US2775760A - Micro wave antenna feed - Google Patents

Description

MICRO WAVE ANTENNA FEED Filed July 28, 1952 2 Sheets-Sheet l INVENTORS. Lowe-1.1. 1?. BROWN Wuvnm KELLY ATTORMEV Dec. 25, 1956 R. BROWN ET AL 2,775,760

MICRO WAVE ANTENNA FEED Filed July 28, '1952 E J UHHH Hl wln 4 i Z\ \NNN. IANNNI WI INVENTORS. Lowe-u 1? BROWN By Wmro/vJ Kan United States Patent ce MICRO WAVE ANTENNA FEED Lowell R. Brown, Royal Oak, andWintonJ. Kelly, Birmingham,.Mich., assip ors to Davis Tool & En -gineering Company,..Detroit,.Mich., a corporation of Michigan Application JulysZS; 1952; Serial No. 301,274

4 Claims. (Cl. 343-781) The present invention relates .to a. wave-guide and more particularly to a micro-wave radio frequency hollow gas filled wave-guide which, in one of its embodiments, may be'utilized as amicro-wave antenna feed. Other applications of the invention will be apparent to those skilled in the art.

In the formation of such micro-wave radio frequency wave-guides, it has been common practice prior to the present invention, to provide-a hollow guide-body which is filled'with a gas under pressure. The present invention in one of its aspectsrelates to improvements in the formation of the hollow guide body and in another of its aspects relates to the provision of a gas-tight barrier or plug in an open end of said body=without introducing electrical anomalies in the system.

Prior to the present invention, it was a common practice to form such wave-guides as hollow bodies which were: generally rectilinear in cross section. In many such instances, it'was-not possible to readily form orma- :chine any tapered portions thereof without creating me chanical irregularities which created conditions'which allowed electrical anomalies to be developed in the system. As distinguished from such prior wave-guides, thepresent invention provides a tubular hollow guide body which is generally circular in cross section and which-may be readily drawn or formed to provide any desired radial reduction therein to. form a tapered portion without causing structural irregularities which are conducive to the development of electrical anomalies in the system.

Sealing the hollow body portions of previously known wave-guides also has been a serious problem. Gas-tight barriers or plugs previously used frequently required mechanical connections which caused electrical anomalies to develop in the system. In other instances, a barrier or plug having properties which made it satisfactory for sealing pressurized gas in the wave-guide body didnot possess the required property of being transparent to the radio frequency waves to be transmitted through the wave-guide. Difliculties also were experienced in the transition areas where the path of the micro-waves through the gaseous medium was changed to a path leading through the gas barriermaterials. As distinguished from these prior attempts. to solve these problems, the present invention provides a gas barrier or sealing element which isfitted into the wave-guide body without introducing mechanically caused electrical anomalies in the system, the gas barrier or sealing element possessing the desired mechanical andv electrical properties to permit postive sealing ofthe gases in thewave-guide body while being satisfactorily transparent to the radio-frequency waves.

The present invention, therefore, resides in the, selection, formation and processing of the respective elements of the micro-wave radio frequency hollow wave-guide so that it possesses both' the required electrical and mechanical properties, without creating conditions conducive to the development'of electrical anomalies in the system. i

2,775,760 Patented Dec. 25, 1956 The principal objects of the present inventioninclude thefollowing:

(1) The provision. of a-hcllow-radio frequency microwave guide having a body conformationz-and crosssectional contour which permits .its ready manufacture within close high precision limits on an efiicient mass production basis without creating a mechanicalenvironment conducive to the development of electrical anomalies in the system.

(2) The provision in a micro-wave guide of a-gastight mechanical. barrier or plug-which is transparent to the radio frequency waves thereinand which may,be-se.- cured-in the wave-guide body and thereafter machined or otherwise formed as determined by the electrical properties desired in the unit.

(3) To provide a micro-waveguide in which the respectiveelements are designed and coordinated to achieve the combination of optimum mechanical and production advantages with minimum interference with the electrical properties thereof.

(4) To provide a micro-wave guide or duct system in which the passage of high frequency radio energy through the guide and the gas barrier or plug is permitted without excessive energy loss or reflection ofthe micro-wave energy.

