US3121204A - Non-reflective liquid termination of a coaxial cable - Google Patents

Description

Feb. 11, 1964 s. GIORDANO NON-REFLECTIVE LIQUID TERMINATION OF A COAXIAL CABLE Filed Dec. 23; 1960 2 6 .nlu, 3 M2 2 M 2 l R\\ Fig.4

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SALVATORE GIORDANO United States Patent 3,121,2tl4 NON-REFLEUTIVE LIQUID TEATION OF A COAXIAL CABLE Salvatore Giordano, Port Jeiierson Station, N.Y., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Dec 23, 1960, Ser. No. 78,193 7 Claims. (Cl. 333-42) This invention relates to a dissipative load for radiofrequency energy and more particularly to a dissipative load for use with a transmission line carrying microwave radio-frequency energy.

In certain applications of radio-frequency power, it is often desirable and convenient to provide a dissipative load while testing or aligning a power source. Such dissi pative power loads may be used with radio transmitters, microwave Waveguides, and power supplies for particle accelerators. Available radio-frequency loads of various types generally include resistance type elements which dissipate energy throughout the audio and into the videofrequency range. However, at microwave frequencies, resistance loads and many of the other existing types of dissipative loads are not satisfactory in that they cannot be used conveniently with very high peak radio-frequency power and further it is difficult to m e meaningful calorimetric measurements of the load dissipation.

In measurement work, it is usually necessary to termiunto the transmission line or waveguide so that the reflected wave is substantially absent over a wide band of frequencies. One simple and efiiective way of obtaining a non-reflecting load impedance is to connect the end of a transmission line to a length of a second transmission line having high loss but the same characteristic impedance as the line being terminated. If the attenuation of the 10583! line is suflicient, an incident wave entering the lossy line will be absorbed completely. Lossy lines are designed so that the required total attenuation can be realized in a reasonable length. For this purpose, flexible cable is commercially available in which the attenuation has been made very high by use of insulation having high radiofrequency losses and by employing resistance wire for the center conductor of the cable.

In particle accelerators of the type using microwave radio-frequency to provide the power, the latter is frequently supplied to the accelerator in a carefully programmed intermittent pattern. In one such type of linear accelerator, the microwave frequency energy is supplied to the accelerator by means of a so-called Magic T. As the linear accelerator is pulsed, there exists periods in time when the energy at the Magic T does not enter the accelerator and must be shunted to a dissipative load. It is for applications such as this one that existing standard radio-frequency dissipative loads are unsatisfactory.

The present invention permits the convenient termination of high energy radio waves in the microwave frequencies with little or no reflected wave, under conditions at which taking of calorimetric measurements is feasible. in one particular embodiment of this invention, a dissipative load for radio-frequency power is provided by suitably coupling to a radio-frequency power source one end of a conventional coaxial cable of the type having an outer conductor separated from an inner conductor by an annular dielectric insulation, modifying the other end of the cable by removal of the outer conductor for a finite distance from the end, terminating the outer conductor in a flare, and submerging the entire bared end, including the flare, in a liquid solution having a suitable dielectric constant and electrical conductivity permitting some current flow to obtain desirable ohmic losses therein. With these conditions satisfied, it is possible to propagate radiofrequency energy along the insulation with a coaxial "ice mode field configuration. This field configuration causes high axial currents to flow in the solution, which in consequence of its high resistivity results in the dissipation of radio-frequency power in the form of heat. Proper calorimetric measurements of the solution under carefully controlled conditions will then indicate the amount of energy dissipated.

It is thus a first object of this invention to provide a dissipative load for radio signals at microwave frequenc1es.

Another object of this invention is to provide apparatus for terminating transmission lines with little or no reflecting wave.

Still another object is to provide apparatus for dissipating high energy radio signals in the microwave range of frequencies with the making of calorimetric measurements being convenient.

A complete and better understanding of these and other objects and purposes of this invention will be had by reference to the accompanied drawings, in which:

FIGURE 1 is a section view of the modified end of a coaxial cable or transmission line according to this invention;

FIGURES 2(a) and 2(b) show two arrangements for submerging the modified ends of the coaxial cable of FIGURE 1 according to the principles of this invention;

FIGURE 3 is a section view of an alternative termination for the cable; and

FIGURE 4- is a section view showing another embodiment of this invention utilizing a tunable element.

Referring now to FIGURE 1, there is shown a transmission line or coaxial cable 1 with an inner conductor 2, an outer electrical conducting sheath 4, and an annular dielectric insulating element 6. A portion of sheath 4 is removed from the end of coaxial cable 1 baring di electric 6 for a distance from the end as shown by A in the drawing, and forming a modified end 3 of cable 1. Outer conductor 4 is terminated by a conical flared portion 1th as shown The modified end 3 of coaxial cable 1 may be provided with a spherical conductor 12 to reduce the possibility of arcing.

