VALVES AT UHF

A REVIEW OF RECENT DEVELOPMENTS

An extract from the March 1954 edition of Practical Television magazine and written by S. Simpson.

By way of a background, British television was in 1954 still broadcast at around 50Mc/s, as had the first transmissions of 1936 when such high frequency was leading edge stuff, at least for domestic equipment. In 1954, new "independant" television stations were planned pushing the frequency up to around 170Mc/s.

THE news that the proposed commercial television transmitters will operate at V.H.F. brings into prominence the question of receiver design at these frequencies. Very little has so far been done in the way of television receiver construction in this country at the frequencies concerned, and it is highly probable that new techniques may be developed. Aerial design is, of course, only one aspect of the matter, and many of the valves specially designed for V.H.F. and U.H.F. work may come into more general use if and when the new transmitters open up. The following data is, therefore, offered for the use of those who may wish to carry out experimental work in the design of wideband receivers suitable for television reception on much higher frequencies than are at present employed.

Not so very long ago the satisfactory operation of radio-communication equipment on frequencies of 60 to 120 Mc/s was considered quite an achievement in this and many other countries, but it is indicative of the trend of development that 120 Mc/s operation is now regarded as a commonplace affair. Today the thoughts and experiments of the development engineers here and abroad are centred on the centimetric held where dipoles have given way to paraboloids, inductors have become straight tubes, and capacitors are only noticeable by their absence in any form with which we have become familiar. Perhaps the most remarkable change in the appearance of typical Kilo-megacycle range equipment lies in the valves used to generate oscillations.

Before considering these tubes, as they are now called, one may ask: What has brought about this concentration on ultra-high frequency operation? Radar, of course, was a very important contributory factor, and the war years saw rapid expansion in this field of U.H.F. Recently, the radio links which carry the BBC television programmes to the north, and the now famous experimental TV relay from France to this country, have drawn public notice to some of the work that goes on behind the scenes in the large radio concerns. Communication interests are now closely watching centimetric radio activities ; in this sphere would seem to lie the answer to the problem of trans-continental telephonic communication in areas where it is impracticable, either for reasons of distance or physiography, to use the orthodox land-lines over the entire route.

These, very briefly, are a few of the reasons behind the recent impetus given to research in the Kmc/s field, but such developments would not have been possible without the introduction of new high-power tubes specially designed to work at these extremely high frequencies--tubes which, to the average reader who recognises a valve by its type number or its base, can be rather awesome at first glance. It is hoped in this review to throw some light on the method of operation of these tubes, to say why they were necessary, and to give a summary of the present position in their development.

Tetrodes at U.H.F.

For several years tetrodes and pentodes were the valves mainly used on V.H.F., and even up into the U.H.F. range the triode was--and still is--used with good effect. As the frequencies increased, however, so the dimensions of the electrodes decreased in order to reduce their effect on the tuned circuits, and the overall effect was a reduction in power which could ill be spared because of the high R.F. losses at U.H.F in the materials from which the electrodes were made. The call has been for greater power to overcome these losses, and it became apparent to development technicians that new designs and new methods of valve operation would first have to be sought before the real work in the Kmc/s range could begin.

New designs were evolved, but the method of electron control is not new. In the closing years of the 19th century several European scientists discovered that powerful magnets could sway electrical discharges in evacuated tubes, and they were quick to realise that the glowing rays of " matter " in their Geissler tubes had similar properties to those of an ordinary current-bearing conductor. Our present-day scientists were also quick to reallse that in the "freak " behaviour of the evacuated Geissler tube lay a possible answer to the problem of cathode current control, and one excellent result of their investigations has been the "Magnetron.''

The Magnetron

Early in the era of magnetically controlled valves this tube provided many of the answers to the centi-metric engineer's problems of obtaining reasonable power at U.H.F. As will be seen from Fig. 1, in construction it is quite unlike the orthodox negative-grid valve. The cathode is an electrically heated hollow tube with end caps which serve to concentrate the cathode emission in a zone of high efficiency. The anode is a circular copper block in the form of a drum surrounding the cathode, from which it is adequately spaced. In the anode block are eight holes, in the same axis as that of the cathode; the holes extend right through the drum, and on one side of each cylindrical hole a slot is cut which extends radially towards the cathode. The arrangement of holes and slots is termed a "cavity resonator."