(5) To provide a micro-wave guide or duct system in which a gas pressure plug or barrier which can support a means for tuning certain elements associated with them:- tenna system of a micro-wave transmitting unit and which permits tuning the system to maintain the standing wave ratio in the micro-wave system within desired limits.

(6) The provision of 'agas-tight barrier or plug for a micro-wave guide or duct system. which ,has suflicient mechanical properties to permit the direct mounting thereon of tuning elements and reflector elements, thus eliminating the need for metallic supporting structures which introduce electrical anomalies in the microwave guide or duct system:

(7) The provision: of a hollow gas-pressurized microwave guide or duct which has suitable mechanical and electrical properties to permit it to be employed as a micro-Wave antenna feed.

Other'objects of this invention will appear in the following: description and appended claims,- reference being had to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.

Referring to the drawings:

Fig. l is an exterior elevationviewof the present invention as embodied in'a micro-wave antenna feed head.

Fig. 2 is a longitudinal cross-sectional view taken centrally' of Fig. l substantially on the section line 2-2 in lh61dl16Ctl0I1 of the arrows and showing the gas barrier or plugin position;

Fig. 3 is an exploded view of the micro-wave antenna feed head in the stage of construction immediately prior to assembly of the units.

Fig. 4 is a cross-sectional view of one preferred form of press machine element useful in: the assembly of the gas plug or barrier in the microwave antenna feed head.

Fig. 5 is a cross-sectional view of Fig: 4-inthe direction of the arrows and showing. a methodof supporting the micro-wave antenna element in position while the gas barrier orplug is inserted.

Fig. 6 is a schematic drawing of one preferred form of heat treating enclosure used for heat treating the assembled unit during its construction.

Fig. 7 is a schematic cross-sectional view of a known type of gas pressure barrier or plug in which electrical anomalies are introduced in the systemand ishere included to-assist-invisualizing the differences between 3 such a barrier or plug and the barrier or plug of the present invention.

Figs. 8, 9 and 10 are fragmentary views, partially in section and showing modified forms of the head of the antenna feed embodying the present invention.

Before explaining the present invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

Referring to Fig. 1, the present invention is shown as applied to a micro-wave antenna feed head as one preferred embodiment. The feed head comprises a hollow tubular body element 1 constructed of any suitable materials, such as a metallic tube having high electrical conductivity. At 2 is shown a base supporting structure integrally constructed with tubular element 1 and used to support the antenna feed head and micro-wave waveguide system with respect to the co-working portions of the micro-wave generator (not shown) and the waveguide system (also not shown).

It is to be understood that uses other than a microwave antenna feed head are contemplated. Since the length of the hollow tubular body element 1 may be any practical desired length, a series of such elements may be utilized to form a duct system.

At 3 is shown the exterior surface of the gas barrier and antenna element to be more fully described hereinafter. At 4 is shown a conducting metallic plate having an overlying circumferential flange 5 as shown in Fig. 2 which is secured to the dielectric antenna head element.

In Fig. 2, the unit is shown in cross section. As there shown, a tuning screw 6 is provided which pierces the reflecting plate 4 and engages in threads cut in the gas barrier or plug of the antenna structure at 7. The gas barrier plug is formed of two initially separate elements indicated at 8 and 9 in Fig. 2. The parts are joined along the boundary indicated at 10 when assembled.

The materials used in forming the element 8 and the element 9 are described in this application as' being formed of the same dielectric materials but it is not essential that the dielectric material of 8 be the same as the dielectric material of 9.

At Fig. 3 is shown a plug 11 of a dielectric plastic For purposes of clarity in describing the present inven- I tion, the gas barrier or plug elements in combination with the other elements of the antenna feed head will be described here in two sections as follows:

(1) Functional operation for use as a device providing a gas barrier or plug which is at the same time transparent to micro-wave radio energy, and

(2) The method of constructing the gas barrier or plug to accommodate the requirements of the functional use of the barrier in a micro-wave radio frequency hollow wave-guide system.

