US2428037A - Tunable high-frequency tank circuits - Google Patents

Description

Sept. 30, 1947. w, Q Q T A 2,428,037

Filed Jan. 22, 1943 INVENTORS [#14 75/? KIN 5. $05557 ATTORNEY Patented Sept. 30, 1947 TUNABLE L'HIGH-FREQUENCY TANK CIRCUITS Walter'van B. Roberts and Samuel G. Frantz, Princeton, N. J assignorsto Radio Corporation of .America, va corporation :of Delaware Application January 22, 194:3,Serial'No. 473,196

The present inventionrrelates: to --.tuned circuits and; more particularly, to high frequency'tanks in the form "of resonant chambers. Iris-known that at ultra high frequencies-a tuned'circuit, in-

cluding a conventional'coilandcondenser, has a comparatively low factor of merit or' Q. In .order toobtain an improved factor of merit a structure in the form-of a: single turn toroid. winding with the tuning capacity in the form of fiat plates at the center of the toroid may be used..

Qoptimum="89R\/ -F where R' is the :radius: of. revolution. of the torus in. centimeters and F is: the.- frequency in megacycles. The radius r-ofrthe-generating circle of the torus must be equal*to .707 Rior the above equation to hold. This relation was given by H. G. Clavier in. Communications magazine, September 1942-. It will'benoted thatit is desirable to makeptheresonant structure quite large but practical considerationsiusually place-a limit on. size.

An object of. the present invention is: thezconstruction of a high. Q tank. circuit which is readily tunable.

A further object of the present invention is the provision of stable adjusting means for tuning a toroidal, resonant circuit for high frequencies,

Still. a further object of thepresent invention is the provision of easily adjusted means for tuning a toroidal tank circuit.

Still a further object ofthe present inventionis the provision of. a high "Q tank circuit which may be easily constructed.

Still a further object of the present invention is the provision of a high frequency tank circuit which may be designed to respond in a predetermined manner to certain. variations in meteorological. conductions, such as temperature or pressure.

The, foregoing objects, and others which. may appear from the following detailed description, are attained by providing a tank circuit in the form of a conductive Walledv chamber having characteristics such that the spacing between a pair of opposing walls determines the resonant frequency. The spacing is adjusted by providing a conductive rod between the Walls for pulling them together orforcing them apart. The length of the rod is so chosen that it has a very small efiective conductance at the operating frequency. Specifically, the tank circuit may be a single turn toroidal Winding having a tuning capacity in the form of a fiat plate at the center of the toroid.

Extending through the center of the flat plates 11 Claims. (01; 178-44) is a'rod having such length as to present a relatively high inductance to the waves within the resonant chamber, said rod being threaded at its ends: and arranged to vary the spacing between the plates as a result of it's rotation,

The present invention also contemplates hermetically sealing the resonant chamber, as described above, so that its dimensions willchange in a predetermined manner with variation in atmospherie pressure. Alternatively, the tuning structure may be so constructed as to have an extremely high coefficient of expansion with temperature changes so thatan enormously increased variation in frequency is attained for moderate variations in temperature.

'The novel features which, it' is believed, are characteristic of the present invention are pointed out with particularity in the appended claims. The invention will, however, be more completely understood by reference to the following detailed description, which is accompanied by'a drawing inwhich Figure 1 illustrates in perpective, and partly in section, a high frequency tank circuit of a type to whichthe present invention may be applied; Figure 2 shows, likewise in perspective and partly in. section, the application of the present invention to the tank circuit of Figure 1, while Figure 3 illustrates a modification of'the form of the invention shown in Figure 2.

The high frequency tank circuit of Figure 1 includes an, inductance in the form of a single toroidal chamber 5: having an annular slit 6 along the inner surface of the torus. Across the edges of the slit 6 are connected a pair of circular plates "8' and 9 providing a tuning capacity for the tank circuit, the inductancev being provided by the interior circumferential length of chamber 5 between one edge ofslit 6 and the other edge. Energy is applied to or abstracted from th tank cir- 'cuit by means of transmission line TL, shown in this figure as including a pair of parallel conductors l0 and H. The conductors Ill and II terminate within the interior of the torus in a coupling loop l2, the dimensions'of which may be adjusted, as desired, to vary the coupling between the transmission line and the resonant circuit. The conductors ofv the transmission line may pass through apertures in the wall of the chamber or the apertures may be sealed by insulators l3 if it is desired to make the tank circuit airtight.

