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By 1900, the fundamentals of microwave transmission, and quasi-optical analogue results were firmly established. The theoretical solutions of waveguide transmission and modes of oscillation on spherical and other conductors were nearly all established; but the subject died completely.

Many Science and engineering students in higher education still make believe how modern and up to the minute their advanced courses in microwaves are. Mainly because of the way modern mass-media approach such matters, they feel at one with all those dishes that sprout on Post Office towers and the microwave systems now used for satellite links around the the world. It seems to be symbolic of being right there and 'with it' in high technology.

It seems to come as a considerable shock to these students when it is pointed out that such technology was nearly all spelled out sufficiently early on for Queen Victoria to have had the possibility of seeing and inspecting the hardware. As the author of Ecclesiastes put it, ^{[★] Ecclesiastes, ch. 1 and 2. (the 'Tao' of the Old Testament.)} 'There is nothing new under the Sun'.

Microwave physics was bound to be realised soon after Clerk Maxwell's equations predicted the possibility of long electromagnetic waves. In 1883, F G FitzGerald was already suggesting that Leyden jar discharges should emit Maxwellian radiation ^{[★] See, Joseph Larmor, ed. 'The Scientific Writings of the Late F G FitzGerald. Longmans Green (London) 1902.} Then in 1888 ^{[★] H Hertz. 'Electric Waves'. Macmillan and Co. 1893. (Reprint by Dover available.)} Heinrich Hertz - at Karlsruhe conclusively demonstrated that such waves existed. The apparatus he devised to generate the shorter of his various wavelengths was broadly resonant at 500 MHz, and with it most of the properties of microwave optics were established. Hertz used a resonant dipole at the focal line of a cylindrical parabolic aerial, together with a short, parallel-wire transmission line to the detector from a similar dipole at the receiver. Working at the same time in Britain and very nearly establishing the space waves, Oliver Lodge was already well advanced in demonstrating powerful high-frequency waves on wires. Lodge, to quote a report of the time, ^{[★] O J Lodge. Signalling Through Space Without Wires. The Electrician Printing and Publishing Co. 1898.} '. . . got quantitative evidence of nodes and loops in wires when working with Mr Chattock in the session 1887-8 (the Bath meeting of the BA) . . . the wires themselves becoming momentarily luminous . . . except at the nodes, thus enabling the waves to be actually seen, having been made stationary by reflection. . . .The wires. . .were very long going five or six times round a large hall . . . '

The reports published on all these spectacular preliminary observations soon resulted in the near-exponential growth of experimental work and the publication of papers, so well described by Derek De Solla Price in his book 'Big Science, Little Science'. ^{[★] Derek De Solla Price. Big Science, Little Science. New York, Columbia University Press, 1963.} Within five years, an identifiable 'invisible college' existed on this subject - a socio-scientific phenomenon also described by Price. The members of this international group were described as 'the Hertzians'. Hertz died at this time, and leadership moved over more firmly to Lodge, A Righi ^{[★] A Righi. LbOttica delle Oscillazioni Elettriche.} in Italy, and to J Chunder Bose from Calcutta ^{[★] J C Bose. Collected Physical Papers. Longmans Green, NY. 1927.} whose millimetre wave experiments were quite remarkable. It was during Righi's public lectures that the young Marconi became acquainted with electromagnetic wave phenomena. Other workers included F J Trouton in Dublin, J A Fleming, Zehnder in Germany, and contributions from Lord Rayleigh strengthened the theoretical base.

Fig. 1. Lodge's radiating cavity with irises is clearly seen in this picture, together with a flanged circular waveguide receiving aperture and detection system. (Lodge's caption: General arrangement of experiments with the Copper 'Hat', showing Metal Box on a Stool, standing outside the Theatre. The Box is not exactly represented, but inside it the Radiators were fixed with a graduated series of apertures; the Copper Hat containing the Coherer is seen on the Table with the Metal Box on the left of the Table containing Battery and Galvanometer Coil connected to it by a compo pipe conveying the wires, as in Fig. 19c; the Lamp and Scale barely indicated at one side of the Table; a Paraffin Prism; and a Polarising; Grid of copper wires stretched on a frame. [This figure is from a thumbnail sketch by Mr A P Trotter, taken at the Lecture in 1894.] ).