Referring to Fig. 2, micro-wave radio energy may be considered as progressing longitudinally through the interior of the tubular element 1 in the direction indicated generally by the arrow 13. The energy may be considered as progressing down the tube in waves having a transverse mode of vibration and will proceed down the tube 1 until it reaches a discontinuity offered to its flow by any internal change in the geometry of the tube. (The term geometry as herein used refers to electrical geometry and does not necessarily include the concept of physical dimensional geometry.) As the wave energy approaches the portion of the tube 14, all of the energy would not flow uniformly down the tube because of a decreasing radius of the tube beginning at this point. The radius decrease for the tube shown in Fig. 2 extends over the tube length from the point indicated by the numeral 14 to a point indicated by the numeral 15. During the traversal by the radio frequency energy, of this tapered section of the tube 1, a portion of the radio energy flowing down the tube in the fashion common to hollow wave-guides will be reflected back toward its source by virtue of the change in electrical geometry. Thus the efficiency of the transmission of energy throughv the tube 1, lacking the elements 8 and 9, would be less than desirable for the reason that energy approaching the exit end of the tube shown at 16 would be reflected back toward the source and would not then radiate through the antenna head 3. The phrase constant electrical geometry best describes the conditions necessary for the radio energy to progress down the tube without suffering an electrical anomaly which will in turn cause a reflection of energy.

Thus if the tube 1 is to be reduced in diameter a mechanism must be introduced in the tapered portion of the tube between the points 14 and 15, which would reestablish the desired electrical geometry of the tube 1 so no reflection of energy would take place at that portion of the tube and the energy could progress forward into the reduced end portion of the tube without suffering energy loss due to reflection. A means for accomplishing this is to change the electrical space constants of the unfilled portion of the tube 1 between the points 14 and 15 so that the velocity of propagation of energy transverse to the tube 1 in the area indicated generally by the numeral 17 is altered in proportion to the radial dimensional alteration occurring between the points 14 and 15 as shown in Fig. 2.

In essence, it is required that the time required for the transverse vibration of energy to effect one complete cycle across the diameter of the tube at 17 must correspond with the time of transverse propagation across the tube in the region of reduced radial dimension between the points 14 and 15. To accomplish this in the present invention, a material is required that reduces the velocity of propagation to the desired extent. In operation, then, as the wave energy approaches the point 14 and passes it, it is required that the time of propagation across the hollow wave-guide be held constant despite the fact that the transverse wave has a physically shorter distance to travel. This is accomplished by incorporating into the wave-guide section between the points 14 and 15 a portion of the total dielectric gas barrier or plug which is capable of decreasing the velocity of the energy wave as required to effect a constant time of transverse vibration as the wave progresses forward. It will be noted in Fig. 2 that this is accomplished by providing at the point 14 a feather edge of suitable material and increasing the thickness of the edge in proportion to the forward travel of the wave until the entire tube is filled adjacent the point 15. Beyond the point 15 as for example in the region 18, the entire tube is filled with the said dielectric material so that the time required for the transverse wave to effect one full cycle from point 19 to point 20 is for all practical considerations identical with the time required for one full transverse vibration from the point 21 to the point 22. Under these conditions such a wave-guide will provide a constant electrical geometry and no reflection of electrical energy in the transition region between the points 14 and 15 will occur.

With the wave proceeding forward, i. e., from left to right as shown in Fig. 2, in the region 18 it is approaching the end of the wave-guide system and is fed into the head section of the antenna. The radio energy leaves the wave-guide at the end o the tube shown at 16 and versed in itsdirection of travel.

wouldjnormally proceed in a forwarddirection and propagate into free space. Under the circumstance of the presentembodiment of theinvention it is desiredthat the radiant" energy be reversed in the direction of its travel to the directions generallyindicated by the arrows 23 and 24. his apparent that the emission of the radio energy from the solidmaterial into free space which occurs at the faces 25 and 26 would introduce an. electrical anomaly into the circuit and cause the reflection of energy back down the tubular system. To prevent this action, a metallic'tuning element-is inserted into the antenna head at 6 and byadjusting the amount of the tuningelernent that is inserted into the head to an optimum value determined experimentally; ithasbeen found that the energy reflected back intothesystem may be reduced to an acceptable minimum.

