US3906402A - Liquid cooled dummy load for RF transmission line - Google Patents

Abstract

A liquid cooled dummy load device or termination for an RF coaxial transmission line. Electrical energy is converted to heat energy in a resistive film that is series-connected between the inner and outer conductors of the transmission line and deposited on a tubular cylindrical dielectric substrate. Liquid coolant being circulated through the device to and from a heat exchanger is moved in an axial direction through an inner annular flow passage defined by the inner cylindrical surface of a low friction tube and by the resistive film to absorb the heat energy, then outwardly through radial ports to an outer coaxial annular flow passage defined by the outer cylindrical surface of the aforesaid tube and by the inner surface of another low friction tube and finally to an outlet fitting. The housing has an interior surface form defining an exponential horn chamber surrounding and coaxial with the resistive film to minimize reflection from the device.

Description

[451 Sept. 16, 1975 LIQUID COOLED DUMMY LOAD FOR RF TRANSMISSION LINE Leo Lesyk, Walton Hills, Ohio [73] Assignee: Bird Electronic Corporation, Solon,

Ohio

[22] Filed: Dec. 13, 1974 [2]] Appl. No.: 532,724

Related US. Application Data [63] Continuation-impart of Ser. No 417,104, Nov 19,

1973, abandoned.

[75] Inventor:

[30] Foreign Application Priority Data Oct. 25. 1974 Canada 212346 Nov. 14, 1974 Germany 2453962 Nov. 19, 1974 United Kingdom 50103/74 52 US. Cl. 333/22 F; 333/81 A 51 m. cm H01? l/26 [58] Field of Search 333/22 R, 22 F, 81 B, 81 A l/l972 Lesyk et al. 333/22 F Primary ExaminerPaul L. Gensler Attorney, Agent, or Firm-Bosworth, Sessions & McCoy 57 ABSTRACT A liquid cooled dummy load device or termination for an RF coaxial transmission line. Electrical energy is converted to heat energy in a resistive film that is series-connected between the inner and outer conductors of the transmission line and deposited on a tubular cylindrical dielectric substrate. Liquid coolant being circulated through the device to and from a heat exchanger is moved in an axial direction through an inner annular flow passage defined by the inner cylindrical surface of a low friction tube and by the resistive film to absorb the heat energy, then outwardly through radial ports to an outer coaxial annular flow passage defined by the outer cylindrical surface of the aforesaid tube and by the inner surface of another low friction tube and finally to an outlet fitting. The housing has an interior surface form defining an exponential horn chamber surrounding and coaxial with the resistive film to minimize reflection from the device.

3 Claims, 4 Drawing Figures PATENIEB SEP I6 3975 LIQUID COOLED DUMMY LOAD FOR RF TRANSMISSION LINE CROSS REFERENCE TO RELATED APPLICATION This application is a continuationimpart of prior application Ser. No. 417,104 filed Nov. 19, 1973, now

abandoned.

BACKGROUND OF THE INVENTION This invention relates to dummy load devices for use as reflectionless terminations for RF coaxial transmission lines. Morepartieularly the invention relates to liquid-cooled dummy load devices which rely on convection to dissipate heat energy. In this type of device a liquid cooling, medium is circulated through the device to carry off the heat energy generated by the dissipation of electrical energy across the load. Normally a heat exchanger and pump are provided to minimize the volume of cooling water required.

Frequently in testing transmitter apparatus or in measuring radio frequency power, a substantially reflectionless termination or dummy load is used to terminate the coaxial transmission line. The termination must be capable of absorbing and dissipating the power in the form of heat. Also the termination must be matched to the electrical characteristics of the coaxial transmission line asdetermined by the physical dimensions of the line in order to avoid reflection of radio frequency waves from the termination. In order to minimize reflection and maximize power transfer from a transmission line to a termination, the load should have a characteristic impedance that is matched to the characteristic impedance of the line. For coaxial lines the characteristic impedance is defined by: Z,,= I L/C where z fljharacteristic impedance L=Distributive inductance C=Distributive capacitance More conventional types of terminations employed for coaxial lines utilize a tapered horn principle to minimize reflection. The microwave signals are passed along a resistive layer defining the tapered surface, and the signal is thus gradually attenuated in an advantageous manner to minimize reflection. Typical examples of such line terminations are disclosed in US. Pat. Nos. 2,556,642; 2,752,572; 2,984,219; 3,300,737; 3,213,392 and $634,784.

