Rail or pedestal mounted meander line circuit for crossed-field amplifiers

Abstract

An improved meander slow wave structure to provide a low cost, broad band ossed field amplification device functioning as a "meander line". The structure includes a conductive ground plane and a meander line shaped conductor being continuous and having lateral and longitudinal segments disposed at right angles to each other, the meander conductor being separated and spaced from the conductive ground plane by rail-shaped dielectric elements which support the longitudinal segments of the meander line conductor, separating it from the ground plane. The dielectric rails may be continuous along the entire device and substantially parallel or may be segmentized (in an alternate embodiment) and termed "pedestals." The structure also allows for the addition of RF shields to increase the band-width of the meander line and an RF matching device which is easily inserted between the meander circuit and the ground plane.

Government Interests

The invention described herein may be manufactured and used by or for the government for governmental purposes without the payment of any royalties thereon or therefor.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to a slow wave, meander line circuit structure and specifically to an improved meander line slow wave structure having reduced fabrication costs and improved structural durability without sacrificing operational performance when utilized as the interaction structure for microwave tubes. The device is especially useful in microwave crossed-field amplifiers.

In many electronic devices, it is desirable to effectively slow down an electro-magnetic of RF wave, which normally travels at the velocity of light. One device for accomplishing effective wave slow down has been termed a "meander line slow wave device". The "meander line" is essentially a conductor having an increased effective length between two straight line points, accomplished by providing a circuitous conductive path (usually a zig-zag type pattern) which in effect increases the time for an electro-magnetic wave to travel between two straight line points. Applications for such a device have been typically found utilizing RF signals in the microwave frequency range. In the past, problems have been encountered in the construction of conventional meander line devices because of the fabrication complexity in that dielectric materials have been coated, deposited or etched on the conductive ground plane (or vice versa) and then the meander line conductor itself has been placed on the dielectric substrate, the uniform dimensional requirements between the ground plane and the meander line conductor being critical. High operating temperatures often require a selection of materials (dielectric and conductive) with compatible expansion and contraction characteristics. Cracking due to expansion is a common failure of conventional meander circuit devices utilizing a continuous planar dielectric substrate.

The instant invention provides a slow wave, meander line structure which may be easily fabricated without reducing desirable operational characteristics when utilized as the interaction structure for microwave tubes. The instant invention is adapted for use in an L-band tube with a frequency range of 1 to 2 gigahertz (with a 20 percent band-width) and is capable of 5 kilowatt peak power and 150 watts average power operating range. The circuit may be designed to include both injected election beam and RF drive modulated operation.

To overcome the problems of the prior art, the instant invention includes a meander line circuit supported beneath continuous or segmentized longitudinally disposed dielectric rails having a high thermal conductivity. The primary difference between the "rail" or "pedestal" supported devices of the instant invention and that of more conventional meander line circuits is that the dielectric material has been removed from the high RF field region of the slow wave structure, thereby increasing RF interaction impedance as well as efficiency. Because the thermal path is no longer directly through the dielectric, the instant invention is limited to moderate average power, the thermal path being along the transverse bars comprising the meander line conductor. The structural improvement of the instant invention provides for exceptional band-width operation and allows for less expensive and less complex fabrication techniques including greater band-width enhancement from shielding vanes between the meander line conductor transverse line elements.

The rail and pedestal mounted structures of the instant invention enhance RF interaction impedance and efficiency at the expense of thermal impedance to the ground plane (which acts as a heat sink), eliminate problems resulting from evaporation and cathodic sputtering (which are highly detrimental to life in conventional dielectric supported circuits), minimize RF losses in the interface between the dielectric and conducting parts of the structure and significantly reduce the cost of construction.

BRIEF DESCRIPTION OF THE INVENTION

A meander line, slow wave circuit device comprising a conductive ground plane, a planar-shaped continuous conductor in the shape of a meander line, the continuous conductor having transversely and longitudinally disposed segments at right angles to each other to form the meander line shaped conductor, and a plurality of elongated, dielectric supports connecting and separating said longitudinal segments of said meander line conductor to said ground plane, said dielectric support being disposed parallel to the longitudinal axis of the device. Conventional signal input and output means are connected to said meander line conductor.