To effect the reflection of the energy which has heretoforeproceeded' axially down thetubular element 1 and beenernitted'inthe directions indicatedby the arrows 23 and 24 it isnecessary to incorporate into the antenna head proper a-plate 4-formed'of-asuitable "reflecting material. The reflecting-material comprises a high conductivity metal against which the radio energy impinges and by reflection mechanismscommon to the art is re- Since the wave shape outside of the tube end 16 is degenerating from a flat planeto a spherical configuration, thedirection of travel is not directly re-entrant into the tubular wave guide 1 but ism-keeping with thenormal laws of incidence and reflection which occur for any reflection occuring at a plane surface.

Theselection of materials for use as the gas barrier or plug in applicationssuch ashas been described is complicated by the requirements that must be met from both the electrical and physical standpoint. Since the velocity propagation constant dicates the extent of physical dimensional change-that thedesigner may incorporate in the wave-guide design, the-matter of the velocity of propagation of radio energy'through the material is of prime consideration; From the standpoint of the velocity of propagation, theselection of materials is limited to a relatively fewdielectric materials such as certain of the silicones or polymers of gaseous tetra-fluoroethylene known commercially as Teflon.

A second prime consideration is the matter of energy loss as the high frequency radio-energy traverses the plastic material. Usually, although not necessarily, the requirement for a proper material in this regard is that it possess an extremely low ohmic conductance and also that its power factor at very high frequencies be low. Usualdesirable powerfactor values are 0.0005 or less.

In addition'the material-must be incapable of absorbing water-since experience has shown that the presence of water-either in liquid ordissolved form in the dielectric material will produce prohibitively high power factors at these frequencies.

Taking-into consideration the foregoing limitations on theselection of a proper materialthe so called Silastic silconeplastics .and the polymers of gaseous tetra-fluoroethylene known as Teflon plastics satisfy all of the requirements with twoexceptions. 111 the case of the Silastics, it is difficult to obtain a homogeneous material that is easily machined'andwhich also possesses high physical strength. With regard to the Teflon materials, all of the'requ-irements are satisfied with the one exception that Teflon has thus far notbeen made so that-it can bemade to adhere to a metallic surface. Thus its use in all phases of the problem, both electrical and mechanical, are satisfied'with the exception that no adhering connectioncanb'e made between the Teflon and the inner wall of the wave-guide as shown in Fig. 2.

The necessity for gas barriers of the type that have beendescribed from an electrical standpoint as heretofore describedparises fromithe usual practice in the art of micro-wave radio wave-guide construction ofproviding a positive gas pressure in the hollow wave-guide syS- tem in order to exclude the entry of moist air or other corrosive atmospheres'which are capable of corroding the inner surface of the'wave-guide, and thereby reducing its conductivity to electrical energy. Usually, the gas is nitrogen under a pressure of the order of 20 pounds per square inch absolute, which, at sea level, provides approximately a five poundper square inch pressuredifference between the interior and exterior of'the wave-guide system. At extreme altitudes, as when the system is used in aircraft, a theoretical maximum of 20 pounds per square inch difference in pressure exists. This gas pressure thus provides a positive means of excluding unwanted vapors from the wave-guide system. In order to maintain such gas pressures, it is necessary-to provide gas barriers or plugs in the wave-guide system which are capable of retaining the gas pressure and which at the same time are relatively transparent to the passage of the radio frequency energy.

The method of providing these gas plugs heretofore has been considerably complicated by the necessity of changing the electrical characteristics of the wave-guide to accommodate the presence of the gas plug.