Oliver Lodge demonstrated radiation from circular waveguides on 1 June, 1894 at the Royal Institution in London. To this end, he invented the radiating iris, and in effect also resonant cavities. He called them bcopper hatsb, and clearly intended them as directive aperture radiators and to raise the 'Q' of the oscillators, as in Figure 1. Figure 2 clearly shows that he was exciting, the TE₁₁ modes in the transmitting guide. By placing his coherer detector crossways in the receiving guide he detected this mode, but he also stated that, 'Sometimes the (coherer) tube is put lengthwise in the hat instead of crossways, which makes it less sensitive, and also has the advantage of doing away with the polarising, or rather analysing, power of a crosswise tube'. This position of his detector can be seen in figure 1.

Fig.2a. Pictured are two of Lodge's oscillators, which would have generated predominantly TE₁₁ mode radiation. Dr Lodge's Hollow Cylindrical Radiator, arranged horizontally against the outside of a Metal-lined Box containing the Spark-producing Apparatus. Half natural size. Emitting 3in. waves.

Fig.2b. Spherical Radiator for emitting a Horizontal Beam, arranged inside a Copper Hat, fixed against the outside of a Metal-lined Box, which contains induction coil and battery and key. One-eighth natural size. The wires pass into the box through glass tubes not shown.

Clearly Lodge understood he was using the circularly polarised TM₀₁ mode in this instance. This mode has a null along the axis, and we find Lodge writing about the receiver as, '. . . a copper hat with its mouth turned well askew to the source . . .' thus receiving the TE₁₁ mode radiation on one of the maxima to the side of the axis. Lodge's microwave demonstration operated at 4 GHz, his 7.5 cm waves were just above 'S' band.

Fig. 3. This microwave bench enabled Bose to investigate polar diagrams, crystal lattice diffraction (or its analogue), total internal reflection, refractive indices and so on, all at 60 GHz. (Bose's caption: Arrangement of the Apparatus. R, radiator; Spectrometer-circle ; M. plane minor; C. cylindrical minor; p. totally reflecting prism; P, semi-cylinders; K, crystal-holder; F. collecting funnel attached to the spiral spring receiver; t, tangent screw by which the receiver is rotated; V, voltaic cell: r, circular rheostat; G. galvanometer).

Fig. 4. It is clear that Bose had all the requisite motions to establish polarisation angles. He must have understood the effect of wire gratings placed parallel, perpendicular and in practice at any angle, to the E-vector of the 60 GHz beam of radiation. (Bose's caption: Polarisation Apparatus. K, crystal-holder; S. a piece of stratified rock: C. a crystal; J. jute polariser; W. wire-grating polariser; D, vertical graduated disc, by which the rotation is measured).

But it is to Bose we owe a considerable advancement in millimetre wave studies. He developed a semiconductor detector, rectangular waveguides and horn aerials (Figures 3 and 4). His microwave bench was put to use in measurements of refractive index, reflection from plane and curved surfaces and many experiments on polarisation.

Bose generated 5 mm wave radiation near 'E' band. His resonator consisted of a conducting sphere set oscillating across a diameter. This was the common form of transmitter employed by virtually all the experimenters. Bose seems to have refined his spherical resonator by partially enclosing it in capacitive cups each side, as seen in his drawing reproduced as Figure 5.

Fig. 5. Reproduced here are Bose's capacitively loaded spherical oscillator, and a curious two dimensional spring contact detector. (left) The Radiator. (right) The spiral spring receiver.