In a typical construction for the devices disclosed in the above patents a tubular cylindrical ceramic mem' ber with a resistive filmor coating applied to the exterior cylindrical surface is mounted in the device to provide the electrical load. The device normally has one end adapted for making the electrical connection to the transmission line and the other end adapted for connection to the inlet and outlet lines for the liquid coolant. As indicated above, the member with the resistive film coated thereon is hollow and a coaxial logarithmic horn is provided around the resistor to minimize reflection. i

Liquid coolant normally is supplied to the interior of the ceramic member at the end with the inlet fitting and flows therethrough to the opposite end and then outward such as through radial ports in theceramic substrate toan annular flow passage that surrounds the resistive filmlThe liquid coolant then reverses flow and proceeds back to the opposite end of the device to the outlet fitting as it absorbs heat energy generated by the resistive film. Thus the circulation of liquid coolant proceeds first through the interior of the ceramic element and then outward and across the surface of the resistive film.

Normally the liquidcoolant must be pumped at a relatively high velocity but in view of the relatively large amount of water within the tubular ceramic element there is considerable resistance to fluid flow when the water moves outward and along the annular flow path surrounding the resistive film. In other words, the path ofcirculating in the above recited type of construction generates a substantial pressure drop. Also contributing to the pressure drop is the surface friction resistance between the cooling liquid and the ceramic sur face defining the respective flow passages. Ceramic surfaces generally have a fairly high surface friction characteristic which contribute significantly to the overall flow resistance.

The device of the present invention, however, reduces the disadvantages described above and affords other features and advantages heretofore not obtainable.

SUMMARY OF THE INVENTION It is among the objects of the invention to improve the heat transfer in a liquid cooled dummy load device.

Another object is to minimize resistance to fluid flow (i.e., reduce the pressure drop) in a dummy load device for an RF coaxial transmission line.

These and other objects and advantages are achieved by the liquid cooled dummy load device of the present invention which includes a cylindrical housing formed of conductive material and a connector at one end of the housing adapted to receive a mating connector from the coaxial transmission line. An elongated tubular dielectric member is mounted coaxially within the housing and a thin resistive film is deposited on the outer surface thereof and series connected between the contacts of the connector to convert electrical energy being transmitted by the transmission line into heat energy. The housing defines a closed annular interior horn chamber surrounding the resistive film for minimizing the reflected energy from the dummy load.

In accordance with the invention, inner and outer cylindrical sleeve elements formed of low-friction insulating material such as TEFLON (polytetrafluoroeythylene) are provided to define inner and outer annular cylindrical liquid flow passages coaxial with the housing. The inner annular flow passage is connected to a liquid inlet at one end and surrounds the resistive film along the entire length thereof to absorb heat energy therefrom. The outer annular flow passage communicates with the inner annular passage through radial ports at the end thereof opposite the inlet fitting and extends back to an outlet chamber with an outlet fitting at the same end as the inlet fitting.

Normally a heat exchanger and pump are provided to complete the cooling system. The space within the di electric member is most advantageously provided with a rod-like filler element adapted and designed to help provide optimum electrical characteristics for the device. An annular space between the filler rod and the interior wall of the cylindrical ceramic dielectric member is filled with liquid coolant by tapping from the inlet, the water being gradually replenished by means of one or more small exhaust ports. It will be noted that it is not necessary, however, to maintain circulation through the interior of the dielectric member since the cooling function is accomplished by' liquid coolant flowing through the annular flow passages.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view along the axis of a liquid cooled dummy load device embodying the invention and including certain associated equipment illustrated in diagrammatic form;

FIG. 2 is a sectional view taken on the line 2-2 of FIG. 1;

FIG. 3 is a fragmentary sectional view taken on the line 33 of FIG. l and with parts broken away for the purpose of illustration; and

FIG. 4 is a fragmentary sectional view taken on the line 4-4 of FIG. 1.