For example, as shown in Fig. 7, a prior device proposed the use of aflanged coupling as here shown to connect two adjoining wave-guides. The flanges 27 are held compressibly together by clamping screws 28 and form a collar-like cavity in the wave-guide proper. Within the collar-like cavity within the wave-guide proper may be placed a barrier 29 which acts as a stop for gas at 30 under pressure and thereby prevents the loss of gas pressure into the section of the wave-guide shown at 31. it can be seen that such a gas barrier provides an electrical anomaly which causes serious reflection of the micro-wave radio energy at the joint. Another disadvantage of a gas barrier of the type shown in Fig. 7 is the difiiculty encountered with high voltage. gradients occurring at the face of the gas barrier at 32. Since the voltage relationships present in wave-guides of this nature are transverse to the axis of the tube any dielectric boundary occurring in the wave-guide has voltages induced across it commensurate with the amount of power being transmitted down the wave guideand the dimensions of the wave-guide. Consequently, arcing will occur if the voltage gradient is suflicient to ionize thegas Within the wave-guide and if the dimensions of the wave-guide dielectric boundary are such to induce high voltage gradients. it can be seen, referring to Fig. 2, that the dielectric boundary occurring at 33 is greatly elongated over that of a common type gas barrier shown in Fig. 7 and the voltage gradient transverse the dielectric boundary is therefore considerably less.

Referring to Fig. 2, it will be noticed that a small projection of the antenna head projects backward along the outside of the wave-guide as shown at 34. This projection provides a material of high dielectric strength to reduce the voltage gradient present at the exit of the wave-guide at 16. The small backward projecting collar 34 also efiectively insulates the end of the tube against voltage breakdown or ionization of such gases that exist exterior to the wave-guide.

In our experience, ,we have found that the most desirable plastic material to use for the gas barrier or plug is a polymer of gaseous tetra-fluoroethylene which is commercially known as Teflon, which term will be used will flow into the confines of a mold but will tend to of the mold is removed. We have observed that the rate at which the material returns to its undistorted shape and the magnitude of the restoring forces are a function of temperature. In the present application this characteristic of Teflon has been used to adapt it to the particular requirements of the problem of micro-wave waveguide systems.

Referring to Fig. 3, at l is shown the metallic tubular wave-guide prior to fixing to it the mounting bracket 2. At 11 is shown a cylindrical slug of Teflon in position to be forced into the hollow interior of the wave-guide 1. At 12 is shown in cross section the unmachined shape of the antenna head into which the end 16 of the wave-guide section 1 is forced into the corresponding cavity 35.

The assembly of the Teflon elements and the tube 1 is accomplished preferably as shown in Fig. 4. As here shown, an assembly fixture is provided which consists of a base 60 which extends upwardly into the sleeve 61 which is provided with an internal circumferential shoulder 62 which engages the top edge 63 of the base 63 The base 60 is provided with a bottom chamber 64 to receive the unmac'nined Teflon antenna head 12. Directly above the bottom chamber 64, the internal portion of the base 60 is provided with a supporting die 65 which is internally machined to provide an external support for the end of the tube 1. The lower boundary of the support die 65 is shown at 65a. The internal opening of the supporting die 65 aligns with the cylindrical opening 66 provided in the top portion of the base 60. The opening 66 is formed to provide an external support for the walls of the tube 1. The punch 37 is guided through the sleeve 61 and reciprocates through the punch guide opening 67 to enter and be withdrawn from the open end of the tube 1.

The Teflon cylindrical plug 11 is here shown in cross section as it enters the wave-guide element 1 and as it is being forced into the restricted or reduced diameter section of the wave-guide at 36. The Teflon cylindrical plug 11 is forced under great pressure into the reduced diameter section of the wave-guide at 36 by the punch ram shown at 37. Under the compulsion of the punch ram 37 the Teflon cylindrical slug 11 is forced to flow through the reduced diameter portion of the waveguide at 36, the cylindrical reduced diameter section of the wave-guide shown at 38, and into the cavity 39 of the previously prepared somewhat cylindrical section of the unmachined antenna head 12. it will be noted that the angle of the hole drilled previously through antenna element 12 is not truly cylindrical but is shaped as a truncated cone as shown at 40. We have found that the desirabiity of perforating antenna element 12 with a hole whose diameter was not constant increased the retentive forces tending to hold the two pieces of Teflon 11 and 12 together after they had been forced into close physical contact under the actuating pressure of the punch ram 37.

