US2158493A - Artificial line for use on radio and other high frequencies - Google Patents

Description

y 1939- J. D. BRAILSFORD ET Al. 2,158,493

ARTIFICIAL LINE FOR USE ON RADIO AND OTHER HIGH FREQUENCIES Filed March 16, 1956 Patented May 16, 1939 ARTIFICIAL LINE FOR USE ON RADIO AND OTHER HIGH FREQUENCIES Joseph Douglas Brailsford and Robert Francis ONeill, London, England, assignors to Radio Corporation of America, a corporation of Delaware Application March 16, 1936, Serial No. 69,098 In Great Britain March 16, 1935 a simple solenoid, wound upon a cylindrical former 2 having radial projections 3 so that the wire of the coil is held away from the main body of the former. This former with its coil is posi- 8 Claims.

This invention relates to artificial lines for use on radio and other high frequencies and has for its object to provide improved variable artificial lines.

According to this invention an artificial line tioned substantially concentrically within a split 5 comprises an inductive coil arranged in associacylindrical copper screen 4 which is spaced a short tion with a metal member in such manner that distance radially from the coil and which has a said metal member in conjunction with said coil split 4a running substantially parallel to the axis. provides the major portion of the efiective dis- The coil is positioned W t th Screen y Suittributed capacity of said coil and means are proable support members 5 which mechanically con- 10 vided for varying the inductance per unit length nect the body of the former with points in the of said coil and/or for varying the distributed screen diametrically opposite the split. Attached capacity per unit length of said coil. to the screen upon the outside thereof is a nut Artificial lines in accordance with this inven- 6, this nut being positioned adjacent one side tion are applicable to a Wide variety of uses and of the split 4a and approximately mid-way of in order that the invention may be the better the length of the screen. Also attached to the understood certain special uses to which said screen but upon the other side of the split and artificial lines may be put will be described. opposite the nut is an abutment member 1, for

In certain forms of radio apparatus, notably example of ebonite. The screen is of springy heterodyne receivers of the type wherein a very material and is sprung in such manner as to 20 high intermediate frequency of the order of 2,000 tend to close its split. Screwed through the kilocycles or more is employed, practical difiinut is an adjustment screw 8 Whose end bears culties arise in obtaining a high impedance load in a suitable recess la formed in the abutment in the anode circuit of valves, for example beat member. Thus it will be seen that by screwfrequency valves. An important application of ing the adjustment screw 8 in one direction the 5 the present invention is to such apparatus wheresplit 4a will be opened against the spring of in an artificial line is adapted for use as the anode the screen while by c w the d Screw in impedance in circuit with a valve of a beat frethe other direction the spring of the screen will quency amplifier operating at a high frequency be allowed to close up the split to an extent limof the order of 2,000 kilocycles or more. As will ited by the setting of the said screw. The former 30 be appreciated, it is a matter of great conven- '2 upon which the coil 1 is wound is a little longer ience to be able to adjust the constants of an than the screen and projects out at both ends impedance employed for such a purpose as that thereof and over the projecting ends of the formdescribed. Accordingly, for the purpose of exer are fitted annular end pieces 9, II), for exempliiying the utility of the invention, there will ample of ebonite, these annular end pieces be- 5 now be described two forms of artificial line, or ing embraced by short split cylindrical copper impedance unit, either one of which may be emmembers ll, [2, each of which is closely held to ployed as a variable impedance. its appropriate end piece by set screws I3 pass- The invention is illustrated in the accompanying through the copper and screwing into the ing drawing in which, ebonite on the two sides of the split. It will be 40 Fig. 1 is a view, partly in section, of a coil conseen that with this arrangement movement of struction which we preferably employ, the adjustment screw in one direction or the Fig. 2 is a transverse section with respect to other will change the efiective distributed capacthe showing of Fig. l, ity of the coil since this capacity is largely con- Figs. 3 and 4 illustrate in elevation and transstituted by the capacity between coil and screen. verse section respectively a modification of the The above described embodiment has the slight invention, disadvantage that the coil is truly concentric with Fig. 5 is a circuit diagram applicable to any the screen for only one position of adjustment. of the embodiments herein shown; and This disadvantage may be obviated, if desired,

Fig. 6 shows a longitudinal section of still anby employing the modified embodiment now to be described and illustrated in sectional elevation and transverse section in Figures 3 and 4. In this embodiment a coil I wound upon a former 2 and positioned within a split springy cylindrical screen 4 is employed as before but other embodiment.

