US3448412A

US3448412A - Miniaturized tunable resonator comprising intermeshing concentric tubular members

Info

Publication number: US3448412A
Application number: US634050A
Authority: US
Inventors: Einar C Johnson
Original assignee: US Department of Navy
Current assignee: US Department of Navy
Priority date: 1967-04-21
Filing date: 1967-04-21
Publication date: 1969-06-03
Anticipated expiration: 1986-06-03

Description

United States Patent US. Cl. 333-82 4 Claims ABSTRACT OF THE DISCLOSURE A miniaturized tunable coaxial resonator for use at high frequencies with high Q comprising intermeshing concentric tubular members in a shielded housing forming a folded coaxial line with capacitive loading and a variable impedance coupling loop.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalities thereon or therefor.

Background of invention The present invention relates to resonant electronic circuits and more particularly to coaxial resonant circuits for use with high frequencies.

At frequencies below approximately 100 megacycles, tuned circuits employing lumped inductance and capacitance are almost exclusively used as resonators. On the other hand, at very high frequencies, i.e., in excess of approximately 1000 megacycles, wave guides and cavities are employed as resonators since they exhibit low losses, high Qs (ratio of reactance to resistance) and their physical dimensions are acceptable. In this intermediate region of approximately 100 megacycles and 1000 megacycles, various combinations of line sections and lumped reactances are employed to provide acceptable size resonators with good Qs. One such device called the helical resonator utilizes a section of coaxial cable with a coiled center conductor. Although this device reduces the overall length of the cable required for resonance at a given frequency, as a result of the smaller diameter and greater length of the center conductor, the losses associated with this device are quite high. Accordingly, such devices have only enjoyed limited success. A great need, therefore, exists for a high Q, low loss miniature resonator which is completely shielded from external circuitry and has a high efiiciency of operation in this region.

Summary of the invention It is therefore an object of the present invention to provide a low loss, high Q resonator of extreme miniaturization with high efficiency at selectably variable frequencies. These and other objects are achieved by providing a device having characteristics of both the radial and axial coaxial lines. A radial line is defined herein as one in which the ratio of the diameter of the outer conductor is greater than the length and conversely for the axial line. To achieve these characteristics, the present invention employs a housing assembly forming a completely shielded enclosure Within which are fixed and movable concentric tubular members in intermeshing relationship forming a serpentine-shaped radially extending chamber which simulates a folded coaxial line resonator by which an appreciable reduction in size is achieved. Relative movement between the tubular members varies the fre quency of operation thereby providing a readily tunable device. By the use of capacitive loading between the base of the movable tubular member and the interior of the housing, the size of the resonator is reduced still further. Input-output coupling to the resonator is provided by coupling loops or caps.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing.

Brief description of the drawing FIG. 1 is a sectional elevation view taken along a diameter of a tunable resonator of the present invention; and

FIG. 2 is a sectional bottom view of the resonator taken on the line 2-2 of FIG. 1 looking in the direction of the arrows.

Description of the preferred embodiment Referring to FIG. 1 of the drawing, there is shown a coaxial resonator 10 of the present invention comprising a cylindrical housing member 11 of thin electrically conductive material having a side wall 11a, a top end cover 11b which may be integrally formed with the housing member and a bottom end cover providing a sealed cylindrical enclosure. A cylindrical member 12a aflixed at one end to the interior surface of the bottom end cover 11c, extends coaxially with the wall 11a of the housing, for about half the distance thereof. As described hereinafter, more coaxially arranged cylinders could be employed if desired. A movable assembly 13 having a base portion 1311 with

cylinders

13b and 13c aflixed thereto, is in intermeshing concentric relation with the fixed cylindrical member 12a. As illustrated, it is preferable that the movable assembly 13 have one more cylindrical memher than is fixed to the bottom end cover 11c so that continuity of the coaxial line is maintained. Although the housing member 11, the cylindrical member 12a and the cylinders 13b and are illustrated as having a circular cross section, it is to be understood that other tubular shapes such as rectangular, square, oval or other combinations thereof could likewise be used without departing from the spirit of the present invention. It is therefore to be understood that the embodiment described herein is merely illustrative and is not to be construed by way of limitation.

Referring now to FIG. 2, the outer cylinder 13b is illustrated as having a radial dimension greater than that of the cylinder 12a and the cylinder 13c is illustrated as having a radial dimension less than the cylinder 12a. The intermeshing concentric cylinders form a serpentineshaped chamber within the housing 11 which simulates a foldedcoaxial line, the length of which may be varied by varying the number of concentric cylinders, or by the dimensions of the housing. The average length of this serpentine path is one-half the sum of the distances between ABCDEFGH and AJKLMN and to a first approximation is between a and 71 wavelength because of the capacitive loading afforded by the capacitance between the top cover 11b and the base 13a, as will be described hereinafter.

For purposes of illustration, the required length of a quarter-wave resonator at one hundred megacycles would be three-quarters of a meter. This length may be achieved from a fixed volume housing by increasing the number of concentric intermeshing cylinders until the length of the serpentine path is equal to three-quarters of a meter. Using the approximation described above, the capacitive loading, provided by the capacitance between the base 13a and the top cover 11b, will reduce this length to between and of a meter. By this arrangement, the ratio of wavelength per unit volume is greatly increased over existing resonators. Whereas the length of the serpentine path afiects the resonant frequency of operation, the height or length of the housing wall 11a has little or no efiect on the resonant frequency so long as the length is less than a quarter-wave length at the frequency of operation.

