Radio Broadcast (May 1928-Apr 1929)

"Our Readers Suggest yy THIS department of Radio Broadcast is utilised each month for the presentation of miscellaneous short radio articles which are received from readers. These abbreviated manuscripts describe "kinks," radio short cuts and economies that the experimenter runs across from time to time and that can be made clear in a concise exposition. Although some of these notes have been submitted by engineers and professional writers, the editors particularly solicit contributions from the average reader. All material accepted, including photographs, will be paid for on publication at our usual rates with extra consideration for particularly meritorious ideas. — The Editor. A Simple IV ave T rap W; HEN wlw opened up with full fifty kilowatts at Mason, about twenty miles from Cincinnati, I experienced a bit of interference from this station. Rummaging around the collection of used parts generally found in the enthusiast's shop, 1 speedily assembled the essentials of a simple wave-trap, and had it operating satisfactorily an hour after wlw came in the air. While there is nothing new in the trap idea, or in the design advocated by the writer, the economy of the arrangement will recommend itself to many readers, particularly in these days of consistently increasing power. The essential parts are a variable condenser, a standard r.f. coil to match the condenser (if coverage of the full wavelength range is desired), a panel, and a dial. The primary and secondary connections to the coil should be definitely located. The primary winding generally can be identified by the initialing P and B + , and the secondary by G and F. If no stamping is apparent, as may be the case on some r.f. transformers, the secondary winding has more turns than the primary coil. The condenser should be mounted on a small panel, about five inches square, and the coil mounted to the condenser frame. The secondary winding is then connected across the variable condenser. This completes the wave-trap, illustrated in Fig. 2. The primary winding is connected in series FIG. I The wave-trap circuit. The primary coil of the r.f. transformer is connected in series with the antenna lead. with the antenna lead to the receiving set, the connections being indicated in Fig. i. D. H. Boyd, Cincinnati, Ohio. STAFF COMMENT The method of operation is simple. First, the receiver is tuned to the station it is desired to eliminate. The wave-trap is then adjusted until this station is weakest. Leaving the wave-trap at this setting, the desired station is now tuned in — generally with little or no interference from FIG. 2 A simple wave-trap made with a standard radiofrequency transformer and variable condenser. the first transmitter. If there is insufficient trapping action, five to ten additional turns of wire should be wound on the primary coil, in the direction of the original winding. Of course, the extra turns should be included in the antenna circuit. A Source of Accurate Meiers EVERY experimenter needs meters to determine accurately the values of the current and voltage with which he is working. However, with reliable meters costing from §7.00 to $20.00 a piece, this important item is often necessarily neglected. Accurate d.c. meters may easily be made from zero-center moving-coil ammeters readily obtainable at any automobile junk yard for fifty cents. Older models of high-priced cars were equipped with moving-coil instruments, such as Weston models 301 and 267. These meters have a 50-miIlivolt movement, and require from 10 to 20 milliamperes for full-scale deflection. The shunt should first be removed from the meter, and the needle swung to the extreme left by moving both the zero adjuster and the corresponding adjustment found at the under side of the movement. The meter may then be calibrated as a low-reading milliammeter. By using shunts of proper values, milliammeters or ammeters of any range may be had. The small operating current required by these meters readily permits their use as voltmeters having a sensitivity of from 50 to 100 ohms per volt by merely connecting the proper resistance in series. Earl H. Miller, Bellefonte, Pa. R.F. Choke Coil AN EFFICIENT radio-frequency choke coil of the plug-in type may be made from a burned-out filament ballast. The ballast tube should be wound with about 250 turns of number 34 silk-covered wire. The wire may be wound in a haphazard fashion and the ends soldered to the caps of the ballast plug. This choke will be effective over the entire broadcast range and down to about 100 meters. For shorter wavelengths a 1 50-turn coil would be better. Allan Hamilton, Houston, Texas. More Output Ideas PUSH-PULL amplifiers generally are provided with an output transformer to which the loud speaker is connected. In some cases better results may be obtained by connecting the loud speaker directly to the primary through two 2-mfd. condensers, as suggested in the diagram Fig. 3. Posts 1 and 4 are used for any single loud speaker, while a very satisfactory combination of cone and horn loud speakers can be effected by connecting one reproducer to posts 1 and 2 and the other to posts 3 and 4. H M. Thompson, Vancouver, B. C. A Band Selector for the Universal Receiver I HAVE always been a booster of the Radio 1 Broadcast Universal circuit, my only possible criticism of the arrangement being the lack of selectivity when operated in congested broadcast localities. However, by utilizing the familiar link circuit, as suggested in Fig. 5, the selectivity was improved to an entirely satisfactory degree with a negligible loss of volume. The following describes the coils indicated on the diagram : Coil A, 10 turns wire on a 2f inch diameter tube; Coil B, 48 turns of wire spaced J inch from coil A; Coils C and D each have 10 turns of wire on 25-inch tubing; -O 1 Sec not used 2mfd 1 1 ' 02 63 04 FIG. 3 An output arrangement that is occasionally superior to the transformer circuit, and which provides for one or two loud speakers. 1 02