First-Hand:The Development (and Subsequent Abandonment) of Low-Frequency Radio
Submitted by Frank R. Stansel
It has been said that the road to progress is lined with junk heaps. And all too often the stories behind those junk heaps are forgotten. May I present the story about a portion of a junk heap I had a part in making?
In 1927, the first commercial transatlantic telephone circuit was established using a radio circuit operating at 60 kilohertz. The service was so successful that AT&T began to look for ways to increase the service. A frequency assignment of 68 kilohertz was available so plans were begun to add a second low frequency channel.
Our group in the Bell Telephone Laboratories, at the field station at Whippany, New Jersey, had finished designing the first 50 kilowatt broadcast transmitter which was installed at station WLW. We were asked by AT&T to design an improved radio transmitter for this new service. It was proposed to establish a new radio station in Maine with two radio transmitters using this new design. One transmitter would serve the new 68 kilohertz channel, the second would replace the 60 kilohertz transmitter then in service at the RCA station at Rocky Point on Long Island.
Our group constructed a model of the proposed transmitter. The final stage, which could deliver 300 kilowatts at 68 kilohertz, initially consisted of about twenty large water cooled vacuum tubes. Later, a double ended water cooled tube capable of dissipating 100 kilowatts became available. The last stage in the new transmitter was changed to use six of these tubes in push-pull.
Both transmitters were to use a common antenna. This antenna, instead of being the conventional, low frequency type on four hundred foot towers as at Rockey Point, was to be a wave antenna type. This type consisted essentially of an array of long lines. Such antennas had been used successfully for the reception of radio signals but had never been used for transmission.
There was uncertainty about whether larger currents used in a transmitting antenna caused some distortion due to possible nonlinear elements in the ground. To clear up this point, AT&T built a test antenna in Maine just across the Pennobscott River from the University of Maine campus at Orano. The antenna was fifteen miles long and consisted of a single wire mounted on thirty foot telephone poles. To measure distortion created in an element of a single sideband transmitter, such as this one, two frequencies are applied and the third order modulation products (of the type 2A-B) are measured. We needed to energize this antenna simultaneously with two frequencies.
For these tests, we built a queer radio transmitter. We were authorized to operate on any frequency between 50 kilohertz and 75 kilohertz except for certain reserved frequencies, one of which was 68 kilohertz. (Sixty-eight kilohertz had been reserved for the new AT&T station.) Our test transmitter thus contained two channels and delivered to the antenna 12 kilowatts at 67 kilohertz and simultaneously 12 kilowatts at 69 kilohertz. The final stage of each channel was a water cooled vacuum tube. The outputs of these tubes were combined and then, to insure an absolutely clean test signal, a band pass wave filter was installed between the transmitter and the antenna. As this filter had to pass 24 kilowatts of power, I doubt there has ever been another wave filter as large as this one built.
Why did this project never see commercial service and eventually land in the junk heap? Economic and political conditions in Europe at that time were in turmoil. The British Post Office found they could not meet the original proposed service date and so the project had to be postponed a couple of times. Then with the commencement of hostilities of WWII, the project was indefinitely postponed.
After the war, it became quite evident that the low frequency radio band with its limited band width could never meet the need for transatlantic communication which was growing larger each year. So the use of low frequency radio was replaced by a newer technique, the cable and later satellite transmission. However, it wasn't a complete loss. Personally, I learned a great deal on this project as, I am sure, the others in our group did. And some of the ideas explored were used in other applications later.