The receiver must be selective yet must not cut side bands, and it must cover the required wavelength range. Lastly mentioned but far from last in order of importance, the set must have high overall voltage amplification, yet keep the amplification of noise at a minimum. THE DESIGN OF THE UNIT THE above resume of what a short-wave set should be, is, of course, general, and outlines the things to be expected from the correct receiver for this job. The set that we propose to develop in this paper is one that fully meets this rigid set of requirements, and at a figure that will not strain the average pocketbook. The set, as seen in the photographs and diagrams, is composed almost entirely of standard commercial parts. The tuning range of the receiver is from 15 to 140 meters, effected by means of three sets of easily interchangeable coils. This range covers to-day's domestic and foreign short-wave broadcasting, and the more important amateur bands. No audio amplifier is included in the set, which should preferably be used in conjunction with two stages of high TfiOR the first time we are glad to be able to present a short-wave receiver using a screen-grid amplifier which is tuned. Up to the present time, designers of such receivers have been content to use the screen-grid tube as a sort of blocking tube which prevents oscillations from the detector getting into the antenna, and which gives a certain amount of amplification, it is true, but not an amount comparable to what is possible when the amplifier input is tuned. In the Laboratory the receiver worked beautifully without any a.f. amplifier, the signals from the detector being as loud as those from an ordinary short-wave receiver using one stage of audio. It is true that such a receiver as this has an additional tuning control, and that for maximum signal strength and selectivity, both the amplifier and the detector circuits must be tuned, but it is believed that it is worth it. The writer recommends that 180 volts be used on the screen-grid plate and 60 volts on the screening grid of this tube. In the Laboratory the receiver worked properly with only 1 j 5 and 45 volts, respectively, on the plate and grid. — The Editor. grade audio, or better still, with a power amplifier using a push-pull circuit. The set requires the use of a 6-volt storage battery or an "A" power unit, and three different B potentials, namely, the plate voltage for the screengrid tube, the screen-grid voltage, and the detector plate voltage. The writers have found that 180, 60, and 45 volts, respectively, work out very well. Impedance coupling is employed in the r.f. stage, as this is the only practical method of matching the plate impedance of the r. f. tube at short wavelengths. A screen-grid tube is employed on account of its high voltage ampli fication, and also because it is the only tube that can supply stability to a short-wave r. f. amplifier. A 200A detector tube should be used. Perhaps a word as to results would not be out of place at this point. During the short time that this set has been in operation in our laboratory it has completely and easily covered the world. Phone reception has been effected with points ranging from Alaska to New Zealand and from England to Russia and Java, not to mention many other foreign stations at lesser distances, a low gain two-stage audio amplifier being used on the output of the set. Quite often signals from 5SW, the short-wave broadcasting station of the British Broadcasting Co., in Chelmsford, England, are strong enough to give loud speaker operation directly off the detector output. For those interested in amateur c.w. reception it will suffice to say that Australian amateurs have been heard in Ithaca, N. Y., at 3 p.m. with the phones on the table, and this also right off the detector, using no audio. CONSTRUCTION IT WILL be noticed that in the design of this 1 receiver one of the shield cans called for is of rather large dimensions, necessitated by the placing of both the r. f. and detector tubes in the same compartment, the overall width parallel with the panel being 13". In some localities difficulty may be experienced in obtaining a shield of this size ready made, and for this reason it was thought advisable to include a description of the shields, for the benefit of those who wish to make their own. Let it be said, however, that unless one has had some experience in work of this nature, he had better take the drawings to a good tinsmith and have him make them, in which case it will be well to impress upon him the importance of having a close fit around the base; when the two halves are put together, the gap should not exceed 3V" at any point if a really good job is done. The material may be either half hard brass or copper and should be slightly less than 3V'. Half hard brass 0.0225" was usec' by the writers. 20