Summary: This paper summarises the factors that affected the development of CW radio communication during the period up to 1918. It shows that most of the important circuits had been invented by 1914. The major technical factor affecting the successful development of CW radios for battlefield communication was the unavailability of robust radio valves: these did not become available until late in 1915 with the introduction of the French TM valve. Up until that time almost all radios were spark transmitters and crystal detector receivers.
Full version of the paper: Technical factors affecting CW radio communication in WW1 [pdf] Part 2 Part 3
Valve development to 1918
- Fleming diode
The diode valve used as a radio wave detector was patented by John Ambrose Fleming in October 1904 . This had the important feature of a solid cylindrical anode that totally enclosed the carbon filament. In 1908 Fleming took out a patent for a tungsten filament . However, the technique had not yet been perfected for ductile tungsten and the filament became brittle after being heated to incandescence.
The Fleming diode only had limited usage and was quickly superseded by the crystal detector which was far more reliable and didn’t require a lead-acid battery to power it. However, the carborundum crystal did require a small bias voltage for optimum sensitivity but this was supplied by a small dry-cell battery. Other crystals, such as the Perikon, did not require a bias but they required frequent re-adjustment.
- De Forest audion
The US engineer, Lee de Forest, believed he could make a more sensitive detector than the Fleming diode. After several unsuccessful attempts the decisive and historic step was made at the end of 1906 when, on 31 December, de Forest inserted a third element, shaped like a grid iron, between the filament and plate. Thus was born the grid audion, a triode valve with an internal control electrode. A patent application was made early in 1907 .
The first grid audions had a carbon filament; both the grid and anode were made of nickel and the glass envelope was spherical. As further valves were made, however, the manufacturer, McCandless, used a variety of materials and there were great variations in the constructional methods and their dimensions. The carbon filament was replaced by tantalum, but this was found to warp. In about 1908, tungsten became available for use in electric lamps. As mentioned above, at the time, the manufacturing process had not been perfected and the material became brittle at the temperature required for incandescence. Tungsten had the advantage that it could provide a higher emission than tantalum, which gave rise to a suggestion made to de Forest by Walter Hudson that he should construct his filaments from tungsten wrapped with fine tantalum wire, a process later modified to cover the tungsten with a tantalum paste. This simplified the manufacturing process and, in this form, the Hudson filament, as it became known, was used for many years by de Forest.
When first produced, the audion valve had many shortcomings, because the technology used for its manufacture was new and poorly developed. Particular problems were the use of unsuitable materials in its construction, a short filament life and poor evacuation of gas from the sealed bulb, resulting in a soft vacuum.
For several years, from 1907 to 1913, the audion was only used as a radio detector, as its conduction mechanism and general principles of operation were not understood. De Forest carried out some experimental work on low frequency amplification for telephone repeaters, but his early efforts were not successful. This was probably because he used r.f. instead of a.f. coupling and did not correctly bias the valve.
The basic problems of the audion were eventually overcome by the industrial laboratories of AT&T, (Western Electric) and General Electric, both companies working independently. The driving force for AT&T was the need to develop a reliable amplifier for their telephone repeaters to improve long-distance telephony. For General Electric the driving force was to provide a speech modulator for their high-frequency alternators, as this would then make radio telephony a practical possibility.
(Note: The BT-H company manufactured the audion valve in 1916 and pumped it to a high vacuum to overcome some of its erratic performance. It was used for a year or so by the Navy in one-valve heterodyne receivers.)
- Valve improvements at AT&T
AT&T had recognised the importance of an amplifier for use with their telephone circuits as early as 1910. Work was carried out by Harold Arnold in their subsidiary company, Western Electric, during 1911 to 1912. The device developed was a mercury vapour discharge tube using magnetic control of the ionised current. Although some success was achieved, the device was never put into production because it was superseded by the possibility of using the de Forest audion valve as an amplifying device.
In January 1912, Fritz Lowenstein demonstrated to Bell staff an amplifier contained within a sealed box. (This box was lead-lined to prevent X-ray photographs being taken.) The performance of the amplifier was erratic and because Lowenstein would not disclose details of his device, there was no way that the Bell Company could judge its suitability. Matters were thus allowed to rest for a while. Later in 1912, John Stone Stone, an ex-employee of the Bell company in America, had seen a demonstration by de Forest of the audion and recognised its potential as a telephone amplifier. At the end of October 1912, he arranged for de Forest to demonstrate the circuit to Bell staff, with a full disclosure of the circuit used. The performance, as with the Lowenstein demonstration several months earlier, was erratic. Nevertheless, the Bell staff were impressed and they arranged for a development project to be undertaken by Western Electric to investigate the audion device and its suitability as an amplifier in telephone circuits .
