Radio Broadcast (May 1928-Apr 1929)

The Radio Frequency Laboratories AN UNUSUAL ORGANIZATION By ROBERT S. KRUSE k BOUT seven years ago there was formed l\ in the town of Boonton, N. J., the / \ Badio Frequency Laboratories, Inc., an organization devoted to research — a task that has long been a proudly accepted function of the university. The first members of the staff were men known to have a permanent interest in the questions "why?" and "how?" Their orders were to get together the necessary apparatus and attack the important problems in radio. I remember clearly the glee with which that prospect was greeted. That laboratory, with the same frame of mind, is the present Besearch Division of B. F. L. As its contributions have reached commerical form they have been put into the hands of licensed manufacturers who maintain their contact through an Engineering Division, created for that purpose. Lately there has also been added an Aircraft Badio Laboratory as another major division. Surprisingly the B.F.L. is not widely known, though it has made fundamental contributions and has for licensees manufacturers whose output is a large share of all that comes to market. Perhaps this is because the contacts have been mainly with the engineers of these organizations, for which B. F. L. is a centralized bureau of research, although working on its original problems as well. Accomplishments BECAUSE of the highly interlocking nature of the research and engineering problems I find it difficult to formulate the work done by these laboratories. However, in the course of various friendly visits made without any such story as this in mind, there has stuck in my recollection some matters that are mentioned in the following paragraphs — the list admittedly being neither completenorfrightfully accurate. The laboratories developed one of the first neutralized radio receivers, and, incidentally, this was also one of the first singlecontrol sets to be produced. Methods were developed for determining the sensitivity, selectivity, and fidelity of a radio receiver and these methods have been adopted by the Institute of Badio Engineers as one of the standard methods for measuring a radio receiver's performance. In collaboration with General Badio. there was developed (and placed on the market by G. B.) a standard signal generator for use in measuring radio receivers. The Laboratories developed a technique of making sound measurements which made it possible to measure the overall receiver performance from the antenna to the sound wave produced by the loud speaker. A basic study of detection was made particularly at high signal levels, and detectors were developed which do not produce distortion and which are not subject to overload under normal conditions. These studies applied particularly to 100 per cent, modulated r.f. signals. The use of 100 per cent, modulation is increasing — or perhaps we had better say that there is an increased tendency for transmitting stations to attempt such modulation. Four-element tubes were designed, constructed, Fig. 1 — This compact receiver was developed for use on airplanes with a seven-foot rod antenna and used in the development of receivers pending the availability of such tubes on the market. The active research problems are quite beyond such a brief account as this: the designs for the next year's broadcast sets of the licensees are still confidential — though I yearn to write about two features thereof. However, B. F. L. is engaged in another task which may be described. In the commercial progress of aircraft, there has developed a need for a reliable means of guiding an airship — a method that will prove equally reliable during day and night and in all sorts of weather. For this purpose radio beacons have been used but there has existed no receiver for airplane use that would provide the necessary sensitivity and at the same time be able to function without a trailing antenna. The Badio Frequency Laboratories were asked to cooperate with the Department of Commerce in developing a receiver that would do these things. The Airplane Receiver AT THE opening of the Aircraft Badio Laboratory on January9, demonstration flights were made with a new beacon receiver. This receiver uses but five tubes of which the last two are resistance-coupled audio and the first, two are of the screen-grid type. It is rather startling to have such a receiver, working with a seven-foot rod antenna, produce a headset signal which, at 30 miles from Hadley Field's beacon station, is far beyond the scale of any ordinary audibility meter and wrecks headsets in short order. With voice modulation at the beacon station, the Wright J-5 motor's roar meekly retreated behind the signal. In I. B. E. language, the set has a sensitivity of 5 microvolts on a 30 per cent, modulated signal. The sets may be used either on the "A 6 N" beacon system or with the vibratingreed system. Of these two systems, we will speak but briefly. In both cases there are sent out two beams, diverging slightly and the course lies down the center of the angle. Fig. 2 This apparatus is used in the laboratories of the R.F.L. for measuring the sensitivity of broadcast receiving sets With the "A & N" system the letter A (• — ) is being sent on one beam and the letter N ( — ) is sent on the other beam. The timing is such that if the two beams are being received equally well the two letters interlock to make a steady signal. In the reed system the two beams are modulated at audio frequency, one at 65 cycles and the other at 85 cycles per second. At the receiving end, therefore, the output of the amplifier carried both modulations equally if one is on the course. If the plane falls off course the 65-cycle modulation may be picked up less and the 85-cycle one more (or the reverse) and one reed spreads out more while the other narrows down, thus advising the pilot as to the direction in which he is off. With the receiver mentioned here the system is dependable up to 150 miles, with a normal 2 kw. beacon station and a seven-foot rod antenna on the plane. A irplane Height Indicator THEBE is at present being developed by the Aircraft Badio Division of the B. F. h. a height indicator. In its present state of development this device is used to give a series of two or three separate indications (such as the lighting of different colored lamps) each of which corresponds to a definite height above the earth or water over which the plane is being flown. It must be clearly understood that the device is not an altimeter, the device at present used in airships and which tells the pilot the height of his plane above sea level. The B. F. L. height indicator will have no reference to sea level but uses the surface under the plane as the datum point — it is of small interest to a pilot how high he is above sea level when he is flying above a mountain and the tree tops are only 50 feet below. This new apparatus was first installed in a D. H. plane and successfully operated over land, fresh water, and salt water. A later installation has been made in a radio test plane of the laboratory. The apparatus is in the forward cockpit of the ship and is housed in an aluminum box, the whole weighing about 7 pounds. The antenna is a doublet stretched between wingtips, and lying beneath the wing. In its present form the device is useful for landing in ground fog, and for landing on smooth water in clear weather. When flying over trees the indicator flickers continually. Of course, it is felt that aviation radio will become of steadily increasing importance. The Laboratories have acquired an airport. For the hangar we can say that it has better than average accommodations, including garages, living quarters, a shop, an office, and an 80 x 100-foot space for planes, which enter through doors with 18-foot headroom. Amazingly enough the place is heated well. It does not stick in my recollection that I have ever been in an airplane hangar that was not several degrees colder than outdoors. In another corner of the field is a laboratory containing living quarters, kitchen, lounge and library, bridge measurement room, a transmitting room, transmitting and receiving laboratories, private laboratories, director's conference room, office, and a finely equipped shop. • march, 1929 page 302 ®