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April 1933 Radio-Craft[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles from Radio-Craft.
Some of the earliest examples of communications via light waves include signaling systems used by mariners to send and receive simple coded messages in ship-to-shore and ship-to-ship modes. Doing so involved candles or bonfires, depending on how far the signals needed to be sent. Costal lighthouses have served an optical communications function for centuries. Even Paul Revere relied on optical communications from the Old North Church in Boston during his "Midnight Ride" to warn colonists of the impending British invasion - "One [lamp] if by land, and two [lamps] if by sea." In the 1930s, Elman B. Myers designed and exhibited the first widely publicized light wave communication system that modulated a powerful mercury vapor lamp (a la many street lights) with information provided by a low power source. A photocell on the receiving end captured the optical energy and converted back to baseband audio. Columbia Broadcasting System (CBS) studios sponsored the project for building-to-building radio show transmission.
Elman B. Myers*
The first published detailed description of the "light beam radio" program recently broadcast over Columbia's national network. This new system utilizes a modulated 50 meter radio transmitter to produce a 375 mega-megacycle "cold-light" in a mercury vapor induction ramp. Note the fields available for this communication system.
On the thirteenth anniversary of Vaughn de Leath's entry into radio broadcasting, her regular program over the Columbia Broadcasting Company's nationwide network was used to inaugurate a new method in radio communication. This new system is known as a "Radio Light Beam."
We have all heard much about the early experimental methods of light beam communication, and of more recent experiments with a 20 mile arc-light beam modulated by a 200 kw. transmitter; and now, through inventions in the Metal-Vapor Induction-Lamp field, this method has been brought to the point where, with only about 1 kw. in the transmitter, it will do all that a radio transmitter will do, but with a great many of radio's disadvantages overcome, - in a manner which is commercially practicable.
The lamp that is the basis of this remarkable new method of communication is pictured in Fig. A; it is sketched in Fig. 1. In the latter illustration the coils L1, L2, and the condenser C1 form the output circuit of a standard, low-power radio transmitter operating on 50 meters. The vaporizing coil L2 is coupled to the larger chamber containing liquid mercury. By the correct coupling of L2 and this mercury vaporizing chamber, mercury vapor is produced and seeks the upper or ionizing chamber. Here the mercury vapor is ionized by the 6,000 kc. (50 meter) R.F. current which circulates through the ionizing coil, L1, and produces a "cold light," of a bluish-white color, having a frequency of 8,000 A.D., or 375 mega-megacycles. Convection currents which are set up in the lamp carry mercury molecules to the second or condensing chamber where they condense, due to coolness of the quartz envelope, and form back into liquid mercury.
All that is necessary to modulate the lamp is to modulate the radio transmitter in the conventional manner. All variations of the transmitter output are followed with absolute fidelity by the mercury vapor lamp! In fact, the same light used in the recent Radio Light Beam tests has been used to reproduce television pictures of 60 lines, 72 elements wide, at 20 pictures per second. This proves that the light will modulate 43,200 cycles per second, for the pictures produced were acclaimed as perfect as the originals in the television studio! It will be seen, then, that frequencies up to the limit of the audio band are well within its scope. Lamps that have been operating 7,000 hours show no signs of fatigue, and it is believed that the life of these tubes is extremely long.
As Vaughn de Leath, 'way back in 1920, climbed the winding stairs of the old World Tower to become the first radio artist of her sex, little did she realize that thirteen years later she would inaugurate a new system of communication that is bound to open up untold transmission bands. A remarkable coincidence is that the author was at the controls of the De Forest radio transmitter which broadcast her voice for the first time!
In Fig. 2A we have a sketch, and in 2B a block illustration, of the system used January 19, 1933. Miss Vaughn de Leath and Freddie Berren's Orchestra furnished the talent, in the observation room on the 71st floor of the Chrysler Tower in New York City. The program was picked up in the regular way by engineers of the Columbia Broadcasting system, and relayed to the 65th floor of the same tower to modulate the "radio light beam." This beam of light was directed to the 16th floor of the Columbia Broadcasting Building, about a half-mile away. At the receiving point the light beam was picked up by a three-foot, water-filled lens and focused on a standard photo-electric cell. The audio output was built up to loudspeaker volume, for a group of reporters and interested spectators, and a relay line took part of the audio to the master control room of the Columbia network where it was transmitted to the ninety-odd stations of the CBS. A transmission line used for all remote pickups was also installed between the 71st floor of the Chrysler Tower and the master control; this permitted the sound of Vaughn de Leath's voice and the music of Freddie Berren's Orchestra to be faded from light beam to wire as the announcer asked the radio audience to try to detect any change in quality.
This was a severe test but the light-beam system was so faithful in reproduction that reports from all over the country states that no difference in quality could be detected.