The length of a serpentine path is adjustable within the confines of the housing by providing an adjustment post 13d having one end fixed centrally to the base 130 and the other end threaded for engagement with a hearing 14 which is electrically and mechanically secured centrally to the bottom cover 110. The bearing 14 is threaded to receive the threaded end of the adjustment post 13d so that a rotational movement of the adjustment post 13d, with the aid of an adjustment slot 13e, causes a translational movement of the assembly 13 relative to the bottom cover 110. This relative movement provides a fine adjustment of the resonant line length to enable operation of the device at a specific frequency.

Electrical connection between the housing member 11 and the movable assembly 13 is provided by a cylindrical sleeve 15 having leaf spring finger contactors 15a. The sleeve 15 is electrically and mechanically secured to the bottom cover 110 around the periphery of the bearing 14 by solder or another appropriate bonding agent. The contactors 15a provide sliding electrical contact between the adjustment post 13d and the bottom cover 110, thereby short circuiting one end of the coaxial resonator. The exact point of contact between the finger contactors 15a and the adjustment post 13d is not critical except that since high electrical currents exist at a short circuited end of a quarter-wave length line (around the bearing 14) and a low resistance threaded contact (between the adjustment post 13d and the bearing 14) is not readily achievable, it is desirable to move the point of electrical contact from this high current point to a lower current point. Accordingly, the exact point of contact of the finger contactors 15a is merely one of design choice.

Electrical coupling to the coaxial resonator is provided by a coupling loop 16 affixed at one end to a point b on the sleeve 15, and at the other end to an electrical connector 17 secured to the bottom cover 110. The impedance which the resonator exhibits to an input signal is a function of the position of the electrical contactor 15b along the sleeve 15; that as, as the point of connection is moved away from the bottom cover 110 toward the finger contactor 1511, the impedance increases. Accordingly, the impedance which the resonator displays to an external circuit connected to the connector 17 can be changed merely by changing the point of connection 1517. It is to be understood that more than one loop could be employed if so desired or that other coupling techniques, such as rotatable coupling loops, could be equally well employed without deviating from the spirit of the present invention.

To further reduce the physical size of the resonator and hence the ratio of length of the serpentine path to the wavelength, reactive loading is provided. In the embodiment described in FIG. 1, this loading is accomplished by the relatively large capacitance existing between the base 13a and the top cover 11b of the cylindrical housing 11. As the movable assembly 13 approaches the top cover 11b, capacitive loading increases inversely with the distance between them. A dielectric material 18 is secured to the inner surface of the top cover 11b so that maximum capacitance can be achieved without fear of short circuiting the resonator; however, the presence of the dielectric material is not essential to the operation of the resonator and may be omitted if desired.

In summary, the resonator of the present invention provides a high Q miniaturized tunable resonator for use at frequencies between approximately 100 and 1000 megacycles. By increasing the number of concentric tubular members, the wavelength per unit volume is increased considerably over previously described devices. Additionally, by utilizing the capacitance between the top end cover 11b and the base 13a, maximum loading per unit area is provided in the same device. Further, the enclosed assembly acts as a shield for the resonator so that coupling between the resonator and adjacent circuitry is minimized without the need for additional shielding.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim:

1. An apparatus for resonating at high frequencies comprising:

an electrically conductive tubular housing having first and second electrically conductive end covers attached thereto;

a trough-shaped tubular member having a base with side walls secured thereto;

a first tubular member having a radial dimension less than that of said side walls and concentric therewith, said first tubular member afiixed along an end to said base;

a second tubular member aflixed to said first end cover and concentrically intermeshing with said first tubular member and the side walls of said trough-shaped member, said intermeshing members thereby forming a resonant serpentine chamber;

an adjustment post secured at one end to said base and movable from the other end relative to said first end cover, whereby an axial movement of said adjustment post varies the resonant frequency of said serpentine chamber; and

means to couple signals to said housing including a sleeve concentric with said adjustment post aflixed at one end to said first end cover and having finger contactors at the other end thereof in electrical contact with said post whereby a sliding electrical contact between said sleeve and said post is provided.

2. An apparatus as recited in claim 1 wherein said means to couple signals to said housing further comprises:

a coupling loop connected at one end to a point on said sleeve; and

an electrical connector mounted in said first end cover and electrically connected to the other end of said coupling loop for providing electrical access to said resonant serpentine chamber.

3. An apparatus for resonating at high frequencies as recited in claim 2 wherein:

said base forms a first plate of a capacitor; and

said second end cover forms a second plate of said capacitor, the capacitance between said base and said second end cover increasing the resonant wavelength of said chambered housing thereby reducing the ratio of the length of the serpentine chamber to the wavelength.

4. An apparatus as recited in claim 3 further comprising:

means to provide a dielectric material between said second end cover and the base of said trough-shaped member.

References Cited UNITED STATES PATENTS 2,111,219 8/1939 Malter.

2,181,901 12/ 1939 Lindenblad.

2,363,641 11/ 1944 Carlson.

2,500,875 3/ 1950 Schupbach.

2,603,754 7/ 1952 Hansen.

HERMAN KARL SAALBACK, Primary Examiner.

PAUL L. GENSLER, Assistant Examiner.

US. Cl. X.R.