Referring to Figures 1 and 2 which are respectively a side elevation partly in section and a transverse section of one form of construction, an artificial line comprises a coil l which may be a somewhat different method is utilised for opening and closing the split 4a in the screen. The former 2 upon which the coil l is wound is somewhat longer than the screen t and projects at both ends beyond the said screen. Upon one of the projecting ends is fixed an end piece it, for example of ebonite, which embraces one projecting end of the former and has an outer face Ma of sloping form, the diameter of the end piece increasing rectilinearly outwardly of the former, as shown, i. e., as it approaches the extreme end of the former. The other projecting end of the former is screw threaded at E5 and over this end is fitted a somewhat similar end piece it which also has a sloping outer face Mia this end piece, however, being able to slide longitudinally with respect to the former. An ebonite ring nut H is screwed on to the threaded end 95 of the former outside the slidable end piece. The copper screen has its ends turned up at 4b and bears at both ends upon the tapered faces Mia Mia of the two end pieces l4, it, as shown. With this arrangement when the ebonite nut ii is screwed in one direction the two end pieces it, it will be brought more close together and accordingly the split 40. in the screen will be compelled to open by reason of the fact that the turned up ends lb of the screen bear upon the sloping faces Ma, lBa, of the end pieces M, It and must accordingly ride up those sloping faces. Movement of the nut in the other direction allows the ends of the screen to come down the sloping faces and accordingly allows the split Ga to close to an extent determined by the setting of the nut.

An artificial line in accordance with either of the specific embodiments just described, when adjusted to be a quarter of the working wave length long, may be used very satisfactorily as a resonant anode impedance in the anode circuit of a beat frequency valve operating in a heterodyne receiver of the type wherein the beat frequency is very high, e. g., of the order of 2,000 kilocycles. The range of variation of the effective distributed capacity of such an artificial line is sufficient to enable trimming adjustment to be made after the artificial line is connected in circuit. For example, as shown in Figure 5, an artificial line AL in accordance with either of the embodiments hereinbefore described may have its coil I connected at one end directly to the anode A of the first detector or frequency changer valve FC of a heterodyne receiver, the other end of the coil being connected to the screen 4 of the line and also to the source (not shown) of anode potential for the valve. Thus the artificial line constitutes a quarter wave length artificial line which is short circuited at one end.

Practical tests with artificial lines as above described have shown that a variation of 3% in the frequency at which the line is resonant is easily obtainable and such a range of variation is sufficient for trimming purposes, e. g., in the best frequency amplifier of a heterodyne receiver.

It will, of course, be appreciated that when the distributed capacity of an artificial line in accordance with either of the embodiments above described is varied, there is a change in the effective length of the line but this represents only a small percentage of the total length and in consequence the alteration in impedance resulting from. such adjustment of distributed capacity is too small to have any important effect upon the valve stage in whose circuit the line is connected.

In a further embodiment in accordance with this invention and illustrated in longitudinal section in Figure 6 provision is made for varying both the distributed capacity and the inductance per unit length, this variation being effected in such manner that the inductance per unit length and capacity per unit length maintain a constant ratio towards one another. A variable artificial line in which the distributed inductance and capacity remain in a predetermined ratio towards one another, is useful for a variety of different purposes; for example, it may be used as a phase changing device in connection with a transmission circuit between an antenna and a remotely situate-d transmitter or receiver.

In the use of long transmission lines a phase changing device is oftentimes required to introduce a certain desired change of phase and since the amount of phase change introduced will obviously depend upon frequency, the phase changing device must be variable for tuning purposes. Such a variable phase changing device can, of course, be constituted by filters having reactances arranged to be varied together in such manner that for all positions of adjustment the phase changing device presents constant impedance, but such filters, with their necessary gang controlled reactances, are somewhat complicated. The present invention may be utilised to provide an improved simplified phase changing device wherein the phase shift may be varied while maintaining constant the characteristic of the impedance.

In the embodiment of the present invention illustrated in Figure 6 and suitable for use for purposes as just described, a variable artificial line construction comprises a cylindrical supporting tube I 8 of insulating material held between end cheeks i9, 20. Enclosed in the tube, and extending over about half its length, is a cylindrical core 2!, for example of the material known under the registered trade name Ferrocart. concentrically surrounding the tube and core and of the same length as the core, is a cylinder 22 of dielectric material, for example glass, slightly spaced from the supporting tube l8 and attached to one of the end cheeks (check 20) Mounted upon the supporting tube i8 is a thin tube 23 of insulating material carrying a single layer helix l which can slide between the supporting tube [8 and the cylinder 22 of dielectric. This tube 23 carries at the end remote from the end cheek 20 a spacing ring 24 of insulating material round the outer edge of which is attached a copper cylinder 25, also of almost the same length as the core. The copper cylinder has a thin slit (which does not appear in the View of Figure 6) out along its length to prevent its acting as a short circuited turn with respect to the solenoid l. The radial thickness of the spacing ring 24 is such that the copper cylinder 25 can slide over the glass tube 22. The dielectric 22 therefore is interposed between the helix and the copper cylinder 25 the arrangement thus constituting a variable capacity.

It will be seen that with this arrangement when the copper cylinder and coil are moved in the direction of the end cheek 20 the effective inductance of the coil is increased since more of the ferro-magnetic core comes within the coil and at the same time the eifective distributed capacity of the coil is also increased in like ratio since more of the dielectric cylinder comes outside the coil. The whole arrangement is preferably (as shown) such that when the copper cylinder is moved to its maximum extent away from the end cheek 20 there is no core material inside the coil and no solid dielectric material outside the coil. With this construction by moving the slider the phase shift effected by the artificial line when used as a variable phase changing device may be varied while maintaining constant the characteristic impedance since variation of inductance per unit length and variation of capacity per unit length takes place at the same rate.