As mentionedabove, the-resonant frequency of the tank circuit is altered Within certain limits by varying the spacing between capacity plates 8 and '9; Heretof'ore, the most practicable Way of doing this has been to provide a clamping structure, such as C clamp [5 having an adjusting screw l6 for pressing plates 8 and 9 closer together or allowing them to spring further apart due-to the spring action of the toroidal portion 5. A practical objection exists to this method of tuning in that the relatively long dimension of G clamp 15 are likely to be considerably varied by temperature variations unless the entire C clamp is made of a material which is substantially nontemperature responsive. However, such materials are relatively expensive and large quantities are necessary due to the size of clamp Hi.

In accordance with the present invention, as shown in Figure 2, an improved means is provided for adjusting the spacing between plates 8 and 9. The plates are made of thick material so as to be relatively free of microphonic vibration and are squeezed together by rod 18 passing each half may be spun from a single sheet of metal. The only jointl iri the structure is the solder joint 33 around the circumference of the device. The details of operation of the tuning means I8 is the same as described with reference to Figure 2.

In the modifications of the invention shown in Figures 2 and 3, in order to prevent variations in atmospheric pressure from varying the tuning through the space between plates 8 and 9. In

portions 22 and 23 of rod l8." It is contemplated as being within the scope of. the present invention to make each of the pipes 19 and 26 equal in length to substantially 'one quarter of the mean operating wavelength of the system so that only a very small effective conductance is shunted across between plates 8 and 9. Thus substantially no energy is absorbed by the adjusting means for tuning the tank. The rod l8, together with the surrounding pipes l9 and 20 will preferably, in this form of the present invention, be adapted to have substantially zero temperature coefficients. For example, they may be made of steel which has substantially a zero temperature coefficient and is silvered or gold-plated in order to provide greater conductivity on the current carrying surfaces.

The threaded portions 22 and 23 of rod [8 may be threaded in opposite directions as is done in turnbuckle construction if only coarse frequency adjustment is desired. However, if it is desired to obtain a very fine adjustment of the operating frequency of the tank circuit the threads 22 and 23 are preferably cut in the same direction but with slightly different pitches so that several turns of rod 18 arerequired to produce a very small change in the separation between plates 8 and 9. A suitable dial and knob arrangement 24 may be provided at one end of rod l8 for the purpose of recording the adjustment of the tank circuit. This dial arrangement may be a more or less conventional tuning dial arrangement or it may be constructed in the form of a micrometer head. A coupling to the tank circuit of Figure 2 is provided by means of loop l2 connected to transmission line TL asin the case of Figure 1. However, if desired, an additional or alternate coupling may be obtained by tapping conductor 26 of a coaxial transmission line CTL to a desired point along rod 18. Impedance matching is provided by varying the point of contact of conductor 26 with rod [8.

The modification of the present invention shown in Figure 3 is similar in operation to that shown in Figure 2 but the structure is more adaptable to construction by a spinning process. For convenience in construction the tank is divided laterally into two shell-like halves 30 and 3!. Each of the halves may be separately spun and then assembled and soldered together, as indicated by the solder joint 33. The pipes l9 and 20 of Figure 2 are replaced by somewhat hemispherical portions 32, 34 in Figure 3. Thus of the tank, a small air leakage should be provided at some point. This may be at the point of insertion of the transmission line, or at some other point. However, advantage may be taken of the efiect of variation in atmospheric pressure upon the frequency of the tank for the purpose of measuring elevation by means of sounding balloons. In this case the tank circuit would be employed as the frequency determining element of an oscillator, the input and output circuits of the tube being coupled to the tank by coupling links such as loop 12 and/or tap 26. The coupling links should pass through airtight insulating seals such as insulators l3,' l3 of Figure 1. If the tank tuning means is made of zero temperature coefiicient material the frequency of the tank will then be a function of atmospheric pressure only and a receiver located on the ground to measure the frequency of oscillations emitted by the transmitter carried by the sounding balloon will obtain therefrom, in' conjunction with a calibration, curve, the atmospheric pressure at the location of the balloon. If desired, an additional transmitter using a similar tank circuit as a frequency determining element may be used in which the tuning rod l8and the pipes l9 and 20 are made of materials having widely different temperature coefficients so that'the generating frequency will vary as a function of temperature; Thus a separate receiver located on the ground to measure the frequency of operations emitted from the second oscillator will obtain therefrom, in conjunction with a. calibration curve, the atmospheric temperature at the location of the balloon. The second mentioned tank should, of course, not be hermetically sealed as, otherwise, the variation in pressure will also influence the general frequency. 7

While we have illustrated some specific embodiments of the present invention it should be clearly understood that the invention is not limited thereto since many modifications may be made in the several elements employed and in their arrangement and it is, therefore, contemplated by the appended claims to cover any such modifications as fall within the spirit and scope of the invention.

