US3914714A

US3914714A - High power dry load in grooved waveguide

Info

Publication number: US3914714A
Application number: US479327A
Authority: US
Inventors: Floyd O Johnson
Original assignee: Varian Associates Inc
Current assignee: PREMIER MICROWAVE OF CALIFORNIA Inc
Priority date: 1974-06-14
Filing date: 1974-06-14
Publication date: 1975-10-21
Anticipated expiration: 1992-10-21
Also published as: CA1020239A

Abstract

A load for absorbing high microwave power flowing in a waveguide without reflecting appreciable power is formed as an elongated massive section of lossy material. The waveguide cross section is shaped to include a slot wherein the electromagnetic fields decrease with transverse distance from the main portion of the guide where most of the power flows. The lossy element starts back in the slot where fields are very low and, proceeding in the direction of power flow, gradually approaches the main guide, whereby the attenuation per unit length gradually increases.

Description

United States Patent [191 Johnson HIGH POWER DRY LOAD IN GROOVED WAVEGUIDE [75] Inventor: Floyd 0. Johnson, Mountain View,

Calif.

[73] Assignee: Varian Associates, Palo Alto, Calif. [22] Filed: June 14, 1974 [21] Appl. No.: 479,327

[52] US. Cl 333/22 R; 333/22 F [51] Int. Cl. H01? 1/26 [58] Field of Search 333/22 R, 22 F, 81 A, 81 B [56] References Cited UNITED STATES PATENTS 2,590,511 3/l952 Craig et al. 333/22 R 2,777,906 l/l957 Shockley 333/81 B X 2,850,702 9/l958 White 333/22 F 3,487,340 l2/l969 Curtis et al 333/8l B [4 1 Oct. 21, 1975 3,560,889 2/1971 Suetake et al. 333/22 R Primary Examiner-Paul L. Gensler Attorney, Agent, or FirmStanley Z. Cole; D. R.

Pressman; R. B. Nelson [5 7 ABSTRACT A load for absorbing high microwave power flowing in a waveguide without reflecting appreciable power is formed as an elongated massive section of lossy material. The waveguide cross section is shaped to include a slot wherein the electromagnetic fields decrease with transverse distance from the main portion of the guide where most of the power flows. The lossy element starts back in the slot where fields are very low and, proceeding in the direction of power flow, gradually approaches the main guide, whereby the attenuation per unit length gradually increases.

11 Claims, 9 Drawing Figures US. Patent Oct. 21, 1975 Sheet20f2 3,914,714

mi w w n HIGH POWER DRY LOAD IN GROOVE!) WAVEGUIDE BACKGROUND OF THE INVENTION 1. Field of the Invention The invention concerns electromagnetic transmission line loads or terminations used to absorb most of the wave power flowing in the line. Complete absorption implies no reflection of incident wave power. Even when some power is allowed to leak through in the forward direction for purposes such as measurement convenience, it is commonly desirable to avoid reflection of incident power. To eliminate reflection over a wide band of frequencies only two methods are known.

In one method the transmission line is abruptly terminated with an impedance equal to its characteristic impedance. This requires a pure resistance (for a nonlossy line) and is feasible only in lines where the characteristic impedance is independent of frequency, such as coaxial or two-wire lines used for low radio frequencies. It.is also limited to low powers because all the energy must be absorbed in a small space.

The other method, suitable for high power, microwave frequencies and hollow waveguides, is to have the absorbing element present to the line an effective shunt admittance whose component conductance (and susceptance if present) increase in the direction of power flow at a slow rate per wavelength of line. In microwave waveguide transmission lines, two types of load are common: the dry load wherein power is absorbed by a solid lossy material and the heat is removed by conduction to a sink, and the calorimeter load wherein the power is absorbed by a circulating liquid coolant, usually water. Temperature rise and flow of the coolant are used as a measure of the power dissipated.

