US2342263A - Electron discharge apparatus - Google Patents

Description

Feb. 22, 1944. 1 H. FREMUN 2,342,263

A ELECTRON DISCHARGE APPARATUS Filed NOV. 27, 1940 Ldlllnmummw B/ 1 l l l l BLMZLQMW Patented Feb. 22, 1944 ELECTRON DISCHARGE APPARATUS John Heaver Fremlin, London, England, assignor to International Standard Electric Corporation, New York, N. Y.

Application November 27, 1940, Serial No. 367,400 In Great Britain December 22, 1939 (Cl. Z50-36) 6 Claims.

This invention relates to high frequency electron discharge apparatus of the kind wherein electrons are caused to oscillate about a positive electrode.

The apparatus of the invention comprises means for directing an electron beam over a finite path the potential distribution along which is substantially parabolic so that the field acting upon the electrons at any point in said path is directly proportional to the spacing of that point from a fixed intermediate point in the path about which fixed point the electrons are to oscillate.

Further features of the invention and the advantages derived by the use of the invention will become clear in the following description of certain embodiments given with reference to the accompanying drawing, in which Figs. 1, 1a, 1b, 2, 3 and 4 are greatly enlarged cross-sectional Views of Vacuum tube elements in accordance with the invention.

The structure of Fig. 1 comprises a cathode C with a control grid G, anode A and anti-cathode L. The figure is a cross-section of the actual structure which consists of a series of parallel rods extended in a direction perpendicular tothe paper.

Now it will be seen that if the electrodes G and L are normally at cathode potential and the anode rods have a considerable positive potential, electrons from the cathode which pass between the rods G will tend to move across the gap towards the anti-cathode L. In a preferred embodiment means are 'provided for setting up a magnetic eld along a line cl running from a point c on the surface of cathode C to a point on the surface of anti-cathode L and through the centers of these elements. This magnetic eld is used to ensure that the electrons do not leave the straight path cl. The shape and position of the various electrodes in the tube are so arranged that the potential distribution along the line gl is as far as possible `parabolic in form g being the point on line cl determined by the intersection with line cl of a line passing through the centers of grid elements G. This being the case, the eld acting upon an electron at any point will be directly proportional to its distance from the point O, this point being approximately midway between g and l. It is assumed throughout that the potentials of G and L are nearly cathode potential. The indicated potentials of A, C, G and L may be maintained' by appropriate connections to a source of potential such as battery B.

Now, it is clear that in this circumstance an electron in the space g to l will experience a force which is proportional to its distance from O and is always directed towards O, i. e., the electron will be naturally urged in generally simple harmonic motion about O, and this motion will be accurately synchronous, even though it may gain or lose energy during the period lof oscillation about O.

The fornr of potential distribution described necessitates the use of a magnetic concentrating field for the working of the valve. (Polepieces M indicate diagrammatically a. means for producing a magnetic field.) This appears from the fact that if the potential distribution along gl is parabolic with a potential maximum at `O, from Laplaces equation it follows that a little way off the line gl there` must be a eld away from the line gl which would tend, if any electron deviates from the centre line, to drive it further away still from that line. No possible arrangement of electrodes could in itself prevent this phenomenon if a parabolic potential distribution is to be taken on the centre line.

In Fig. l the parabolic field distribution along the line gl is attained by making the anode rods of large diameter perferably greater than their spacing and comparable with the dimensions Ol, Og, the grid and anti-cathode rods being of relatively small diameter. If the anode rod diameter is reduced and the distance between centres remains unchanged the anode potential .has to be increased. The precise relation between dimensions and potentials can be found by means of a rubber sheet model, probes corresponding to the electrodes being adjusted against opposite sides of the sheet until the sheet conforms to a parabolic template placed along the axis of symmetry of the probes.

The new requirements are compared with the usual conditions in Barkhausen Kurz oscillators in Figs. 1a and 1b. In Fig. la the equipotential lines (dotted) and the corresponding parabolic voltage curve (chain line) are shown for the structure of Fig. 1, while Fig. 1b' shows the potentials in the case of a, planecathode CZ,` positive mesh grid GZ and plane anti-cathode L2.`

The'mode of action of the tube lof Fig. 1 is as follows: l Y

Imagine a continuous emission of electrons from the cathode C between the two' wires of the grid" G. These electrons will normally travel across to L where some of them will be collected and the rest will be reflected back towards the cathode. Now suppose that the electrode L is fore will be removed permanently from the nurnber of electrons oscillating in the space gl. the other hand, electrons which pass O as the electrode L begins to go negative with respect to its normal potential will lose energy to the oscillation, and in consequence a larger number of them than usual will be reflected andv fewer will be collected by the electrode L.