DESCRIPTION OF THE PREFERRED I EMBODIMENT Referring more particularly to the drawings and initially to FIG. 1 there is shown a system for terminating a coaxial RF transmission line. The system includes a reflectionless type dummy load device with integral means for dissipating the heat generated across the load. A signal generator 11 is connected by a coaxial transmission line including an inner conductor 12 and an outer conductor 13, to the device with a suitable connector. The system also includes a heat exchanger 15 and a pump 16., both serially connected to the de vice 10 by an inlet pipe 17 and an outlet pipe 18. The pump 16 circulates liquid coolant (e.g., water) between the device 10 and the heat exchanger 15 and provides the necessary flow velocity to achieve the necessary cooling. The signal generator 11 produces signals at any frequency from DC up to the microwave region and most typically it constitutes a transmitter.

As will be apparent from FIG. 1 the device is constructed so that the fittings for cooling fluid connections are all at the right hand or upper end of the device whereas the electrical connections are made at the left hand or lower end of the device which will be referred to herein as the power connection end. The power connection end includes an inner connector sleeve 21 adapted to receive the inner conductor 12 of the transmission line and an outer connector flange 22 which is adapted to .receive the outer coaxial conductor 13 of the transmission line. The outer connector flange 22 functions as a swivel follower and is brazed to an outer connector sleeve 23 coaxial with the inner connector sleeve 21.

An annular swivel plate 24 is telescoped over the outer connector flange 22 and has a plurality of circumferentially spaced holes that receive mounting bolts for securing the device 10 to appropriate structure. The swivel follower flange 22 may be rotated relative to the mounting plate 24 to position the device about its axis as desired.

Another annular flange 25 is brazed to the opposite end of the outer connector sleeve 23, and is adapted to be bolted to another annular flange 26. The flange 26 is brazed to the main body of the device and specifi cally to a main housing section 30 which constitutes one of the three housing sections including a secondary housing section 31 and a housing end section 32.

The assembly within the housing includes outer and inner flow tubes 33 and 34 respectively formed of TEF- LON or other suitable electrical insulating material with a low-friction characteristic; a dielectric cylinder 35 within the flow tube 33, and with a thin resistive film coated on the outer surface thereof, an insert rod 36 within the sleeve 35 (TEFLON). a resistor socket 37 (TEFLON) anda:'resistor ground fitting 38 (TEF- LON), all of which are coaxially mounted relative to one another. The main housing section 30 has an upper (right hand) end with external threads adapted to be engaged by internal threads formed at the lower (left hand) end of the secondary housing section 31. The upper (right hand) end of the secondary housing section3l is also provided with internal threads adapted to be engaged by external threads formed in the housing end section 32.

The lower (left hand) end of the outer flow tube 33 is received in an annular recess 42 formed in the resistor socket 37, and is provided with an inwardly extending flange 41. The upper (right hand) end of the outer flow tube 33 seats snugly within the inner surface of the main housing section 30 and the respective surfaces are sealed by an O-ring seal 48. An inwardly extending flange 43 at the upper (right hand) end of the main housing section 30 serves to retain the outer flow tube 33 in its axial position between the flange 43 and the resistor socket 37.

A fluid tight seal is maintained between the flange 41 on the outer flow tube 33 and the conductor 44 by means of an O-ring seal 47 which is received in an an nular groove in the flange 41.

A conductor rod 44 of a generally cylindrical form extends through the resistor socket 37 and the flange 41 on the outer flow tube 33 and has a threaded stud which is used to bolt the inner conductor sleeve 21 thereto with a nut 46. This places the inner conductor sleeve 21 in electrical contact with the conductor rod 44.

