Radio broadcast .. (1922-30)

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STRAYS FROM THE LABORATORY A.R.C. Radio Altimeter The picture herewith shows a special indicator panel insta'led in the pilot's compartment of a Fokker cabin mono- plane, one of several planes employed in performance studies on the reflection altimeter of the Aircraft Radio Corpo- ration, one of the Boonton (N. J.) Associ- ates. The series of studies, continuing over many months, has included investigations of the effect of different wavelengths, an- tennas, airplane constructions, soils, to- pography, weather, and obstructions on the ground. The commercial form of the device can be made to give its indications by a needle traveling over a calibrated scale or by a sequence of lamps, each of which lights at a predetermined height above the ground or water over which the plane is traveling. In the experimental installation shown in the picture, the functions have been divided for ease of obtaining data. Two indicating instru- ments are provided, each of which is illuminated by a colored lamp, the two lamps lighting alternately at predetermined heights above ground. The height can be read from the instrument which is at the moment illuminated. The dial below the center of the panel enables the ob- server to make a balancing adjustment which is useful during tests on the effect of voltage variation, tube ageing, and the like. Linear Detection An interesting advantage of linear de- tection, as pointed out by Professor Ter- man in this issue of RADIO BROADCAST is its ability to discriminate automatically against unwanted signals weaker than the desired signal. Fig. 1 shows how this occurs. At (a) is the weak undesired envelope, in (b) is the desired envelope. These are envelopes of in- audible carriers and the beat note between these carriers must be inaudible; i.e., the two signals must differ by more than 10,000 cycles. The first thing that happens, then, is the production of an inaudible beat frequency. When the envelopes are in phase this beat frequency has a high amplitude; when they are out of phase, the amplitude is low. Thus the amplitude of the beat fre- quency varies between (c) and (d) in Fig. 1. The aver- age value of this change in amplitude is the desired audio signal. If there is any non-linearity the increases (say) above this average will be greater than the de- creases below this average and thus the undesired weak signal will affect the desired strong envelope. Audible Output , of desired Characteristic Resultant Inaudible beat Frequency Fig. I Thus it appears that not only is the linear detector advantageous from the standpoint of decreased harmonics, from which the square-law detector suffers, but it is a sort of interference View of radio equipment employed in ex- periments with the reflection altimeter. eliminator too. Static may be reduced by the use of a linear detector, providing the modulation of the desired carrier signal is sufficiently great. Advice to Students Robert S. Kruse—who doesn't know Kruse?—has written the following letter to a reader who wants to be a radio engi- neer. Every word of it is true. "No school does, or can, educate a man to be an engineer—it can only give him fundamental principles and general meth- ods of thought and action to use as tools in attacking the special problems he meets. Problems are all special in one sense or another and no book can anticipate every- thing that progress brings. If such a hand- Ixxik were possible we would always find the answer in a handbook, and engineering staffs could be replaced by a five-foot shelf of books. Manifestly this is not the way things are—we do not anticipate and congeal our methods into a handbook. Instead we train men who regard the handbook as a toolrack in which concrete facts are stored but who themselves see to the use of the tools—and forge new ones as the need arises. These men are our researchers and engineers. The former deals with things exclusively—with the use by man not a controlling factor—the latter works con- stantly with things and men as applied to the pressing immediate needs of men. If you wish to enter either of these two fields, learn all the mathematics you can, see all the radio equipment of any sort (it is all within 30 miles of you) and handle all you may of it, operate an experimentiil station of your own (but do it studiously and not as a social activity or a fad), at- tend I. R. E. meetings and make all pos- sible personal contacts, and above all learn how people think, feel, and act, for the greatest danger that con- fronts the student engineer is that in learning how to deal with scientific things he loses contact with people —without whose aid he is useless." Screen-Grid Ratings Although the rating on the screen-grid tube has not been changed, we under- stand that the majority of set manufacturers using this tube are fixing their own control- and screen-grid vol- tages. With a 1.5-volt nega- tive bias the control grid in good tubes begins to draw current at about 0.8-volt negative. This means that an incoming signal of about 0.7-volt peak would cause the grid to take current and its input resistance to (Concluded on page CO) NOVEMBER 1929 •