Of the continuous motion of electrons from the cathode C more of these electrons which are moving in such a phase as to give energy to the On n oscillation are preserved than there are of elec- Y trons moving in such a phase as to absorb energy from the oscillation. In consequence of this, the oscillation on balance gains energy from the electron beam, and will build itself up further.

The output may be taken from the anticathode rod L and one of the other electrodes for example by extending the rods to form a tuned Lecher wire system.

The structure may be made symmetrical as shown in Fig. 4 by providing additional rods G1, A1, L1 similar to those shown in Fig. 1 on the opposite side of the cathode and similarly connected to potential source B. There are, then, two oscillating electrodes, L and a second L1 symmetrically disposed with respect to the capacities of the rest of the valve, and these two electrodes may be used as output electrodes. They may be sealed straight from the top of the closed bulb in which the valve is built and tuned directly as a pair of Lecher wires to any desired frequency.

In such case it is found that when this external Lecher wire system is tuned to suitable` wavelengths, the valve itself begins to oscillate. The frequency of oscillation is found to coincide very well with the calculated frequency of oscillation of the electrons in the space gl, thus showing that the valve is operating in the expected manner. The frequency of oscillation depends rst upon the linear dimensions of the valve, and

secondly upon the potential of the accelerating electrodes-A. It is possible, then, with a good valve, by varying the potential of A, to obtain oscillations of a wide range of wavelengths. This gives the valve a marked advantage over the valves of the Barkhausen type, in which the frequency range is very much limited. It is quite possible to obtain oscillations with potentials between 100 volts and 1000 volts on the electrodes A, which gives us a variation of frequency by a.

factor greater than 3. It is also possible in some special circumstances to obtain waves of this type in which apparently the oscillation of the Lecher wire system has a frequency double that of the electrons in the space gl. This would extend the frequency range ofthe valve still further.

It is not proposed that thisv valve should be used for the production of high powers. Its construction, with thin metal rods, and its necessarily small size preclude the possibility that very high powers can be distributed safely. Its Wide frequency range. hQWeVerimakesit Suitable.

that it is very dicult mechanically to construct ale/12,263

for use for example as a receiving valve. In one suitable receiving arrangement, it is proposed to work super-regeneratively, using a quenching oscillation at about 5 megacycles, which can best be put on the grid of the valve. It has been found that if the grid potential is changed considerably from the optimum for oscillations, the oscillations are suppressed. If, then, superimposed upon the direct grid potential, we have an oscillation of frequency some 200-300 times smaller than the "frequency of the valve, itself, there will be only certain, periods, during this relatively low frequency oscillation when the valve is in a position to oscillate.

If areceived signal is now impressed upon the Lecher system the valve will break into o-scillation much more rapidly than if there is no such signal and the-'start of oscillation has to depend entirely upon random thermal variations. In consequence, if a signal is being received the valve will be oscillating on the. average. fora greater proportion ofV its timethan` wouldbe the case if no signal exists. The oscillation ofthe valve can be detected by measuring the direct current to. the Lecher Wire systems. Itis clear that with the. mode, of oscillation which. occurs, the number of. electronsarriving at L willbe considerablyaffected by theexistenceof oscillations. Consequentlyeither a sensitive meter or tele. phones in theV circuit of L willregisterchangesin the current to Lin a` waysuitableV either for direct detectionor. for further amplification. The valve in fact when used as a super-regenerativeV receiver actsA also, as its owndetector.

A number of further variations within the scope .between g andi we have nowtwo different parabolic distributions, one betweenO and l and the.

other between O and y. In order to effectthis, it is necessary that` the two` anodeA rods A should be very closetogether, to ensure that the spaces on either. sidel of. these. rods should be well.

shielded l,from one. another. In this case, the mode, of, phase selection ofthe. electrons. at the Lecher wire lanti-cathodel. is unchanged, and

an electron, instead 4of crossing` from O to Gand.

back to,.O again in half Athe period of the. oscillation of the Lecher wire, nowV does it in ,l1/2 periods It then.. returns, in the same phase as. itwould have if. the `distance O to G were still identical with thev of the oscillation ofthe Lecherwire.

distance() to L. Thishas, however, the mechan-` ical advantage that thevspace O toGcan be made. considerably larger forl a given desired waver length Without making it necessary for the anodeI potential to be, excessively high. Valves Vof this lop-sided typev oscillateV satisfactorily down to wave lengthshof 10 cm. atanode potentials not greater than, 400 to 500 volts.,