The inner flow tube 34 is positioned and radially spaced within the outer flow tube 33 with a spacer ring 49.'The inner flow tube 34 extends axially to the upper (right hand) end of the device and is sealed at the upper end against the secondary housing section 31 using an O-ring seal 50 which is positioned in an annular groove located in the inner surface of the secondary housing section 31. The inner flow tube 34 is retained in its axial position by the resistor ground fitting 38 which seats against the upper (right hand) end of the inner flow tube 34 and thus retains its opposite end against the flange 41 of the outer flow tube 33. The resistor ground fitting 38 has a radial flange 51 that seats in a matching annular recess 52 in the secondary housing section 31 and is retained therein by the lower (left hand) end of the housing end section 32. The housing end section 32 is sealed against the adjacent portion of the secondary housing section 31 by still another 0- ring seal.

As will be apparent from FIG. 1 the resistor ground fittting 31 and the housing end section 32 define a liquid cooland inlet chanber 53that communicates with the inlet pipe 17 through an axial fitting 54 threaded into a matching threaded receptacle in the end of the housing end section 32. The resistor ground fitting 38 is provided with radial ports 55 that communicate between the in'let chamber 53 and an inner annular flow chamber 56=defined between the dielectric cylinder'35 and the inner flow tube 34. Liquid coolant flowing through the-inner annular flow chamber 56 is in intimate contact with the resistive film on the dielectric sleeve 35 and thus absorbs heat generated by the dissi pation of electrical energy.

From the inner annular flow chamber 56 liquid coolant flows radially outward to an outer annular flow chamber 57 through radial ports 58 in the inner flow tube 34. The outer annular flow chamber 57 is coaxial with the inner annular flow chamber 56 and is defined by the outer surface of the inner flow tube 34 and the inner surface of the outer flow tube 33.

Liquid coolant proceeds through the inner annular flow chamber 57 as indicated by the flow arrows in FIG. 1 and then through radial ports 59 in the main housing section 30 to an annular liquid coolant outlet chamber 60 defined between the outer surface of the main housing section 30 and the inner surface of the secondary housing section 31. Liquid coolant exits the water outlet chamber 60 to the water outlet pipe 18 through an outlet fitting 61 extending through the wall of the secondary housing section 31.

The interior wall at the lower (left hand) end of the main housing section 30 is formed with a tapered exponential surface of revolution to minimize reflection from the device and to reduce the VSWR. The electrical characteristics of the device as thus designed are in balancing relation so that the Z,, of the device varies from that of the transmission line down to zero.

As indicated above an insert rod 36 formed, for ex ample of TEFLON, is positioned within the dielectric sleeve 35 in accordance with the teachings of U.S. Pat. No. 3,300,737 to assure that the desired electrical characteristics are achieved. The diameter of the rod 36 is slightly less than the inner diameter of the dielectric cylinder 35 so that a thin annular space 63 is defined therebetween. In order to achieve the desired electrical characteristics for the device, the space 63 is filled with liquid coolant from the inlet chamber 53. The coolant enters through an axial opening 65 (FIGS. 3 and 4) in the end of the resistor ground fitting 38 and then radially outward through a slot 62 formed in the end of the insert rod 36 to the space 63.

While it is not necessary to circulate the coolant through the space 63, some movement is beneficial and accordingly a radial bleed port 66 is provided in the resistor socket 37 to permit some escape of coolant and thus a small amount of continuous movement of the liquid coolant indicated by the flow arrows in FIG. 1.

With the construction thus described, the liquid coolant contacts and absorbs heat energy from the resistive film on the ceramic element 35 during the initial part of its circulation through the device and during an interval when it is at its highest velocity. This affords a more efficient and more uniform heat transfer. An ad ditional advantage is that compared with prior art devices the device of the invention affords a much lower resistance to fluid flow and thus a much lower pressure drop. By forming the two annular passages 56 and 57 with sleeves comprising a low friction material, resistance to liquid flow is minimized and the device provides a minimum pressure drop for the cooling liquid being pumped therethrough. This factor is extremely important in view of the high flow velocities that must be used in order to afford the necessary heat tranfer.