As the punch ram 37 forces the cylindrical Teflon plug 11 into the reduced portion of the wave-guide shown at 36 and 38 and out of the end of the wave-guide at 16 into the hollow portion 39 of the unmachined antenna head 12, it distorts the antenna head 12 so that the boundary 41 rises slightly as shown by the dotted line 42 and also distorts the shape of the truncated cone 40 into a shape approximating those shown by the dotted lines 42 and 43. When the Teflon has been thus forced into the wave-guide as described in the previous paragraphs, and distorted necessarily in order to be forced through the wave-guide and a portion of the slug extending into the hollow portion 39, the top portion of the Teflon slug is at approximately the location shown by the dotted line 44. The punch ram may then be retracted and the wave-guide with the Teflon within it may be removed from the press.

Referring to Fig. 5, the Teflon material is forced into the wave-guide as shown at 11. The wave-guide at 1 in Fig. is surrounded by the die section 45. The die section is constructed in such a fashion that it may be opened by removing the die bolts 46 and 47 after the encircling case 61 is slipped from around the die 45.

Because of the persistence of memory in the Teflon there is a constant tendency for the slugs to return to the initial forms they possessed before the pressure was applied thereto. Therefore, the assembled but unmachined wave-guide and antenna element is then placed in a heating chamber 49 as shown schematically in Fig. 6. The chamber is equipped with heating elements 50 and 51 with a temperature sensitive element 52 for controlling the energy available to heat the chamber. We have found that a curing temperature of approximately 225 F. is satisfactory to erase the memory of the materials and hold them in their assembled positions. The temperature is held at this value for approximately one hour, at the end of which time the wave-guide element is removed and allowed to return to room temperature.

When the assembled wave-guide has returned to room temperature, it may be machined into any desired shape. Referring to Fig. 2, the specific embodiment described in this disclosure shows that the Teflon insert in the end of the wave-guide has been provided by any suitable means such as a gun drill, with a conical cavity as shown at 33 to facilitate the transmission of energy from the entrant to the exit portion of the wave-guide section described, and the antenna head has been shown machined in a somewhat hemispherical shape as shown at 25. In addition the machining of the antenna head described in this disclosure provides a threaded hole at 6 into which a threaded tuning slug may be inserted and held in position by the excellent machining abilities of Teflon. The reflector disc 4 is held in position by the flange 5 rolled over the edge of the hemisphere and by the tuning slug 6. It may, however, be accomplished in any other suitable manner.

Three modified forms of antenna heads embodying the present invention are shown in Figs. 8, 9 and 10, respectively. They differ from the head previously described in the particular mounting of the reflector cap and the tuning arrangements provided therefor. In Fig. 8, the end surface of the head is machined as indicated by the numeral 70 to provide a shallow recess into which a metallic spacer 71 is mounted. The reflector cap 72 is seated against the end surface of the head and in intimate contact with the metallic spacer 71. A circumferential flange 73 is crimped over the circumferential edge portion of the head and holds the assembly in position.

In place of machining the head as in Fig. 8, the head shown in Fig. 10 is similar to that shown and described in Figs. 1 and 2 except the reflector cap 4 is spaced from the end of the antenna head by a metallic spacer 75 whose thickness is selected to provide the desired spacing of the reflector cap 4 relative to the end of the antenna head.

The antenna heads of Figs. 8 and 10 are designed primarily to provide a wide band preturned antenna. The head as shown in-Figs. 1 and 2 is primarily designed where a narrow band variable tuned antenna is desired. Thus the present invention will provide a reflector cap mounting with either a fixed or variable tuning as may be desired.

In the modification shown in Fig. 9, the antenna head portion is provided with a thin metallic skin or layer 76 which acts as the reflector cap and is adhered to the end surface of the antenna head. When using Teflon as the dielectric material, the metallic skin or layer 76 is applied thereto by causing the vapors of boiling metal to condense thereon the process being effected at reduced atmospheric pressures common to the art. This provides a means for effecting a tightly adhering metal skin or layer without mechanical attachments such as the circumferental flanges previously described.

It is to be understood that if desired. the head may have a reflector cap such as shown in Figs. 8 and 10 secured directly to the metallic skin or layer 76 as here shown.