The term waveguide is used to mean a transmission line for electromagnetic waves in which the wave is substantially enclosed in a hollow conducting tube. It includes coaxial and shielded two-wire lines as well as hollow pipes with no separate internal conductor.

The objective of eliminating reflections requires that, where the wave first encounters the lossy material, only a small fraction of the wave energy flow in the lossy material. On the other hand, to achieve high attenuation in a short distance requires that a large fraction of the energy flow in the lossy material.

2. Description of the Prior Art The mutually competing requirements for low reflection and for high attenuation in a short length have resulted in prior art dry loads typically using a long tapered wedge or pyramid of lossy material situated in the waveguide with its apex pointed in the direction of the power source. For low power the lossy material is commonly a thin resistor board of plastic or glass coated with a high resistance layer such as graphite or a thin metal film. High power dry loads typically use a solid lossy member made of material such as solid silicon carbide, porous alumina ceramic impregnated with carbon, or a beryllium-oxide-ceramic-containing dispersed silicon carbide particles. To conduct away the dissipated heat, the lossy member is fastened to one or more waveguide walls by an adhesive or by a metalceramie brazed bond. These high power'loads are difficult to make because the useful lossy materials are brittle and tend to break in the processes of grinding them down to a slim point and bonding them to the waveguide. A further difficulty is that near the point of the load the incident wave is not significantly attenuated. The resultant radio frequency, high electric field at the surface of the lossy material often starts arc discharges due to the microscopic roughness, and to the nonuniform conductivity of the surface which can create incandescent spots. Prior art water loads have taken several forms. The most widely used broadband wave guide water load is a cylindrical glass tube, through which the water circulates, extending inside the waveguide for a length sufficient to absorb the power. The leading end of the water tube cannot be tapered to a point due to the requirement for water flow. The high dielectric constant of water causes this blunt leading end to act as a discontinuity of impedance in the waveguide, with resultant wave reflection. The wavereflecting discontinuity is minimized by displacing this end into a region in the waveguide where the electric field is low. In conventional rectangular guide carrying the TB mode this region is next the short E-plane wall. US. Pat No. 3,147,451 issued June 6, 1960 to George K. Merdinian describes this technique of reducing leading-end reflections.

When prior-art water loads as described above are designed for high power levels, the water flow requirement requires the leading end of the tube to be a significant fraction of the waveguide cross section. The electric field falls to zero at the waveguide wall as a sine function of lateral distance, hence much of the leading end of the tube must be in a region of appreciable field, causing wave reflections inevitably to result. Another problem is that at frequencies below 1 gigahertz, the wave attenuation in water decreases, so in order to limit the physical length of the load, it is desirable to have as much of the waveguide as possible full of water. In the Merdinian patent cited above, this is done by forming the water tube as a longcone with truncated apex facing the power source. Such a structure is expensive to make and is fragile.

OBJECTIVES A principal object of my invention is to provide a high power waveguide load of rugged, simple construction which is well matched over a broad band of frequencies.

Another object is to provide a waveguide load which is free from arcing at high power.

Another object is to provide a load in which the attenuating element is easy to fabricate and attach to the waveguide.

Another object is to provide a load in which the dissipation per unit length is uniform.

SUMMARY My invention utilizes a novel combinatioin of elements to meet the objectives for a high power waveguide load. The disadvantages of prior art loads are overcome by using a waveguide of unusual cross section, in which there is a region where the electric field falls to zero in a quasi-exponential manner. The best known of this class of guides is the so-called groove guide described by F. J. Tischer in the Institute of Electrical and Electronic Engineers Transactions on Microwave Theory and Techniques, pages 291-296,

Sept. 1963. Similar guides are disclosed in US. Pat. No. 3,315,187 issued to Tsuneo Nakahara et al., April 18, 1967. I discovered that in such a guide one can use a massive lossy element which starts, at the end nearest the wave source, in the region of very low field and over an axial extent of several wavelengths gradually penetrates into regions of successively higher field. The starting end need not be tapered in size because in the low-field region it reflects negligible wave energy, eliminating a major difficulty of previous loads. Also, at the axial position where the lossy element reaches a very high-field region the wave has already been attenuated considerably, so the danger of arcing is eliminated.