A drawback` whichhasgbeenrfound in constructing the valves with yelectrodes arranged symmetrically about thepathode lies lin the fact* the two sides Vof' the valve accurately` alike, Smallv differences between the two sides lead to the pro'- duction of'4 Voscillations inA two differentV wave lengths fer the. .Semeoieeiialpeihe anode rede;

These do not, of course, co-exist at any one time, but each can be obtained by tuning the Lecher wire system suitably. This lack of symmetry is often inconvenient as the peaks of` current of the Lecher wires, as the external circuit is tuned, often over-lap. This has been avoided by coating one side of the cathode only (in any case, the cathode is normally coated only in narrow strips opposite to the gap between the grid wires G). The necessity for constructing both sides of the valve and then using one side only is however a disadvantage and it is best to revert to the single sides structure of Fig. 1 in which the pair of electrodes which are being continued outside the valve as a tuned Lecher wire circuit consist oi the grid and electrode L. This also takes up less space, which is important when it is remembered that the magnetic eld is essential to the working of the valve. With overall dimensions of the order of 1 cm. or less small permanent magnets as indicated by reference characters M can be used and an adequate eld still obtained. The 'working length of the electrode system, that is the length of each rod electrode may be about 2 cm.

It is also proposed to construct the whole tube in metal such as copper, e. g., as shown in Fig. 3. En this gure the parabolic field distribution is set up inherently because of the shape of the conductive envelope. This envelope is shown to comprise two generally cylindrical portions F and I-I, joined by two preferably integral longitudinally extending reentrant portions X. The metal envelope is given a positive potential, and the two points X thus correspond to the anode rods of Fig. 1, as far as function is concerned. It is then possible for the electrode L to be tuned as the centre of a concentric line, the surrounding envelope being tuned as the outside of this line. It would be possible of course to have two cathode and control grid systems, each as shown in Fig. 3, opening orf the concentric line on opposite sides. It would thus be possible to obtain oscillations of greater amplitude from the system. Since, howeVeI, the Silbe iS Primarily for use as a receiver,

it is probably not convenient or particularly advantageous to do this, as it increases the complication of the construction, and it is easier rather to amplify the signal after detection than to produce a high amplitude initially.

What is claimed is:

1. Electron discharge apparatus including a cathode, a pair of grid members each spaced equally from said cathode and spaced from each other, a pair of accelerating electrode members i..

each spaced equally from said cathode and spaced from each other, the spacing from one of said grid members to one of said accelerating electrode members being equal to the spacing of the other of said grid members from the other of said accelerating electrode members, an anti-cathode member spaced equally from each of said accelerating electrode members, and means producing a magnetic field extending from said cathode to said anti-cathode and generally perpendicular to all said members.

2. Electron discharge apparatus including a cathode, a pair of grid members each spaced equally from said cathode and spaced from each other, a pair of acceleratingelectrode members each spaced equally from said cathode and spaced from each other, the spacing from one of said grid members to one of said accelerating electrode members being equal to the spacing of the other of said grid members from the other of said accelerating electrode members, an anticathode member spaced equally from each of said accelerating electrode members, means producing a magnetic field extending from said cathode to said anti-cathode and generally perpendicular to all of said members, a pair of Lecher wires, one of said Lecher Wires being connected to said anticathode, and the other of said Lecher wires being connected to another electrode of said tube.

3. Electron discharge device comprising a generally cylindrical cathode member, a plurality ci pairs of grid members each spaced equally from said cathode member, said members of said pairs being spaced from each other, a corresponding plurality of pairs of accelerating electrode members, in each of said corresponding pairs the spacing from one of said accelerating electrode members to one of said grid members being equal to the spacing of the other of said accelerating electrode members from the other of said grid members, and a. plurality of anti-cathode members corresponding to said plurality of pairs ci grid members and accelerating electrode members, each of said anti-cathode members corresponding to a pair of said accelerating electrode members being spaced equally from each of said last-mentioned accelerating electrode members.

4. Electron discharge apparatus according to claim l, in which said cathode and said members are each generally cylindrically shaped and disposed in their respective axial directions parallel to one another.

5. Electron discharge apparatus according to claim 1 wherein said cathode and said grid members are enclosed in an envelope of conducting material, and wherein said pair of accelerating electrode members are formed by two parallel oppositelir disposed reentrant portions formed in said envelope.

6. An electron discharge device according to claim 2 wherein one of said Lecher wires is connected to said anti-cathode and the other to said grid members.

JOHN HEAVER FREMLIN.

Images