Experience gained in the operation of micro-wave antenna heads and micro-Wave gas barriers constructed in this fashion revealed that they satisfactorily and completely sealed the inner gas pressure against escape at the end of the micro-wave wave-guide and also by their physical characteristics serve in every other desirable way. for the transmission of radio frequency energy at these frequencies.

The process as described effects a high unit area pressure between the Teflon and metallic wave-guide thus effectively providing a seal not requiring the adherence of the Teflon to the inner portion of the wave-guide but obtaining a seal by simple pressure action. We have determined by experiment that if an extraordinarily high pressure is to be maintained in the hollow portion of the wave-guide system the inner surface of the wave-guide may be slightly roughened to provide additional shearing forces necessary to force the Teflon plug out of its position. Another method of increasing the retentive force involves plating of a suitable metal on the external periphery of the plug by evaporative means. Soldering or similar processes may then be used to effect a firm fastening of the device in place. Under normal operating conditions, however, these precautions are not necessary. Additionally, the conical shape of the Teflon slug at 33 in Fig. 2 transmits, by virtue of its shape, the additional pressure of the gas pressure within the wave-guide element in a radial direction thus increasing the unit pressure of the Teflon against the inside surface of the wave-guide element. This further decreases the chance of gas seepage in the boundary between the Teflon" and the metallic wave-guide.

From the foregoing, it is apparent that we have been successful in overcoming the mechanical limitations of Teflon by a unique method of construction and processing, thereby making available for the art the excellent electrical and chemical characteristics of Teflon for use in the construction of gas barriers, antenna and tuning elements and similar appurtenances for radio micro-wave hollow wave-guides.

Having thus described our invention, we claim:

1. In the process of forming a gas barrier in a hollow micro-wave antenna feed, the novel steps of deforming a fused homogeneous blank of polymerized gaseous tetrafluoroethylene under pressures sufficient to effect its flow into the restricted end of a confining body portion of said feed and thereafter curing it at a temperature of approximately 225 F. for approximately one hour to induce the stable retention of said blank in its constrained shape.

2. In a micro-wave guide having a hollow electrically conductive body, a gas-tight dielectric barrier disposed in the end of said hollow body and projecting therefrom and a dielectric antenna head disposed on the projecting end of said barrier, the improvement consisting of the provision of a portion on said antenna head projecting backwardly along the outside of said hollow electrically conductive body in the region of the high voltage gradient existent at the metallic terminus of the hollow body.

3. In a micro-wave guide having a hollow, electrically conductive body and a dielectric antenna head aflixed on one end of said body, the improvement consisting of the provision of a portion on said antenna head projecting backwardly along the outside of said hollow electrically conductive body in the region of the high voltage gradient existent at the metallic terminus of the hollow body.

4. In the process of forming a gas barrier and an antenna head on a hollow micro-wave antenna feed wherein said gas barrier is formed from one blank of dielectric material and said antenna head is formed from a separate blank of dielectric material, the novel steps of supporting a fused homogeneous antenna head blank of polymerized gaseous tetrafluoroethylene adjacent one end of said hollow antenna feed, deforming a fused, homogeneous, gas barrier blank of polymerized gaseous tetra-fluoro-ethylene under pressure suflicient to effect its flow into said hollow Wave guide from the opposite end thereof and into mechanical locking engagement with said antenna head blank, curing said blanks at a temperature of approximately 225 F. for approximately one hour to induce the stable retention of said blanks in their constrained shape, and thereafter machining said gas barrier blank and said antenna head blank as desired.

References Cited in the file of this patent UNITED STATES PATENTS 2,400,099 Brubaker et al. May 14, 1946 2,429,640 Mieher et al Oct. 28, 1947 2,449,570 Violette Sept. 21, 1948 2,496,643 Smith Feb. 7, 1950 2,509,196 Cork et al May 23, 1950 2,577,158 Rosencrans Dec. 4, 1951 2,617,151 Rubin Nov. 11, 1952 2,698,901 Wilkes Jan. 4, 1955 2,702,858 Foster Feb. 22, 1955 FOREIGN PATENTS 623,756 Great Britain May 23, 1949

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