To dissipate the RF energy in accordance with this invention, modified end 3 of coaxial cable 1 would be im mersed to level .14 in a liquid having the desired dielectrio and resistance properties, as to be described more particularly below in connection with FZGS. 2(a) and 2(b). The other end of coaxial cable 1 is connected to a suitable high-frequency power source in customary fashion (not shown) not forming part of this invention.

FIGURE 3 shows an alternate flared termination for the outer conducting sheath 4- of coaxial cable 1'. In this arrangement outer conducting sheath 4/ is terminated at a suitable point and a truncated conical conductive element 2t}, is suitably fastened, as by soldering or welding, to the terminal end of the conductive sheath 4'.

In FIGURES 2(a) and 2(b), two arrangements for immersing modified end 3 of coaxial cable 1 (or 1) in a container 15 of high dielectric-high resistance solution 17 are shown. In FIGURE 2(a), modified end 3 of the coaxial cable 1 is shown with an unmodified portion 16 having the terminated outer conductor flare ill submerged below the level of liquid 17 and modified end 3' of coaxial cable 1 coiled in loops under the surface of liquid 17. In FIGURE 2(b) modified end 3" of coaxial cable 1 is shown entering liquid container 15 from underneath.

In the use'of this invention where the radio-frequency energy to be dissipated is of a high frequency, the modified end having the outer conducting sheath removed, as represented by A in FIGURE 1, can be short. When modified end 3 is short, it is possible to insert this termi nal end of coaxial cable 1 without bends. Cable 1 can enter through the air liquid interface or through a container wall, provided the container wall is of a non-conducting material having a high dielectric. As shown in FlGURE 2(a), when the modified end is long, it is possible to coil the modified end of the coaxial cable into several loose loops without any detrimental effect, provided the spacing between adjacent loops is suflicient (i.e., on the order of 2 or 3 inches for microwave frequencies) to prevent coupling between adjacent loops.

Where outer conducting s reath 4 is made up of woven or braided material and the outer conductor flare ill is made by expanding the terminal end of the retained position of conducting sheath 4, there are no disadvantages to inserting modified end 3 through the air-liquid interface as shown in FIGURE 2(a). However, if outer sheath 4 is of non-woven material or if the flare is provided as shown in FIGURE 3, it is preferable to use the embodiment as illustrated in FIGURE 2(1)). With nonporous flare material, flare it serves to act as a collector for air, if used as shown in the embodiment of FIGURE 2(a) and a discontinuity is formed, which discontinuity interferes with the operation of this invention. When the immersion shown ni FIGURE 2(b) is used, the tendency to trap air within the confines of the flare it} is eliminated. 7

It has been found that aqueous solution 17 in FIGS. 2(a) and (b) must have certain electrical properties to obtain the desired results in accordance with the principles of this invention. A minimum of 1% by weight of NaCl dissolved in Water has proved to be particularly applicable, to obtain a dielectric constant slightly greater than that of the insulation used in the cable and a resistivity much less. Pure Water has a dielectric constant of 75 as compared to a value of 2 for the element 6 used in typical transmission lines. The addition of a minimum of 1% by Weight of NaCl produces a sufficient conductivity of solution 17 which will permit continuation of the coaxial mode of the high axial current in the solution which dissipates the RF power effectively under the conditions desired, as previously described.

In one example of this invention, illustrated by FIG- URES 1 and 2(a), distance A was equal to 2 feet and liquid 17 was an aqueous solution of sodium chloride con taining about 3% by weight of the sodium chloride. A 200 megacycle microwave frequency power source was terminated and found to have a voltage standing wave ratio of 1.6 to 1.

Where improved performance of the dissipative load is desired, as measured by a reduction in the voltage standing wave ratio, it has been found that a tunable arrangement illustrated in FIGURE 4 can be used. In this embodiment, a tunable element 2- 2 is added to modified end 3, slidably disposed on the dielectric insulation 6 in that region from which the outer conducting sheath 4 has been removed. Tunable element 2.2: is provided with an upper flared end 24 and a lower flared end 26- and is tunable by movement along the length of the exposed por tion of 6 for adjustment to a minimum value of the voltage standing wave ratio. 'With a coaxial cable having tunable element 22 of FIGURE 4 and an exposed length of 2 feet, and using 200 megacycle power, as described above, the voltage standing wave ratio was found to be 1.2 to 1. It is also possible with two such tunable elements 2% to adjust the voltage standing wave ratio to even lower values. Elements 2.2 can be constructed by selective removal of portions of the outer conductor sheath 4 with suitable flaring of the ends of the retained portions.

