1938: Radio

Vacuum Tubes.

The successful development of a tiny radio tube whose possible applications include mechanical 'ears' for the deaf that are four times as sensitive as the present type, and police and foot-soldier radio receiving sets, have been announced by the Hytronic Laboratories at Salem, Mass., after two years of research. Developed primarily for the construction of an improved hearing aid, the miniature tubes are one and five eighths inches long and nine sixteenths of an inch in diameter. Hair-like filaments, small grids, and other parts correspondingly small make the use of magnifying lenses necessary during manufacture. The tubes operate from a small battery no larger than flashlight cells. It can be incorporated into a hearing aid no longer than four inches. Operating on a filament voltage of 1.4 volts, the tiny tubes have a drain of 0.070 ampere. The tubes are made in a triode, tetrode, input pentode, and output pentode.

These small tubes have reawakened interest in battery receivers; and some manufacturers have increased production, since such receivers are quite practicable for the farm and other places where electric power is not available.

Receiving Sets.

There have been several new developments in receiving sets. For example, in a development by the General Electric Company in collaboration with the Warren Telechron Company, known as 'time-tuning,' it is possible to preselect any one of five stations for any 15-minute period over a full 24-hour span. Once the day's and night's programs have been selected and set up in advance, it is not necessary to go near the receiver. It will turn on and off, change stations, and perform manual functions automatically. By means of a simple switch, the clock feature may be made inoperative when clock operation is not desired.

Also, the General Electric Company has brought out the 'beamascope,' a built-in shielded-loop antenna that eliminates the nuisance of outside antenna and ground connections. This antenna also has an added advantage in that it is shielded from the electrostatic component of reception and responds only to the electromagnetic component. Since noise such as static is stronger in the electrostatic component than in the electromagnetic component, this troublesome factor is materially reduced. This elimination is accomplished by rotating the beamascope until the electromagnetic field of the noise source is either eliminated or reduced to a minimum.

With one exception, all the new General Electric receivers are equipped with keyboard, push-button tuning. The tone monitor is a newly-perfected circuit which greatly extends the reproducing range, both high and low, in proper proportion. It corrects the effect of peaks, boom, and distortion of individual tones to give a balanced richer tone.

In order to realize the full capabilities of the high-fidelity receivers, a new triple-cone speaker has been designed by R.C.A. For handling the high-power input, a relatively heavy cone is necessary but this limits the high-frequency response. To overcome this disadvantage, a smaller and lighter auxiliary diaphragm is placed within the larger cone and fastened to it by a flexible base support. A compliant coupling in the large cone permits the small cone to be driven independently at high a-f. frequencies by the wire coil to which it is attached. A small metal dome is used to load the apex of the inner diaphragm to resonate with the stiffness between the apex and the voice coil. This speaker has a substantially flat response characteristic of from 50 to 7,500 cycles.

Probably the most novel innovation in receiving sets is the remote-control automatic tuning device developed by Philco, the details of which have not as yet been released.

The station selecting-and-controlling device has no conductive connection with the receiver, but contains a small short-wave transmitter which sends the appropriate signals to the receiving set. The station is selected by means of a dial on the small transmitter. This causes the proper impulse to be sent to the receiver, whose mechanism is set in motion to tune in the selected station. Also the volume may likewise be controlled, and the receiver turned off at will. The control device may be carried at will about a home.

Aeronautics.

Outside those connected with the aircraft industry, few realize the extent to which the commercial air lines are using radio facilities for maintaining regularity of schedule and safety in flight. The transmission of messages to and from the plane has improved greatly in reliability, partly due to increased power in both ground and plane transmitters and in more reliable receiving sets. For example, one manufacturer supplies a heating device with a thermostat to remove moisture, a frequent cause of failure. The most difficult obstacle in radio reception under all weather conditions is 'snow-static,' a general term for static caused by charged particles, whether they be due to direct snow, sleet, rain or fog. Snow-static has a hissing character. A quite successful attack has been made by the Bureau of Air Commerce by enclosing the loop in a circular metal tube which allows the magnetic component of reception to influence the loop, not the static, but when the pilot runs into snow-static, he switches from the conventional straight-wire antenna to this last. This loop may not be satisfactory when the static level is high, but even then reception is greatly improved.

Receivers are almost universally of the superheterodyne type, with a range from 150 to 15,000 kc. Another major improvement is the newly-developed simultaneous radio-range and weather-reporting station which transmits beam signals and voice signals at the same time. Formerly it was the custom for the pilot to silence the radio-range signals during the broadcast of weather reports, since both are assigned to the same frequency in any given locality. Transmission is effected by the use of from 50 to 4,000 cycles for the voice modulation frequency range with a band of 830 to 1252 cycles suppressed by means of a band filter, this band being reserved for the radio-range signals. The removal of this band does not seriously affect the intelligibility of speech.