As a further benefit of my invention, I have found that I can choose easily fabricated dimensions and materials such that the power dissipated per unit length is constant. Thus, the overall length of the load may be minimized without exceeding the dissipation capability of the structure at any point.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 through illustrate a dry load in a modified grooved waveguide in which:

FIG. 1 is a longitudinal section of the load. FIGS. 2 and 3 are cross sections of the load of FIG. 1 at two different longitudinal points. FIG. 4 illustrates the electric field pattern of the dominant wave mode in the waveguide. FIG. 5 is a graph of the field strength along the mid-plane of the guide.

FIG. 6 is a longitudinal section of a water load in grooved waveguide.

FIG. 7 is a cross section of the load of FIG. 6.

FIG. 8 is a cross section of a dry load in coaxial waveguide.

FIG. 9 is a cross sectional view of the dry load of FIG. 8, at the plane 9-9 in FIG. 8.

DESCRIPTION OF THE INVENTION It will be obvious to those skilled in the art that many embodiments of the present invention are possible and useful. The invention will be described using some typical examples.

In FIGS. 1, 2, and 3, a rectangular hollow metallic waveguide 10 having open end 11 for attachment to the source of power to be attenuated is joinedat its other end 12 to a double-grooved waveguide 13. For minimum wave reflection at this joint, a tapered or stepped transition section can be interposed, but for simplicity this is not shown in FIG. 1. Grooved guide 13 consists of a metallic wall 14, as of aluminum, enclosing an axially extended opening which comprises a rectangular passage 15 of dimensions approximating those of input waveguide 10 and two rectangular grooves 16 opening from the centers of the wide walls 17 of rectangular passage 15. Within grooves 16 are elongated bars 18 of lossy material, such as silicon carbide. At the end 19 of bars 18 nearest input waveguide 10, the bars occupy an area recessed from rectangular passage 15, as shown in FIG. 2. As bars 18 extend in the direction of wave flow, they are tapered so that their inner surfaces 20 progressively approach and enter rectangular passage 15, as shown in FIG. 3. At terminal ends 21, bars 18 largely fill passage 15. The shape and taper of bars 18 is chosen to provide approximately constant power dissipation per unit length, the relative attenuation per unit length increasing as the power flow decreases due to preceding attenuation.

In FIG. 4 are plotted in cross section, the electric field lines in an empty grooved waveguide. The rectangular passage 15 has width b between one-half and one free-space wavelength to propagate the dominant mode corresponding to the TE mode in purely rectangular guide. To make the electric field fall off as desired in grooves 16 their width d is chosen less than one-half of a space wavelength. Grooves 16 alone cannot propagate the dominant mode. FIG. 5 is a plot of field strength as a function of height along the midplane 22 of guide 16. For grooves very deep compared to their width, the field falls exponentially with height except for local effects at the open and closed ends. With the dimensions used in practical loads, the end effects cover much of the area so a quasi-exponential decay results, decreasing with distance from passage 15 at a rate which itself decreases with distance. The attenuation is a function of the field strength at the inner surface 20 of bars 18. The concave-downward variation of field allows the attenuation to start at a low value and increase at an increasing rate with axial distance, as required for uniform dissipation, while using simple uniformly tapered bars 18. Near the terminal end 21 where bars 18 enter the high field passage 15, the transverse field variation becomes less, and bars 18 accordingly penetrate passage 15 at a greater rate with respect to axial distance.