Referring once again to FIGURE :1, distance A determines the frequency of energy above which this coaxial cable termination can be used as a dissipative load. For instance, when the distance A is etual to 2 feet, a 200 megacycle load can be accommodated as noted. If the distance A is equal to 1 foot, however, only frequencies above 4 mcgacycles can be accommodated. Similarly, if the distance A is equal to 4 feet, all radio-frequencies above megacycles could be accommodated. it has been found therefore that the critical limiting frequency is a linear function of the distanceA, in accordance with the following empirical relationship:

where A is in feet and f is the lowest frequency which can be accommodated in accordance with this invention.

Because all of the incident microwave energy is dissipated as resistive heating of the solution in which the transmission line is immersed, suitable temperature measurement of this solution can be used to make meaningful calorimetric measurements of the load dissipation in accordance with well-known techniques.

In applications for high peak radiofrequency power, where coaxial line terminations employing a resistance wire core and lossy insulation gene-rally suffer breakdown of the insulation or failure of the resistance wire, the coaxial dissipative load termination of this invention is particularly useful since the coaxial cable insulation is not modified by the addition of lossy material which could lower its breakdownpotential, and the resistance element for energy dissipation is distributed throughout a large volume and not concentrated in a small element having a limited heat capacity.

Where high average radio-frequency power dissipation is required, the liquid which is heated by resistive heating can be cooled, as is well known in the art, by provision of means for circulation of a coolant through a heat exchanger immerscd in the heated liquid or for circulation of the heated liquid through suitable auxiliary cooling facilities.

Whereas, this invention has been described as nonreliecting terminal ends for coaxial transmission lines, it will be readily apparent that this invention can be used to terminate waveguides by suitably coupling a coaxial trans mission line to a waveguide by conventional means.

While further, an aqueous salt solution has been described as one example of a liquid having suitable electrical characteristics, it is understood that other liquids and solutions having the herein specified properties of high dielectric constant and high electrical resistivity may be used under appropriate conditions.

It should be understood therefore that the foregoing disclosure relates to only preferred embodiments of the invention and that numerous modifications or alterations thereof may be made therein without departing from the spirit and the scope of the invention as set forth in the appended claims.

I claim:

1. A non-reflecting terminator for a coaxial cable having inner and outer conductors separated by an annular insulating element comprising a prepared portion of the end of said cable, said prepared portion consisting of the inner conductor and annular insulating element extending beyond the end of the outer conductor for a predetermined length and a flare formed at the end of said outer conductor and a tank of liquid into which said cable end is immersed, said liquid completely covering the exposed insulating element and said flared end of the outer conductor, said liquid having a predetermined relatively high dielectric constant.

2. The terminator of claim l in which the predetermined bared length of said cable is determined in accordance with the function where A is said length in feet and f is the lowest radiofrequency carried by said cable.

3. The terminator of claim 1 in which said liquid is solution of at least 1% by weight of NaCl in water.

4-. The terminator of claim 1 in which a slidable conductive element surrounds and is movable on the length of said cable with the outer conductor removed to permit tuning of said cable to reduce the voltage standing wave ratio.

5. The terminator of claim 1 having means to prevent arcing, the latter said means including a spherical element of conductive material mounted on the tip of the prepared portion of said cable covering the exposed ends of the inner conductor and insulating element.

6.. The terminator of claim 1 in which the hated end of said cable is coiled in loops and the distance between 10 7. The terminator of claim 1 in which said tank is made of non-conductive material and said cable passes through a wall of said tank.

References Cited in the file of this patent UNITED STATES PATENTS 2,158,271 Buschbeck May 16, 1939 2,498,589 Steinke Feb. 21, 1950 2,945,913 Conangla July 119, 1960 FOREIGN PATENTS 815,501 Germany Oct. 1, 1951

Claims (1)

1. A NON-REFLECTING TERMINATOR FOR A COAXIAL CABLE HAVING INNER AND OUTER CONDUCTORS SEPARATED BY AN ANNULAR INSULATING ELEMENT COMPRISING A PREPARED PORTION OF THE END OF SAID CABLE, SAID PREPARED PORTION CONSISTING OF THE INNER CONDUCTOR AND ANNULAR INSULATING ELEMENT EXTENDING BEYOND THE END OF THE OUTER CONDUCTOR FOR A PREDETERMINED LENGTH AND A FLARE FORMED AT THE END OF SAID OUTER CONDUCTOR AND A TANK OF LIQUID INTO WHICH SAID CABLE END IS IMMERSED, SAID LIQUID COMPLETELY COVERING THE EXPOSED INSULATING ELEMENT AND SAID FLARED END OF THE OUTER CONDUCTOR, SAID LIQUID HAVING A PREDETERMINED RELATIVELY HIGH DIELECTRIC CONSTANT.

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