The method of operation depends on the effect of an intense magnetic field from a permanent magnet on the field produced by an electric current, and also on the fact that the resonators will oscillate quite readily at a frequency dependent on their physical dimensions, which can be altered by external mechanical means. Electrons, forming the current upon which the magnet will act, leave the cathode (held highly negative) enroute to the earthed anode block. Some of these electrons will reach the anode region, and in doing so come under the influence of the permanent magnet which gives the tube its name. At this stage the path of the electron is bent away from the anode, and the electron follows a course which is the resultant of the two forces pulling on it. In addition, the electron has to contend with an oscillating feld due to the R.F. energy in the cavity of the resonator. Two things can now happen to the electron: (a) it may arrive out of phase to assist the oscillations, in which case it is immediately swung back by the magnet to the cathode, or (b) it may swing into step with the oscillations in such a way as to part with.its energy in assisting them, before coming to rest on the anode.

In this tube lay the first solutions to many of the difficulties found with the negative-grid tube. Anode/cathode potential could be quite high, since anode/cathode spacing had little or nothing to do with frequency of oscillation. In the negative-grid valve, the spacing affected transit-time, and as a result the frequency obtainable. The Magnetron cathode could be as large as was considered necessary, since here also size did not affect frequency. Greater emission was there for the asking--and with it greater power. In this respect a pleasant surprise lay In store for the research engineers ; it was found that the heater current could be cut off once the tube had started operation, but cathode current continued because of the secondary emission, due to the heating
of the cathode, by the repelled electrons.

The tube is relatively easy to cool compared to lower-frequency tubes of the same power capabilities, and it has proved reliable in operation. The wave-length obtainable can be as low as 0.5 cm/s., as was more or less expected; What was rather unexpected was the enormous power realised from the Magnetron, a typical experimental result being 5 mega-watts at 10 cm/s., and efficiencies in the region of 70 per cent.

The Magnetron, splendid performer as it was and still is, has proved to be only the forerunner of several other "cavity"-type tubes, prominent among them being the "Klystron."

The Klystron

Difficulties in tuning arrangements, the liability to sudden change in oscillation mode of the Magnetron, and the need for heavy magnets, are perhaps the main reasons underlying the introduction of the Klystron. Here again magnetic fields can play a part in the functioning of the tube, but only to assist in focusing the electron stream passing through the valve. The general arrangement of a Klystron takes the form as shown in Fig. 2.

A cathode, the emission from which is prefocused by an electrostatic shield, is placed in alignment with a "cavity resonator," a form of hollow cylinder containing two gaps through which the cathode stream must pass. This resonator-is termed the "buncher." The electrons in their travel now pass into the ''drift-tube" and finally enter a second cavity resonator, referred to as the "catcher."

Thee operation of the tube is somewhat as follows: Electrons leaving the heated cathode are prefocused at the shield, and in some cases further focused magnetically, resulting in a very sharp beam. The electrons now pass into the buncher through the first aperture. Between the first and second apertures of the buncher is the U.H.F. oscillatory field, and its effect is to retard or speed up the incoming electrons depending on the instant of their arrival as related to the phase of oscillation.

The usual method of action is to consider an electron - the "idle" electron - which arrives at an instant when the R.F. field neither assists nor opposes its transit. The idle electron will move across the gap, out of the second aperture and into the drift-tube at a speed dependent on the pull of the catcher. If now a second electron arrives at an instant when the polarity of the field assists the catcher, then that electron is hurried along and will overtake the idle electron somewhere in the drift-tube; the two electrons will then proceed together along its length.

Considering a third electron which arrives at an instant of retarding polarity it will be slowed down and eventually overtaken by the next electron which is now, at the completion of the R.F. alternation, an idle electron moving under catcher influence only.

There is, therefore, a tendency for the electrons leaving the first resonator to crowd together in groups,or bunches; thus the term "buncher." By the time the electrons have "drifted" to the far end of the drift-tube the bunching is very pronounced, and they now emerge from the tube to pass through the single aperture into the catcher in sharp pulses, timed to assist the oscillations already existing there.

A very small power will cause the bunching and subsequent pulsing. In effect, because of the flick excitation at the resonaator the Klystron can be considered as a Class C amplifier, and in common with Class C operation it is possible to tune the catcher to a harmonic of the input frequency. The Klystron has the advantage, however, of greater output at high harmonics perhaps not so readily developed in the negative-grid valve.

In common with the Magnetron the output can be taken from the Klystron by means of a probe or into a waveguide designed for the operational frequency. For use as an oscillator the output is coupled back to the input by means of a short, internal loop, and under these conditions the tube is most useful at frequencies in the 3,000 Mc/s range.