The dielectric supports may be made in the form of continuous elongated rail-shaped members which are disposed and connected to longitudinal segments of the meander line conductor or in an alternate embodiment may be segmentized into a plurality of small rail-shaped elements which are parallel to the longitudinal axis of the device and support only the longitudinal segments of the meander line conductor, thus allowing for spacing between adjacent supports. Continuous elongated supports along the entire length of the device may be termed "rails," while if segmentized may be termed "pedestals." In either embodiment, the dielectric material may be chosen from a variety of different materials depending on the frequency and RF power requirements. In many operational situations beryllia (BeO) would be the optimum material because of low RF loss, moderate dielectric constant and high thermal conductivity. In both embodiments the interior portion between the meander line conductor and the conductive ground plane are essentially open, eliminating the interior dielectric material as shown in the prior art. The pedestal-mounted meander line has in some applications an advantage over the rail-mounted type in that the thermal expansion problem, inherent in long, continuous rails can be overcome by providing short, segmentized dielectric supports. In the rail-mounted method co-expansive alloys must be used to prevent cracking of the dielectric rails. The pedestal-mounted device may employ "plasma spray" as a construction technique which would involve a one-step process. The dielectric material in either the rail-mounted or pedestal-mounted embodiments is bonded to the meander conductor.

Either device may employ the mounting of conductive shields parallely spaced between the transverse meander line conductor segments which improve the band-width of the circuit by decreasing the capacitive coupling from one segment of the conductor to an adjacent segment. The shielded embodiment is easily fabricated because of the open spacing between the meander conductor and the ground plane. Also a dielectric RF matching element can be utilized in either the rail or pedestal mounted circuits, the matching material being inserted between the first transverse segment of the meander line conductor and the ground plane. This embodiment provides a practical means for RF matching and loading of the anode circuits. Various dielectric materials and thicknesses can be utilized to provide an additional flexibility in RF matching methods.

It is an object of this invention to provide an improved meander line circuit device.

It is another object of this invention to provide a method of fabricating a high performance, low cost, broad band, crossed-field amplifier circuit element suitable for use in high efficiency crossed-field tubes.

And yet still another object of this invention is to provide an improved meander line circuit structure having reduced fabrication costs, increased efficiency of operation, and improved structural integrity.

But still yet another object of this invention is to provide a low cost, rail or pedestal mounted meander line device which allows for the utilization of shielding techniques and RF matching and loading of the anode circuits.

In accordance with these and other objects which will be apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show perspective views of meander line devices found in the prior art.

FIG. 3 shows a perspective view of the instant invention.

FIG. 4 shows a perspective view of an alternate embodiment of the instant invention, utilizing shielding techniques for improving circuit efficiency.

FIG. 5 shows a perspective view of an alternate embodiment of the instant invention using pedestal-mounted dielectric elements and includes a dielectric RF matching element.

FIG. 6 shows a graph of certain operating characteristics of the instant invention depicting the delay ratio (C/Vph) as a function of frequency, where C equals the velocity of light and Vph equals the measured phase velocity of the circuit.

FIG. 7 shows a graph of certain operating characteristics of one embodiment of the instant invention depicting interaction impedance in ohms as a function of frequency.

PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings and particular FIGS. 1 and 2, conventional, prior art meander line structures are shown (generally at 10) in which a conductive ground plane 12 has mounted thereupon a dielectric 16 which supports the meander line shaped conductor 14. In FIG. 1, the dielectric material 16 is shaped like the meander line to provide continuous dielectric material between the conductor and the ground plane itself. FIG. 2 shows a conventional meander line structure with the meander shaped conductor 20 being supported by a plurality of dielectric elements 22 which are disposed transversely of the longitudinal axis of the device. The ground plane 18 supports the dielectric material.

Referring now to FIG. 3, the instant invention is shown having a ground plane 24 and a meander line shaped, flat conductor 26 which has both longitudinal and transverse segments shaped at right angles to each other, with a pair of dielectric supporting rails 28 disposed under the longitudinal segments of the meander line conductor 26. The rails 28 may be constructed from a variety of different materials depending on the frequency of RF power requirements and are bonded to the conductor ground plane 24 and the meander conductor 26. The transverse segments of the conductor 26 have an open space between them and the ground plane.

FIG. 4 shows the instant invention having the same supporting structure shown in FIG. 3 but additionally includes conductive shielding vanes 30 vertically disposed between adjacent transverse segments of the conductor 26 to further enhance the operating characteristics of the device, increasing its broad band operational width. Because of the absence of dielectric material under the transverse segments of the conductor 26, the shields 30 may be easily fabricated and inserted between the adjacent transverse segments of the conductor.