The major airports are now equipped with traffic control stations operating at 278 kc. which contact each airplane as it comes within the vicinity of the airport, and direct its movements thereafter.

The Bell Telephone Laboratories have developed a new device which determines the direction from which the radio transmitter of an approaching airplane is sending signals, even though the weather conditions may make the plane invisible. A tiny green light speck is located on a frosted glass screen in accordance with the direction from which the plane is coming. The points of the compass are marked around the edge of the screen. The system provides for any of ten wave lengths which may be selected remotely. As any single pilot talks, the spot of light moves to its correct position on the screen of a cathoderay tube. A pick-up antenna of special design which may be located at any remote point is employed, and a single telephone line connects the antenna and the dispatcher.

The Bureau of Air Commerce has instituted a radio-range beacon system which transmits directional signals along four courses, two along the regular way and two to mark cross airway flying. In the opposite quadrants the signal A is sent by a directional antenna system, and in the remaining quadrants the signal N is sent. At the boundaries the A and N signals synchronize to produce a continuous tone which is used by the pilot as a guide. When the pilot is immediately above the towers of the station the tone ceases for the short interval when he is in the 'cone-of-silence.' This silent region can be used to determine the location of the airport, but with the disadvantage that the signal is negative, and silence may be due to failure of the signal or reception. To obviate this, the Bureau of Air Commerce is installing ultra-high-frequency beacon stations which give the pilot a definite signal when he is in the cone-of-silence. In the matter of blind landing, the National Bureau of Standards has experimented with an ultra-high-frequency antenna operating underground in the center of the airport to establish a satisfactory landing beam.

Altimeter.

The single most important contribution of the year to safer flight is the absolute altimeter. The altitude indicator generally used operates on the principle of the aneroid barometer and hence it cannot indicate absolute altitude with respect to the ground. It merely shows altitude with respect to sea level or some other initial reference level at constant pressure. If the barometric pressure or temperature vary, an error of several hundred meters may occur, and such an indicator is not adapted to blind landing. With the altimeter a 500-megacycle radio wave is transmitted from a small T-shaped antenna on the under side of the right wing. On this wave is impressed an 80-cycle audiofrequency wave. The combined wave is sent to earth and reflected by the earth back to the plane where it is picked up by a similar antenna under the left wing. A portion of the transmitter output goes as a direct wave to the receiving antenna. The receiving circuit essentially counts the number of 80-cycle beats between the time that the direct wave and the reflected wave is received. The 80-cycle wave is used because it is easy to count and the apparatus is simple. The ultra-high-frequency wave is used since it is easily directed and is free from static. The instrument will be extremely valuable in flight, for it will indicate the presence of hills and mountains, the cause of several airplane disasters. Statistics show that with such an instrument practically one-half of the major airplane accidents that have occurred during the past few years would have been avoided. The instrument will also be useful in nautical navigation during fog, to detect the approach of land and other ships.

Television.

Since 1935, field tests have been conducted in New York City by RCA-NBC, the broadcasting antenna being located on top of the Empire State Building. For example, on June 7, scenes from Susan and God featuring Gertrude Lawrence were broadcast. Picture quality both from the studio and the motion picture films have improved steadily and the receivers have also improved in simplicity and performance. With the opening of the World's Fair scheduled television programs will be on the air in New York and in other cities. The receiver will produce 441-line images. There will be several types of receivers offered, the picture tubes varying in diameter from 7 in. to 12 in. The price must necessarily be considerably higher than those for sound receivers since they are much more complicated, involving from 18 to 33 tubes. There is considerable activity among the manufacturers, and a definite interest awaits the reception of television by the public. (See also TELEVISION.)

P. T. Farnsworth has brought out an entirely new image-amplifier pick-up tube for use in the sending station of the television studio. It combines several important features of the iconoscope (the tube having an activated screen on which the image to be sent is focused) and in addition utilizes the principle of the control grid for amplifying the picture signal before the image is scanned rather than after it is scanned, as in all prior pick-up devices. The resulting tubes are free from the spurious 'shading' signal, and one form has a sensitivity about 10 times that of the conventional iconoscope.

General.

John Shepard of the Yankee Network is having installed on the top of Mount Washington, N.H., a steel broadcasting tower nearly 7,000 ft. above sea level and commanding a line of view into five states. The tower is to be used for experiments in frequency modulation invented and now being developed by Major E. H. Armstrong.

Westinghouse is radio-proofing pin-type high voltage power insulators by means of a newly-developed copper-oxide glaze which maintains good contact between tie-wire and insulator to eliminate charging current arcs. The object of the glaze is to reduce the high-voltage static which interferes with radio reception.