While the invention has been shown and described with respect to a specific embodiment thereof, this is intended for the purpose ofillustration rather than limitation and other modifications and variations of the specific machine herein shown and described will be apparent to those skilled in the art all within the in tended scope and spirit of the invention. Accordingly, the patent is not to be limited to the specific embodi ment herein shown and described nor in any other way that is inconsistent with the extend to which the progress in the art has been advanced by the invention.

1 claim:

1. A liquid cooled dummy load device for an RF coaxial transmission line, the device having a connector with first and second contact means for the inner and outer coaxial conductors respectively, and a cylindrical housing;

an elongated tubular dielectric member coaxially mounted in said housing,

a thin resistive film deposited on the outer surface of said dielectric member and series connected between said first and second contact means, said film being adapted to convert electrical energy being transmitted by said transmission line to heat energy,

means defining a liquid inlet, and a liquid outlet at one end of said housing,

a first cylindrical sleeve member formed of low friction insulating material surrounding and coaxial with said tubular dielectric member and defining with said thin resistive film an inner annular cylindrical liquid flow passage communicating with said liquid inlet,

a second cylindrical sleeve member formed of low friction insulating material surrounding and coaxial with said first cylindrical sleeve member and defining with the outer surface of said first cylindrical sleeve member an outer annular liquid fiow passage communicating with said liquid outlet, said second cylindrical sleeve member defining with said housing an exponential horn chamber adapted to contain an air medium for minimizing the re flected energy from said termination,

means defining radial ports connecting said inner liquid flow passage to said outer liquid flow passage at the ends thereof opposite said inlet and outlet,

whereby liquid flowing in said inner flow passage ab sorbs heat energy from said resistive film,

means defining a liquid chamber within said tubular dielectric member and communicating with said liquid inlet to provide a liquid dielectric medium within said tubular dielectric member, and

means for pumping cooling liquid through said passages.

2. A device as defined in claim 1 wherein said first and second sleeve members are formed of polytetrafluoro ethylene.

3. A device as defined in claim 1 wherein said tubular dielectric member defines a radial bleed port at the end thereof opposite said inlet and outlet whereby liquid coolant in said liquid chamber bleeds out into said inner flow passage and is continously replenished.

Claims (3)

1. A liquid cooled dummy load device for an RF coaxial transmission line, the device having a connector with first and second contact means for the inner and outer coaxial conductors respectively, and a cylindrical housing; an elongated tubular dielectric member coaxially mounted in said housing, a thin resistive film deposited on the outer surface of said dielectric member and series connected between said first and second contact means, said film being adapted to convert electrical energy being transmitted by said transmission line to heat energy, means defining a liquid inlet, and a liquid outlet at one end of said housing, a first cylindrical sleeve member formed of low friction insulating material surrounding and coaxial with said tubular dielectric member and defining with said thin resistive film an inner annular cylindrical liquid flow passage communicating with said liquid inlet, a second cylindrical sleeve member formed of low friction insulating material surrounding and coaxial with said first cylindrical sleeve member and defining with the outer surface of said first cylindrical sleeve member an outer annular liquid flow passage communicating with said liquid outlet, said second cylindrical sleeve member defining with said housing an exponentIal horn chamber adapted to contain an air medium for minimizing the reflected energy from said termination, means defining radial ports connecting said inner liquid flow passage to said outer liquid flow passage at the ends thereof opposite said inlet and outlet, whereby liquid flowing in said inner flow passage absorbs heat energy from said resistive film, means defining a liquid chamber within said tubular dielectric member and communicating with said liquid inlet to provide a liquid dielectric medium within said tubular dielectric member, and means for pumping cooling liquid through said passages.

2. A device as defined in claim 1 wherein said first and second sleeve members are formed of polytetrafluoro ethylene.

3. A device as defined in claim 1 wherein said tubular dielectric member defines a radial bleed port at the end thereof opposite said inlet and outlet whereby liquid coolant in said liquid chamber bleeds out into said inner flow passage and is continously replenished.

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