FIGS. 6 and 7 illustrate a waveguide water load incorporating the features of my invention. The grooved waveguide 23 is similar to that in FIG. 1 except that in this example only one groove 16 is present. The attenuating member is a dielectric tube 24, 25, as of glass, carrying a circulating liquid, as water. At high microwave frequencies, such as 1 GHz and above, the dielectric loss of the water provides the power attenuation directly. At lower frequencies the water may be made conductive by addition of an ionized salt, or lossy material, as silicon carbide grains, may be placed in the tube. The active portion 25 of the tube enters waveguide passage 15 slowly from groove 16. Fluid flow is measured by flowmeter 26 and temperature rise by inlet and

outlet thermometers

27 and 28. From these measurements, the dissipated power may be calculated.

FIGS. 8 and 9 show another embodiment of the invention. The waveguide 29 is a coaxial transmission line formed by center conductor 30 and closed outer conductor 31. Most of the wave energy travels in passage 15 near the center conductor. Groove 16', of width d less than half a free-space wavelength wide, opens into passage 15'. Groove 16 is illustrated as having width d less than the outer diameter D or coaxial line 29. The width d may, however, be greater than or equal to line diameter D. The inner edge of lossy member 18 tapers from a position 20' in the weak field remote from the center conductor 30 to a close position 20" where the field and attenuation are high.

It is thus seen that my invention provides a rugged, easily fabricated, high power load that does not reflect wave energy, occupies minimum space, and reduces arcing danger.

It is apparent that many embodiments other than those illustrated are possible within the scope of the invention. For example, it can be applied to any form of enclosed waveguide and any elongated form of power absorbing element. Accordingly, the true scope of the invention should be determined only by the following claims and their legal equivalents.

What is claimed is:

1. A waveguide power absorber comprising:

a length of enclosed waveguide;

a wave attenuating member within said waveguide extending generally in the direction of wave propagation; said waveguide being so shaped in cross section transverse to the direction of propagation as to include a region in which, as a function of transverse distance from the locus of maximum electric field, the electric field decreases at a decreasing rate, and a first end of said attenuating member nearest the source of wave energy is entirely located in said region; said region being a slot of width less than one-half of a free space wavelength in the band of interest, said slot being symmetrical about a midplane of said waveguide.

2. The apparatus of claim 1 wherein the electric field at said midplane is parallel to said midplane.

3. The apparatus of claim 1 wherein said attenuating member, as a function of distance in the direction of propagation, extends gradually into transverse positions of increasing local electric field with respect to the maximum electric field at said distance.

4. The apparatus of claim 3 wherein said attenuating member extends gradually into a transverse position where said local electric field is substantially equal to said maximum electric field 5. The apparatus of claim 3 wherein said transverse positions of said attenuating member are arranged to produce substantially uniform power dissipation per unit length.

6. The apparatus of claim 1 wherein said first end of said attenuating member is blunt.

7. The apparatus of claim 6 wherein said attenuating member is of substantially uniform cross section.

8. The apparatus of claim 1 wherein said attenuating member is a rod of solid lossy material.

9. The apparatus of claim 8 wherein said rod of lossy material is affixed throughout a substantial portion of its length to at least one wall of said waveguide, providing a thermally conducting path.

10. The apparatus of claim 1 wherein said attenuating member comprises a dielectric tube capable of conducting a flow of liquid.

11. The apparatus of claim 10 wherein said liquid is the attenuating material.

Claims

1. A waveguide power absorber comprising: a length of enclosed waveguide; a wave attenuating member within said waveguide extending generally in the direction of wave propagation; said waveguide being so shaped in cross section transverse to the direction of propagation as to include a region in which, as a function of transverse distance from the locus of maximum electric field, the electric field decreases at a decreasing rate, and a first end of said attenuating member nearest the source of wave energy is entirely located in said region; said region being a slot of width less than one-half of a free space wavelength in the band of interest, said slot being symmetrical about a midplane of said waveguide.

4. The apparatus of claim 3 wherein said attenuating member extends gradually into a transverse position where said local electric field is substantially equal to said maximum electric field

5. The apparatus of claIm 3 wherein said transverse positions of said attenuating member are arranged to produce substantially uniform power dissipation per unit length.

11. The apparatus of claim 10 wherein said liquid is the attenuating material.