Radio age (Jan 1927-Jan 1928)

RADIO AGE for April, 1927 Portable Direction Finder Has Many Government Uses UNIQUE in that it is probably the only portable radio direction-finding device in the world that operates over such a wide band of wave lengths — from 39 to 3,300 meters — a new apparatus developed by Francis W. Dunmore of the Radio Laboratory of the Bureau of Standards may be transported and installed with quite the facility a large camera is put to service. Equally as simple in manipulation, this novel radio direction-finder is governed with but two controls. When the entire United States is to be eventually crisscrossed with radio beacons for guiding aircraft along predetermined courses and with every ship depending upon radio for finding its port of safety, this new equipment for locating the source and direction of a radio signal makes its appearance at an opportune moment. Other radio direction-finders may be equally as efficient but the Dunmore device is said to excel in the particulars of operating at relatively short wave lengths and in the simplicity of control, two knobs, one for tuning the radio receiving set and the other for obtaining a balancing effect. These features, too, are not insured at the sacrifice of portability, Photograph showing the completed portable direction finder developed by Mr. Dunmore, having a wavelength range from 39 to 3,300 meters By S. R. WINTERS because the apparatus is relatively compact. Experimenters and members of the "Homemade Radio Set Builders' Club" will be interested in knowing the popular fad of shielding radio receiving sets is adopted in the construction of this direction-finder. An aluminum box affords a cozy home for all of the radio receiving units, including batteries. The single exception to this sweeping statement is that the coil antenna is mounted on a bakelite shaft extending through the aluminum box. This pick-up coil is revolved in different directions of the compass by turning a handwheel under the shielding aluminum box. The shielding of circuits in radio receiving sets may be carried to extremes in some instances in response to a popular fad, but in designing this radio direction-finder the Bureau of Standards ventures the contention, "This shielding is important, especially when receiving the higher frequencies, since the directive effects are blurred if any circuits except the direction-finder coil pick up power from the wave." GAM-ACTUATED tuning condensers, an idea of Mr. Dunmore's for which he was granted a patent during 1926, has made possible a reduction of the tuning controls of the super-heterodyne receiving set used with this direction-finder to a single one. This is accomplished by mounting the main tuning condenser and the heterodyne generator tuning condenser on the same shaft. The latter condenser has connected in parallel with it an auxiliary condenser of slightly smaller capacity operated by means of a cam which may be slipped on the shaft carrying the two tuning condensers. The movable plates of the balancing condenser are connected to an antenna, which is an integral part of each directionfinder coil. This antenna consists of a brass rod the height of the coil which passes through the center of the coil. Another rod is telescoped into this one, which, when extended, doubles the height of the antenna. Such an antenna, according to the Bureau of Standards, gives sufficient antenna effect for good balancing. It was found this small antenna connected to the movable plates of the balancing condenser was much more effective than grounding them. Seven cams of different shapes are necessary to the proper functioning of this direction-finding device. The operation of this new apparatus over such a wide band of wave lengths — from 39 to 3,300 meters — is made possible by a set of seven interchangeable plug-in coils. These vary in size from 12}4 inches square to 24 Yz inches square, the former having only two turns of wire, and the later sixty turns of wire, space wound in four layers. Each of these seven direction-finder coils has imprinted on it the wave-length range over which it operates, thus facilitating the plugging-in of different coils when wave length variations are desired. Each coil has a socket containing four terminal plugs, two connected to the end of the coil, one to the center of the coil, and the fourth to the brass rod antenna. The coils are enclosed, being wound on box frames made of wood. (Continued on page 31) Schematic diagram of the all-wave direction finder which has but two controls for its operation