This must have increased the charge stored, therefore the energy, by capacitive loading. Also, the radiation must have been reduced by the partial screening effect, thus raising the 'Q' of the system, which yielded many more cycles of oscillation per discharge than must have been usual. (The bandwidth of the radiation must have been reduced.) It had been reported elsewhere by other members of the 'Hertzians' that the damping of an open oscillator of this type was such that normally only one or two complete oscillations were obtained.

The ingenious detector evolved by Bose is also shown in Figure 5. It most likely consists of a space-irradiated multi-contact semiconductor (using the natural oxide of the springs) plus some cohering action. But from Bose's reports this action, unlike most coherers, appeared to be self-decohering.

One of his experiments involved Bradshaw's Railway timetable, interleaved with sheets of tinfoil in the pages, as a cut off metal plate grating. Also, one of his millimetre aerials used a sulphur lens, shaped to the required curve by using the refractive index as measured on the bench at 60 GHz.

A most remarkable development carried out by Bose has already been mentioned. His use of microwave horn aerials occurred well before the turn of the century. No doubt he considered that the larger collecting area of a horn aperture would increase the energy incident on his detector - reasoning precisely in the same way as a microwave engineer now designing his receiving aerial for a communication, links or satellite ground station.

All these workers generated radio frequency powers of considerable magnitude, so they were not energy limited. Oliver Lodge estimated that one of the shock excited oscillators that were in popular use at the time developed a peak power of some 70 kW. He goes on to say that sparks could be drawn from all sorts of metal pipe wires and fittings, and that fuses were regularly blown by the received power that was picked up by the electrical system of the building, when the sending apparatus was operating.

In 1896, Lord Rayleigh ^{[★] Lord Rayleigh. On the Passage of Electric Waves Through Tubes . . . Phil. Mag. vol. 43, pp 125 to 132, February 1897.} published a complete solution to Maxwell's equations yielding all the possible modes in rectangular and circular waveguides, complete with Bessel's functions and all. Thus the stage was set for point to point communications links with parabolic aerials, waveguide feeders, increasingly sensitive detectors - and even microwave Radio Astronomy of the Sun as Oliver Lodge proposed, and actually attempted.

Yet one of the most remarkable mysteries in science and technology is that none of this occurred. The subject faded rapidly from the scene. Hertz was dead, the others seemed to switch to new fields of work. Microwave electronics was before its time and had to wait half a century until just before the second world war for Southworth, Chu, Schelkunoff and others, to make the re- discovery and begin the applications.

Lodge, who was the outstanding British figure in this work, became increasingly involved in running a University, and took an increasing interest in the paranormal, becoming in fact President of the Society for Psychical Research. He continued to write many instructive articles on 'wireless' in the journals such as the early editions of Wireless World, and was elected President of the Radio Society of Gt Britain for the year 1925.

Bose moved on to investigations of plant growth and the effects of EM radiation upon biological structures. All the other workers faded from view. Perhaps it was Gugielmo Marconi's great success in using extremely long Hertzian waves for telegraphy that swung all the young engineers away from microwaves. But whatever the historial cause of the moratorium in microwave science, nothing can detract from the lustre of these first pioneers of microwaves at the end of the 19th century.

References

1. Ecclesiastes, ch. 1 and 2. (the 'Tao' of the Old Testament.)
2. See, Joseph Larmor, ed. 'The Scientific Writings of the Late F G FitzGerald. Longmans Green (London) 1902. Page 100 contains the details of comments FitzGerald made at the 1883 British Association meeting in Dublin.
3. H Hertz. 'Electric Waves'. Macmillan and Co. 1893. (Reprint by Dover available.)
4. O J Lodge. Signalling Through Space Without Wires. The Electrician Printing and Publishing Co. 1898.
5. Derek De Solla Price. Big Science, Little Science. New York, Columbia University Press, 1963.
6. A Righi. LbOttica delle Oscillazioni Elettriche.
7. J C Bose. Collected Physical Papers. Longmans Green, NY. 1927.
8. Lord Rayleigh. On the Passage of Electric Waves Through Tubes . . . Phil. Mag. vol. 43, pp 125 to 132, February 1897.