Within a short period of time, Arnold, van der Bijl and other engineering scientists of AT&T had turned the primitive audion of de Forest into a reliable telephone relay amplifier. Their main improvements were:
- The production of a high vacuum device, where the conduction was governed by electron current rather than ionisation. This high vacuum was achieved by the use of the Gaede molecular pump that was introduced from Germany in April 1913.
- A more reliable filament. This was made from a strip of platinum coated with barium nitrate to improve emission and thereby allow a lower temperature to be used. The coated filament was a direct development from the pioneering work of Wehnelt.
- A stronger internal construction to support the electrode assembly.
One of the first valves to go into production at Western Electric was the Type A. The first telephone repeater to use this valve went into service on 18 October 1913 and this provided the necessary amplification to achieve long distance line communication. By 1915, improved valves were produced which were claimed to have a filament life of 4500 hours.
AT&T bought the de Forest audion patent for a total sum of $390,000, but this was made in three separate payments. Initially, in 1913 for $50,000, they bought the rights in all fields except wireless telegraphy and telephony. Then in 1914, for $90,000, the company bought a non-exclusive licence in the wireless telephony field. Finally, in 1917, AT&T paid $250,000 for an exclusive licence to all remaining rights.
The amplifier that Lowenstein demonstrated to the Bell staff in January 1912 was later shown to be an audion valve without a grid blocking capacitor, but with negative grid bias. Lowenstein took out a patent for this method of bias in 1912, which he subsequently sold to AT&T for $150,000 [5, 6]. In later years the negative bias patent assumed great importance and was the subject of much litigation.
The most important requirements for repeater valves was long life and constancy of their characteristics. Manufacturing costs was of secondary importance. The valves were used in the benign environment of telephone offices and would not be subject to severe shock, vibration or extremes of temperature.
With the possibility of the USA entering WW1, Western Electric began the development of valves for military use and these had to be mechanically robust, have a reasonable life and withstand extremes of temperature.
America entered the war in April 1917, 22 months after the British and 19 months before the armistice which brought the war to an abrupt end.
Two of the valves mass-produced by Western Electric were the VT-1 and VT-2. The VT-1, was a general-purpose triode for use in radio receivers as a detector, amplifier or low-power oscillator. Initially, the valve was deemed too fragile for use under battlefield conditions but over the ensuing months there were significant structural changes. The VT-2 was designed as a 5-watt transmitting valves and became available in 1917.
The following is an interesting abstract from A History of Engineering and Science in the Bell System. The Early Years: 1875–1925, p.368:
The War came to Europe before the vacuum art had been applied to telephony and as a consequence the vacuum tube received only limited use in the European nations and only for radiotelegraphy. In the United States it was possible to continue peacetime development for several years after the war began and as a consequence this country’s technicians were in a much better position to apply radiotelephony to the war effort.
- Valve Improvements at General Electric
General Electric became interested in the audion valve through the desire to provide speech modulation for their high frequency alternators. The company was a major manufacturer of electrical equipment, including large power generators. Their interest in high-frequency alternators was stimulated by Reginald Fessenden, a brilliant, enigmatic engineer who headed a small company called NESCO (National Electric Signalling Company). Amongst his many inventions was the heterodyne method of reception, whereby the incoming radio frequency signal was beat against a local oscillation to give a low, audio frequency that could be applied directly to headphones (described later in this paper).
Fessenden was one of the pioneers of radio telephony and his ideas were many years ahead of the time. His earliest attempts to transmit voice signals were in 1900 when he used a microphone to modulate a rotating spark generator, although articulation was poor. He was aware of the work of Elihu Thomson and Nikola Tesla with high frequency generators. However, with these, the frequency achieved was no higher than 5 kHz. In 1900, Fessenden wrote to Charles Steinmetz, the consulting engineer of General Electric, enclosing a specification for a high-frequency alternator. The first machine, produced by Steinmetz in 1903, had an output power of 1kW, but the maximum frequency was only 10 kHz, making it unsuitable for direct transmission purposes.
The second G.E. machine for Fessenden was designed by a young Swedish engineer, Ernst Alexanderson, who had recently settled in the United States. This machine, delivered to Fessenden at his Brant Rock radio station in August 1906, could generate a modest output power of 500W at a frequency of 76 kHz. Initially, the range achieved for telephonic signals was only 11 miles, but in July 1907, speech was transmitted between Brant Rock and Jamaica, a distance of nearly 200 miles .