We claim: g

1. A tank circuit including inductance and a pair of plates providing capacity connected across said inductance, and means for varying the spacing between said plates comprising a metallic adjustable spacing bar, said bar being arranged to act as a low loss shunt inductance connected between said plates, said bar being threaded through and in conductive connection with each of said plates.

2. A tank circuit including inductance and a pair of plates providing capacity connected across said inductance, and means for varying the spacing between said plates comprising a metallic adjustable spacing bar, said bar being arranged to act as a low loss shunt inductance connected between said plates, said bar being threaded through and in conductive connection with each of said plates, the threads being in opposite directions.

3. A tank circuit including inductance and a pair of plates providing capacity connected across said inductance, and means for varying the spacing between said plates comprising a metallic adjustable spacing bar, said bar being arranged to act as a low loss shunt inductance connected between said plates, said bar being threaded through and in conductive connection with each of said plates, the threads being in the same direction and of different pitches.

4. A resonant circuit including a conductive walled resonant chamber, the operating frequency thereof being determined by the spacing between a pair of opposing walls and means for adjusting said spacing including a conductive rod passing through said chamber from one of said walls to the other, the length of said rod being such as to present a high impedance at the operating frequency.

5. A resonant circuit including a conductive walled resonant chamber, the operating frequency thereof being determined by the spacing between a pair of opposing walls and means for adjusting said spacing including a conductive rod passing through said chamber from one of said walls to the other, the length of said rod being such as to present a high impedance at the operating frequency and means for varying the point of attachment of at least one of said walls to said rod.

6. A resonant circuit including a conductive walled resonant chamber, the operating frequency thereof being determined by the spacing between a pair of opposing walls and means for adjusting said spacing including a conductive rod passing through said chamber from one of said walls to the other, the length of said rod being such as to present a high impedance at the operating frequency, said rod being threaded through said walls, the threads passing through said walls being in opposite directions.

7. A resonant circuit including a conductive walled resonant chamber, the operating frequency thereof being determined by the spacing between a pair of opposing walls and means for adjusting said spacing including a conductive rod passing through said chamber from one of said walls to the other, the length of said rod being such as to present a high impedance at the operating frequency said rod being threaded through said walls, the threads being in the same direction but of different pitch.

8. A resonant structure including a pair of complementary circular plates each having a concavity in the center, a flat annular intermediate portion and a semi-circular trough around the edge and means for securing the outer edges of said troughs together to form a toroidal inductance with the flat portions of said plates cooperating to form' a tuning capacity for said inductance, there being threaded apertures at the center of each concavity to accept a threaded rod for adjusting the spacing between the fiat portions of said plates.

9. A resonant structure including a toroidal conductive structure having an annular slot on the inner surface thereof, flat conductive parallel plates connected to adjacent edges of said slot, each of said plates having a tubular member attached to its midpoint, said tubular members being directed in opposite directions and closed at their opposite ends, and a conductive rod passing through said tubular members from one end to the other and threaded through said closed ends.

10. A resonant structure including a toroidal conductive structure having an annular slot on the inner surface thereof, fiat conductive parallel plates connected to adjacent edges of said slot, each of said plates having a tubular member attached to its midpoint, said tubular members being directed in opposite directions and closed at their opposite ends, and a conductive rod passing through said tubular members from one end to the other and threaded through said closed ends, the length of said rod being such as to present a high inductance to energy of the operating frequency of said structure.

11. A resonant structure including a toroidal conductive structure having an annular slot on the inner surface thereof, flat conductive parallel plates connected to adjacent edges of said slot, each of said plates having a tubular member attached to its midpoint, said tubular members being directed in opposite directions and closed at their opposite ends, and a conductive rod passing through said tubular members from one end to the other and threaded through said closed ends, the length of said rod being such as to present a high inductance to energy of the'operating frequency of said structure, each of said tubular member having a length of the order of one quarter of the operating wavelength.

WALTER. VAN B. ROBERTS. SAMUEL G. FRANTZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,737,741 Thomas Dec. 3, 1929 2,085,223 Kolster June 29, 1937 2,242,275 Varian May 20, 1941 FOREIGN PATENTS Number Country Date 244,009 Great Britain Dec. 10, 1925

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