Fields for Radio Light Beams
1. For any communication system up to fifty miles that has not been able to secure a wave length from the Federal Radio Commission. 2. For a communication system between engine and caboose on some of our one and four mile freight trains. 3. Telephone and telegraph communication to and from moving passenger trains. A wide-open field. 4. In large industrial plants as a combination source of illumination and call system. 5. Shore-to-island and island-to-island telephone and telegraph communication, where the price of a submarine cable would be too costly. 6. Communication between plane and ground, and plane for the passengers who are not allowed the use of present radio service, which is meant primarily for weather map data for the pilot. 7. Train dispatching. 8. Grade crossing warnings. 9. Fire-boat dispatching. (The entire harbor can be seen from the Chrysler Tower.) This service is now by radio, and these radio channels could be relieved for other services. 10. Police services could be handled much the same as air light-beacons. Constant communication from headquarters could be maintained by talking on successive lights along our city streets. The dead spots that now exist in radio areas may obviously be completely eliminated. 11. Communication with large liners that sometimes have to wait hours to get through Quarantine. Two-way service could be used and parties on the boats connected with the Bell Telephone service and plane throughout the land. 12. Broadcasting paid advertising, similar to our present radio advertising programs. The use of "black light" (invisible, infrared rays filtered from the mercury light) would satisfy the City Fathers.
Figure C is a photo of the receiving equipment. The large lens was used to ensure enough light even under the most adverse conditions. It was found to be very much larger than necessary; a good signal could be picked up on a six-inch lens, at this distance. A standard color-sensitive photoelectric cell, peaked in the blue region of the color spectrum, was purchased from the American Photoelectric Corporation, and was used to convert the audio modulations of the light beam into sound. This audio signal was then sent by the regular wire line to the various stations of the CBS network, and from there into the millions of homes throughout the United States and Canada.
The distance over which this test was conducted was one of circumstance and not choice. The one-half mile in most cases would not seem "commercial," but this happened to be the distance between the two buildings. It was advantageous to use these two points - the Chrysler Tower being the locus of our laboratory, and the Columbia Building housing the CBS master control room, the main feed for the entire network. Conservative estimates, based on measurements, have shown that distances up to 10 and 15 miles can be covered with no loss in quality.
With the intrinsic brilliance developed at the power now available, we produce about 50,000 beam candle-power which is modulated 100%. Measurements are under way at the present time to establish the field intensities of the beam in the New York area. Work has been done in broad daylight with as good results as at night. Cell rush caused by the great amount of sunlight falling on the photo-cell (also, "interfering" rays, such as those of an electric-light sign, etc.), can be balanced out by a very simple method used in all laboratories when accurate work is required.
As the driver or power-to-light converter is a standard radio transmitter, it is possible to multiplex the beam by multiplexing the transmitter by any of the well-known methods. It is possible to use heterodyne detection on the receiving end as the carrier and its sidebands appears in the output of the vacuum-type photoelectric cells. This means that the extreme sensitivity of this method of detection would greatly increase the distance over which telegraphic communication can be carried on, without increase in power at the transmitter.
A Light-Beam Radio Broadcast System
When we stop to think of the tremendous bands of frequencies that are available by this new method, we wonder why more effort has not been put on systems to relieve the terrible congestion that now exists in the present broadcast channels. The frequency of the ionizing current gives us one channel to work on. Now, if we take another searchlight and change the ionizing frequency, say, 10 or 20 kc., we have another channel - and so on for the entire broadcast band. Mercury has three spectral lines, yellow, green, and blue. In the recent broadcast we used the blue line. This means that we can have 600 searchlights using the 600 channels on the blue line, with 600 more channels on the yellow line of 600 other searchlights, and still 600 channels on 600 more lights using the green line. At first this statement may seem wild, but anyone who will take the time to investigate the system will find that things have not been misrepresented.
This opens up a tremendous field for Service Men because one of the best ways to take care of an apartment house would be to have one large reflector on the top of the apartment and the audio developed in each cell could be "piped" down through the building at 500 ohms and rented to each subscriber so he could plug the program into the phonograph jack of his present broadcast receiver.
It is only necessary to stop and think for a moment to see the tremendous value of this system to armies and navies, both in time of peace and in time of war.
In summing up this brief outline, it would seem that inventions in the metal-vapor induction-lamp field have set a new mark for high quality communication systems. The directive qualities may be used if desired, or the complete , 360 degree servicing of any area by well-known lens systems. The horizon being the boundary of interference between cities, together with an almost unlimited frequency response for television, augurs well for this system of communication which will be bound to show startling advances in the near future.
*President, Myers Electrical Research Corp.
Posted March 17, 2015