Radio Broadcast (Nov 1926-Apr 1927)

84 RADIO BROADCAST ADVERTISER reasons for the golden opportunities for radio operators free quit ough [en who need money fojx^ college courses learn radio, take] an operator's position with good pay, with board and room. They as soon as they save enc for their purposes. Gpportunity for new men. Man| operators find^t iat it pays to learn radio fcr the pleasure of a few ocean voyages t'p foreign lands. Then they leave for shore jobs. Opportunity for ne<w men. 3 Biggei • and better pot itions that require technical radio knowledge and experience are constantly calling operators from the ranks! Opportunity for new men]. Learn Radio Now With a few months of R4dio Institute or America's expert instruction, yoiL too, can qualify for your U. S. Government Commercial or Amateur Radio License — and start to see the far corners of the world as a radio operator. Learn at Home Radio Institute of America gives the finest radio instruction you can obtain. And you can learn at home. Send coupon for complete information. SEND THIS IN Radio Institute of America K 328 Broadway. New York City Please send me full information about your Home Study Course of radio instruction. I I I am interested in the complete course including code instruction. I I I am interested in the technical course without code instruction. Name Address Radio Institute of America (formerly Marconi Institute) Established in 1909 328 Broadway New York City A Way of Increasinj A Higher. tivity edied Exposed Antenna Will Improve SemsiTuning — How This May Be Renting a "Selectivity Unit" to Any Receiver By HAROLD JOLLIFFE OW many of you fans who are always building and rebuilding your receivers into all manner of trick circuits, buying more tubes, and accessories, and generally striving to pull in more dx, ever pause long enough in your experimenting to reflect that by erecting a long, high antenna you are virtually adding the equivalent of another stage, of radio frequency amplification to your set? The distance-getting ability of a large antenna versus a small one may be compared to the difference between a good headset and an insensitive one; the good headset brings in signals from distant stations quite clearly and dis \ tinctly, while the poorlydesigned phones reproduce them so faintly that you have to strain your ears to find out what it's all about! And it's analogous with antennas. A long unsheltered antenna will bring in signals from far greater distances than a small, low one can ever hope to pick up; stations heard on the small antenna will come in with a remarkable increase in volume when the set is coupled to a longer, higher antenna. And this simply because the antenna, being high and covering a greater area, is collecting considerably more energy, thereby resulting in a more pronounced radio frequency delivery to the detector. True, indeed, there will be a not FIG. 1 r FIG. 2 able decrease in selectivity ordinarily, but we're coming to that now, and it is this factor with which we are concerned in this article. There are several types of receivers in use to-day which employ a tuned antenna circuit. By this is meant that the primary coil is capable of being tuned to the exact frequency of the incoming signal, either by means of a variable condenser of the proper capacity, a system of taps, or a combination of both. The well-known three-circuit variometer set is an example of a receiver of this type. There are also many receivers which employ the "shock excitation" method of coupling, i. e., the untuned, or aperiodic primary, as in some forms of neutrodyne receivers. Each system has certain advantages. The first, the tuned primary, provides greater signal strength, since the antenna coil may be tuned exactly to the frequency of the desired signal and, in turn, the secondary may be brought into resonance with the received signal. The second, the untuned primary form, makes for greater selectivity at some sacrifice in volume because the primary coil, having a low value, and not variable, cannot be tuned to the signal frequency but depends for its operation upon "shocking" the grid coil where the selection of the desired signal is accomplished. The greater the number of turns the broader will be the tuning, with an increase in volume; likewise, as the number of turns is reduced, selectivity is more pronounced, with a decrease in signal strength. In the course of some experiments with a fourtube Teledyne receiver, the writer found that, by cutting down the antenna coupling coil from ten turns to one, and then loading this one turn with a specially designed loading coil, thereby making it a combination of the two coupling methods referred to above, the selectivity was as sharp as if the one turn alone were used, while the volume was practically the same as with the ten turns. The antenna used during these tests, and which was subsequently used with great success last winter, was 175 feet long and 1 1 5 feet high at the main support — a water tank. The results were so pleasing in this case that the idea was tried with a standard singlecircuit receiver; and, in view of the fact that no radio frequency amplification was employed that would aid in boosting the selectivity, the results were eminently satisfactory. The author therefore decided that it is possible to use a long, high antenna system, with the resulting high energy-intercepting qualities, without sacrificing that degree of selectivity which is generally obtained only by the use of a small antenna, provided that the proper apparatus is employed. The first thing to do is to build the variable loading unit; the constructional details are