Abstract:
A calorimetric load for very high microwave power at very high frequencies is formed by a metallic, cylindrical chamber into which the wave-guide carrying the power opens. Inside the metallic cylinder is a coaxial dielectric cylinder, with a space between full of circulating wave-absorbing fluid such as water. The incoming wave may be in a higher-order mode. To make it disperse rapidly into the absorbing fluid, a conical reflector is located inside the dielectric cylinder to reflect the wave outward.

Description:
FIELD OF THE INVENTION 
     The invention pertains to high-power calorimetric loads for absorbing microwave energy in waveguides. Such loads are used to measure microwave power in testing components and systems. Also, in some circuit applications, a wave attenuator or a complete matched termination is needed. 
     PRIOR ART 
     Calorimetric loads have always been useful elements of radio-frequency (rf) power equipment. They convert rf wave energy to heat a circulating liquid (usually water). The power is measured as the product of the rate of flow of the liquid, its temperature rise, and its specific heat. At low frequencies, loads have absorbed the wave energy in resistive materials which in turn are cooled by the liquid. For very high power densities, the surface heat transfer from the resistive material to the liquid becomes a limitation. 
     At microwave frequencies the attenuation in pure water is high enough that the wave is generally absorbed directly by dielectric loss in the water. The load then consists of: an input waveguide, a wave-propagating chamber filled with circulating liquid, a low-loss dielectric window separating the liquid and the waveguide, and instruments for measuring the flow and the temperature rise of the liquid. 
     Many geometrical arrangements have been used, some of the problems being to distribute the power dissipation over a suitable volume of liquid and to provide a broadband match of the wave into the high-dielectric-constant liquid. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a waveguide calorimetric load for a wave with circular electric field. 
     A further object is to provide a load which will handle very high powers at very high frequencies. 
     A further object is to provide a compact, rugged load. 
     A further object is to provide a load which is well matched to its waveguide over a wide band of frequencies. 
     A further object is to provide a load which is easy to manufacture. 
     These objectives are realized by a load having a cylindrical window at the outside of the waveguide surrounded by a jacket of water. Wave energy propagating down the guide is deflected outward through the window by a conical, metallic reflecting member coaxial with the circular waveguide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an axial section of a prior-art load. 
     FIG. 2 is an axial section of another prior-art load having extended absorbing area. 
     FIG. 3 is an axial section of a load embodying the invention. 
     FIG. 4 is an axial section of a different embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the prior-art load of FIG. 1, a waveguide 10 starting at a flange 12 for connection to a power source is sealed off by a dielectric window 14 behind which waveguide 10 is filled with water 16. The end of waveguide 10 is closed with a metallic baffle 18 through which water is circulated via input and output tubes 20,22. Instruments (not shown) are used to measure the temperature rise and flow rate of the water. As described in U.S. Pat. No. 3,445,789, issued May 20, 1969 to G. D. Rossini, the water chamber may have a baffle septum to direct the water flow over window 14. Waveguide 10 may be either circular or rectangular. 
     For a broadband waveguide match between the air-filled waveguide 10 and water 16, window 14 is preferably of a dielectric constant which is the geometric mean of those of air and water and is one-fourth of a guide wavelength in thickness. High-alumina ceramic has the preferred dielectric constant, about 9, and has excellent physical and dielectric properties. 
     Another prior-art waveguide load is shown in axial section in FIG. 2. Here waveguide 10&#39; is cylindrical and the dielectric window 24 is in the shape of a hollow narrow cone. Water circulates through inlet 20&#39; near the tip of cone 24&#39;, over the surface of window 24 and through outlet 22&#39; near the base of cone 24. The load of FIG. 2 distributes the power over a larger area of ceramic-to-water interface, so this load is capable of handling more power than the simple load of FIG. 1. However, ceramic cone 24 is an expensive part and difficult to manufacture to the required tolerances. Grinding the inside of a narrow cone is particularly difficult. 
     Rapid advances are presently being made in generating very high powers at very high microwave frequencies. The foremost generator is a &#34;gyrotron&#34; crossed-field electron tube. The output of such a tube is typically in a circular waveguide transmitting a mode with transverse, circular electric field TE on . The power and frequency levels are too high for most of the prior-art water loads. Loads have been proposed in which the power leaks out gradually from a long length of waveguide. However, the high-order modes involved tend to continue largely in a forward direction (to &#34;beam&#34;) in the waveguide whose dimensions are large compared to a free-space wavelength. Thus, such loads are bulky and expensive. 
     FIG. 3 is an axial section of a load embodying the invention which solves most of the problems of prior-art loads. It is compact, easily fabricated, and can be designed for any suitable density of power dissipation. The wave enters through a waveguide 30 which may be of rectangular or preferably circular cross-section. The absorbing body of the load is in a closed, metallic, cylindrical shell 32 which is typically, but not necessarily, somewhat larger than input waveguide 30. Cylinder 32 is closed at both ends by metallic end-plates 34,36. Inside cylinder 32 and coaxial with it is the dielectric window 38, which is a hollow cylinder, preferably of ceramic, sealed at its ends to end-plates 34,36. The absorbing liquid 40 is circulated between shell 32 and window 38 in a cylindrical passage 41 which is of radial thickness to substantially absorb the wave in one passage outward and reflected back inward. 
     A high-order circular-electric-field mode would ordinarily beam through the length of window 38 without sufficient spreading to divert most of its energy into fluid 40. To provide the desired spreading over the desired length (to keep the power density within desired limits), a conductive cone 42, as of copper, is disposed coaxially within window 38, its base sealed to end plate 36 and its tip pointing toward the entering wave. The angle α of cone 42 is chosen to provide the desired axial length of the power dissipation area. The entering wave is reflected by the outer surface of cone 42 outward through window 38 into absorbing fluid 40. Particularly for a TE on  mode whose electric field is parallel to the surface of cone 42, the wave reflection is quite specular. Arrows 44 indicate direction of wave energy flow. To remove heat generated by rf current flow in reflector 42, fluid 40 is circulated through its hollow interior 46 via inlet and outlet pipes 48,50. This fluid flow may be in series with the flow through the main absorbing passage 41, leaving through exit pipe 52. Alternatively, the two flow paths may be in parallel. With cooling by parallel flow paths, reflector 42 may be made of a high-resistance conductor such as austenitic stainless steel to help absorb some of the power. 
     Reflector 42 need not be of a true conical shape. Indeed, if the pattern of the mode to be absorbed is known, the shape may be calculated to provide the most uniform distribution of dissipation, hence, the shortest length of the load. FIG. 4 illustrates schematically a shape which might be used for the TE 01  mode. There is no electric field on the axis, hence, no power flow. The nose 54 of reflector 42&#39; which reflects the low, paraxial field may be blunt as shown to reflect this power in a short distance. The blunt shape is advantageous for making reflector 42&#39; by hydroforming. 
     The advantages of the inventive load include: short axial length due to control of the energy distribution, ruggedness, ease of manufacture, particularly of the cylindrical dielectric window which is easy to make of precision-ground ceramic, and a good match to the incoming wave. 
     The above embodiments are intended to be exemplary and not limiting. Many other embodiments will be obvious to those skilled in the art. The invention is to be limited only by the following claims and their legal equivalents.