The input terminals are constituted by one end of the coil and the copper cylinder and the output terminals are similarly constituted by the other end of said coil and the said copper cylinder.

The expedient of varying distributed capacity by varying the amount of dielectric in association with a coil may be adopted in artificial lines in accordance with this invention wherein variation of distributed capacity is all that is required; in other words, the first two specific embodiments hereinbefore described may be modified by substituting for the expedient of varying distributed capacity by varying the space between a coil and a screening member, the expedient of varying the distributed capacity by varying a dielectric which in effect forms part of the distributed capacity. Similarly in artificial lines in accordance with this invention wherein variation both of inductance and capacity is required, the expedient adopted in the first two embodiments specifically described herein may be employed for varying the distributed capacity.

It has been found in practice that, to obtain maximum effectiveness of artificial lines in accordance with this invention, the capacity between turns of the coil should be less than the capacity of each turn to the screen and if the inter-turn capacity is allowed to be larger than the turn-screen capacity the effectiveness of the line is apt to be much reduced. For lines which are to have a low value of distributed inductance there are no difficulties in the way of obtaining a sufiiciently high ratio of distributed capacity to inter-turn capacity since the number of turns per unit length will be low. Where, however, higher values of distributed inductance are required, i. e., where the design calls for a relatively large number of turns per unit length, the use of coils wound in the ordinary way with round sectioned wire may not be as satisfactory as is desired since it may result in the inter-turn capacity exoeeding the turn-screen capacity. In such cases, therefore, it is preferred to wind the coil with flat strip with its flat face presented to the screen so that the inter-turn capacity is due to the capacity between adjacent narrow edges of the strip winding while the wide face of the strip co-operates with the screen to provide the turn screen capacity.

We claim:

1. An artificial line comprising a cylindrical coil, a split cylindrical screen substantially surrounding said coil and in electrostatic association with the same, and means for widening or reducing said split so as to vary the distributed capacity due to said screen, said line being further characterized in that the screen is of springy material and has the split parallel to the coil axis, said screen being fixed relative to said coil at a point or points opposite said split, the means for varying the width of said split comprising a screen member bridging said split and adapted to force said split to widen against the springiness of the screen.

2. An artificial line comprising a cylindrical coil, a split cylindrical screen substantially surrounding said coil and in electrostatic association with the same, and means for widening or reducing said split so as to vary the distributed capacity due to said screen, said line being further characterized in that the screen is of springy material and has the split parallel to the coil axis, means including end-supporting members for positioning said screen, said means having oppositely tapered faces, one of said end members being fixed on said coil structure and the other being longitudinally movable so that by bringing said end members nearer to one another said screen may be forced to ride up on said tapered faces thereby to widen its split against the springiness of said screen.

3. An artificial line comprising a tubular guide member, a ferro-magnetic core co-axial with and inserted into one end of said guide member and extending over a fraction of the length thereof, a tubular dielectric member of insulating material also co-axial with said guide member and surrounding the same over a length substantially corresponding to the length of insertion of said core, and a coil structure consisting of a tubular coil having a spaced metal screen carried from one end thereof and co-axially surrounding said coil, said coil structure being of approximately the same length as the length of said core insertion and being slidably mounted on said guide, the whole arrangement being such that by moving the coil structure towards the end of the guide member Where the core is inserted said coil will pass inside and said screen outside said tubular dielectric member both inductance and capacity being thus simultaneously variable.

4. A line as claimed in claim 3 and wherein the arrangement is such that the inductance of the coil and the effective distributed capacity thereof may be variably adjusted in like ratio by moving the coil structure along the guide member.

5. An impedance unit comprising a conductive helical coil, a magnetic core member within said coil, a split cylindrical metallic shielding member closely adjacent and substantially surrounding said coil, and providing a greater capacitive reactance with respect to the convolutions of said coil than the total distributed capacitance of the coil itself, and means for simultaneously adjusting the relative positions of the shielding member and of the coil with respect to said core member, thereby to vary the effective over-all capacitance of the impedance unit, while maintaining a predetermined ratio between the distributed inductance and the distributed capacitance of said unit.

6. An impedance unit according to claim 5 and having said helical coil constituted by a single layer of flat wire, the edges of which are in juxtaposition while one broad face thereof is opposed to said shielding member.

7. An impedance unit comprising a stationary magnetic core member, an insulating sleeve surrounding said core member and of greater axial length than said core member, a conductive helix surrounding said sleeve and adjustably positionable thereon, a split metallic shielding member substantially surrounding said helix and insulated therefrom while being mounted integrally therewith, and means including a sleeve of dielectric material mounted in fixed relation to said ,5' core member and insertible between said helix and said shielding member for determining the capacitance between said helix and said shielding member.

8. An impedance unit in accordance with claim '7 and having the dimensions of the several parts so fixed in relation to one another that variations in the position of said helix and shielding member relative to said magnetic core and dielectric sleeve respectively result in a phase shift effect upon the circuit in which said unit is connected, while maintaining constant the impedance characteristic of said unit.

JOSEPH DOUGLAS BRAILSFORD. ROBERT FRANCIS ONEILL.

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