FIG. 5 shows an alternate embodiment of the invention in which the continuous, elongated support rails have been replaced by a plurality of relatively short, dielectric rail-like segments (hereinafter termed "pedestals") 34 which support only the longitudinal segments of the meander line conductor 36 on each side of the device and above the ground plane 32. A dielectric RF matching element 38 is inserted under the first transverse meander conductor segment adjacent a conventional RF input conductor 40 to adjust and properly match and load the device for a particular signal range. The RF matching element dielectric material may be chosen from a variety of different materials which provide for flexibility in RF loading and matching of the circuit. The ground plane is constructed of a conventional conductive material, usually copper. The ceramic or dielectric pedestals 34 are sized in height to provide proper separation distance between the conductor 36 and the ground plane 32 and are of sufficient length to support the longitudinal segments of the meander line conductor. The pedestals 34 are on opposite sides of the device to each other and to the longitudinal axis of the device.

The dielectric RF matching element 38 may be utilized with either the pedestal mounting elements shown in FIG. 5 or with the rail mounting elements shown in FIGS. 3 and 4.

The rail and pedestal mounted circuits shown in the instant invention can be designed to include both injected electron beam and RF drive modulated operation.

A device shown in FIG. 3 was constructed in which the effective length of the meander line conductor 26 was 3.62 inches (straight line distance from one end of the meander line to the other), a transverse width of 1.45 inches and a thickness of 0.005 inches. The distance between adjacent transverse segments of the meander line conductor was 0.0625 inches while the width of the conductor itself was 0.0625 inches. The meander conductor was spaced above the conductive ground plane by a distance of 0.010 inches. The test results of this particular device constructed with these dimensions are shown in FIGS. 6 and 7. FIG. 6 shows the delay ratio which is a ratio of the speed of light divided by the measured phase velocity of the circuit as the function of the frequency in megahertz. FIG. 7 shows the interaction impedance in ohms as a function of the operational frequency of the device. The device was designed to operate in L-band, i.e., 1,000 to 2,000 megahertz or 1 to 2 gigahertz. The delay ratio was found to vary from 14.0 at 1,000 megahertz to 15.2 at 2,000 megahertz, thus showing that the phase velocity of the wave was approximately 1/14 to 1/15.2 slower than the speed of light. The interaction impedance shown in FIG. 7 varied from 60 ohms at 1,000 megahertz to 25 ohms at 2,000 megahertz. The higher the interaction impedance the more efficient the device is.

The instant invention has an additional advantage in that all meander line slow wave devices become impractically small as the operating frequency becomes higher. The instant invention has the advantage of becoming smaller at a slower rate than the prior art devices. Therefore, this device can be designed to operate at higher frequencies without the disadvantage of being impractically small when compared with other meander line prior art slow wave structures.

Although not shown in FIG. 5, the device in FIG. 5 would have an output conductor similar to the input conductor but disposed at the opposite end of the meander line conductor itself.

The instant invention has been shown and described herein in what is considered to be the most practical and preferred embodiment. It is recognized, however, that departures may be made therefrom within the scope of the invention and that obvious modifications will occur to a person skilled in the art.

Claims

What I claim Is:

1. A meander slow wave structure comprising:

a conductive ground plane;

a meander line conductor means for propogating a wave in a predetermined direction, said conductor means including a continuous conductive material having first and second sets of longitudinal conductors and a set of transverse conductors;

said first and second sets of longitudinal conductors each having a plurality of linearly aligned conductors extending along a line parallel to said predetermined direction;

said set of transverse conductors including a plurality of equally spaced parallel conductors each extending transverse to said predetermined direction and each connecting the end of a different one of said conductors from said first set to the end of a different one of said conductors in said second set;

a plurality of dielectric supporting means mounted on said ground plane for supporting said longitudinal conductors and for spacing said transverse conductors above said ground plane.

2. The device in claim 1, including:

a plurality of electrically conductive shields disposed between adjacent parallel transverse conductors of said meander line conductor means and connected to said ground plane.

3. A meander slow wave structure as in claim 1 including:

a dielectric RF matching element disposed between said meander line conductor means and said ground plane at one end of said meander line conductor means.

4. A meander slow wave structure as in claim 1 wherein:

said dielectric supporting means consists of first and second dielectric rails, each said rail extending along a line parallel to said predetermined direction.

5. A meander slow wave structure as in claim 1 wherein:

said dielectric supporting means consists of a plurality of dielectric pedestals, each said pedestal mounted between said ground planed and a different one of said longitudinal conductors.