Over the next few years, Alexanderson developed larger h.f. alternators capable of delivering several hundred kilowatts at a frequency of 100 to 200 kHz . For wireless telephony applications, these alternators required a high power modulator. There were two possible approaches: one was to modulate directly with a microphone, but this resulted in excessive power dissipation (specially cooled microphones were developed for this purpose). The second approach was to insert an amplifier between the microphone and the alternator. In 1912, Alexanderson developed a magnetic amplifier with some success, but he was not entirely satisfied with its performance.
Two of Alexanderson’s h.f. alternators were delivered to John Hammond Jr at his Massachusetts laboratory in 1912. Whilst there, Alexanderson was told of the audion valve. In spite of its inherent weaknesses at that time, he thought it might be adapted into an amplifying device to overcome his modulation problem. General Electric were ideally equipped to develop the primitive audion valve into a robust product. They were a major manufacturer of electric lamps and had extensive research and development facilities. Amongst the staff at that time were many brilliant engineering scientists, including Coolidge and Langmuir (who in 1932 received the Nobel Chemistry Prize). Coolidge had recently perfected a process for manufacturing ductile tungsten for use as filaments in electric lamps, which gave a greater light output per watt and was also more reliable [9, 10]. The task of developing the audion into a satisfactory device was given to Langmuir, assisted by William White.
Langmuir recognised that one of the main limitations with the audion was its poor vacuum; this was quite contrary to the strongly held view of others at that time. He was familiar with the work of Richardson, Fleming and other leading scientists of the day, and he immediately set out to understand the operation of thermionic emission.
In 1913 he published an important paper in the Physical Review in which he verified Richardson’s equation for emission from a hot cathode, but also showed that as the cathode temperature was raised, the increased emission of electrons gave rise to a space charge that formed between the cathode and anode . This had the effect of repelling the electrons back to the cathode. Thus, when the space charge was present, the actual current that flowed to the anode was less than that given by Richardson’s law. This current was found to be proportional to the anode voltage raised to the power of 1.5. Langmuir established that this relationship between anode current and anode voltage was true irrespective of the shape of the electrodes: a similar result had been found by C.D. Child in 1911 for the flow of positive ions between two parallel plates .
Langmuir expanded on this work in his famous paper of 1915, ‘The Pure Electron Discharge’ .
According to White in an unpublished document, dated 1 March 1929:
‘Until the outbreak of War [presumably April 1917], all work on tubes was almost entirely of a research and experimental nature. … Prior to the War, owing to the patent situation, there was no commercial outlet possible for receiving tubes.
…The coming of the War changed this because it was early recognized by the Signal Corps of the Army that vacuum tubes would probably play an important part. [This is a 60-page document and makes very interesting reading, particularly the sections covering tube development for the Army and Navy, together with the manufacturing problems and how these were overcome].
This lengthy discussion so far has been to show how all the fundamental problems associated with the de Forest audion, with its poor vacuum, erratic performance and fragility had been solved by the two US companies, Western Electric General Electric by 1916.
1. Fleming, JA, British patent 24,850, appl. 16 November 1914.
2. Fleming, JA, British patent 13,518, appl. 25 June 1908.
3. de Forest, Lee, US patent 879,532, appl. 29 January 1907.
4. Aitken, Hugh GJ, The Continuous Wave: Princetown University Press; 1984, pp.246–7.
5. Lowenstein, US patent 1,232,764, appl. 24 April 1912.
6. Ref 4, p.228.
7. Fessenden, RA, ‘Wireless Telephony’, Proc. AIEE, 27, 1908, pp.1283–58. (See p.1309.)
8. Coursey, PR, Telephony Without Wires. London: The Wireless Press; 1929, p.195.
9. Coolidge, William D, US patent, 1,082,933, appl. 19 June 1912.
10. Hammond, JW, Men and Volts. New York: JB Lippincott Co.; 1941. See Ch. 37 ‘The Taming of Tungsten’.
11. Langmuir, Irving, ‘The Effect of Space Charge and Residual Gases on Thermionic Currents in High Vacuum’, Phys. Rev., 2, 1913, pp.450–86.
12. Child CD, ‘Discharge from hot CaO’, Phys. Rev., 32, 1911, pp.492–511.
13. Langmuir, Irving, ‘The Pure Electron Discharge’, Proc. IRE, 3, September 1915, pp.261–93.