US2761059A

US2761059A - Wave polarization discriminating duplexing system

Info

Publication number: US2761059A
Application number: US243134A
Authority: US
Inventors: Edward M Purcell; Dorothy D Montgomery
Original assignee: Individual
Current assignee: Individual
Priority date: 1943-10-02
Filing date: 1951-08-22
Publication date: 1956-08-28
Anticipated expiration: 1973-08-28

Description

g- 28, 1956 E. M. PURCELL AL WAVE POLARIZATION DISCRIMINATING DUPLEXING SYSTEM 2 Sheets-Sheet 1 Original Filed Oct. 2, 1943 FIG.4

FBG.5

' RECEIVER IN EN T OBS EDWARD M. PURGELL DOROTHY D, MONTGOMERY TGOMER Inventor dececspd dministrutnx CAROL G. MON Y,

NTG MERY,A

DOROTHY D. MO

Aug. 28, 1956 E. M. PURCELL ETAL 2,761,059

WAVE POLARIZATION DISCRIMINATING DUPLEXING SYSTEM Original Filed Oct. 2, 1945 2 Sheets-Sheet 2 RECEIVER IN YEN TORS EDWARD M. PURGELL DOROTHY D. MONTGOMERY CAROL G. MONTGOMERY, Inventor deceus'ed DOROTHY D. MONTGOMERY Adminisirctrix T ORNEY Ofi ice 2,761,059 Patented Aug. 28', 1956 WAVE PULARIZATION DISC" DUPLEXING SYSTEM Original application October 2, 1943, Serial No. 504,776,

new Patent 2,607,849, dated August 19, 1952. Divided and this application August 22, 1951, Serial No. 243,134

till- ATING 20 Claims. (U. 250-l3) This invention relates to systems for transmitting and receiving high-frequency radio waves and in general to systems for conveying high-frequency oscillatory electric energy. In particular the invention relates to the control of the polarization of guided radio waves in apparatus for conveying such waves and to systems making special use of the control of the polarization of such waves for various useful purposes more particularly described bellow.

This application is a division of copending application Serial No. 504,776, filed October 2, 1943, and entitled Control of Polarization in Wave Guides and Wave Guide Systems, now Patent No. 2,607,849 granted Aug. 19, 1952.

In the past some difiiculty has been encountered in controlling the polarization of guided waves in high-frequency radio apparatus, particularly in cylindrical wave guides which because of their axial symmetry permit the plane of polarization of the wave guided therein to change during transmission. In such wave guides, bends in the wave guide systems which do not lie exactly in the plane of the electric vector or in a plane exactly perpendicular thereto tend to cause the polarization of the wave transmitted to become elliptic because of the different path lengths for different plane-polarized components of the wave.

Means have been found for correcting ellipticity of polarization caused by bends in the wave guide system and for restoring plane polarization and has further been found that such means may be'adapted to produce from a plane-polarized wave any desired degree of ellipticity of polarization and, in particular, circular polarization. It has further been discovered that by organizing a system for sending out pulses of circularly polarized radiation and then receiving such pulses, it is possible to discriminate against the, transmitted signals and in favor of the received signals at a suitably located junction of the apparatus, thus providing a new form of radio transmitting and receiving system employing a common radiator interceptor or antenna. It has further been found that it is possible to combine polarization-controlling means in accordance with the present invention to provide phase shifters, rotating joints and other useful apparatus.

Among the objects of the present invention are the provision of means for controlling the polarization of waves in guided wave systems and for the organization of such polarization-controlling means for (l) effecting discrimination between a transmitted signal and a received signal in a radio transmitting and receiving system, (2) providing an adjustable phase shifter for plane or otherwise polarized waves (3) providing a transition between plane and circularly polarized waves, and for other purposes.

The invention is illustrated in the annexed drawings, in which:

Fig. 1 is a perspective view, partly diagrammatic, showing means for providing a transition between plane and elliptically or circularly polarized waves in a cylindrical wave guide;

Fig. 2 shows, partly in cross section, another arrangement for producing interchange of energy between a planepolarized wave and an elliptically or circularly polarized wave in a cylindrical wave guide;

Figs. 3 and 4 show, in cross section and in side elevation respectively, an arrangement similar to that of Fig. 1 which includes in addition a fine adjustment;

Fig. 5 is a plan, partly diagrammatic, of a system for radiating and receiving circularly polarized radiation as aforesaid;

Fig. 6 is a plan view, also partly diagrammatic, showing another form of system of the same general type as that shown 11 Fig. 5;

Fig. 7 is a cross section along the line 77 of Fig. 6, looking downwards; and

Fig. 8 shows in cross section a form of antenna or radiator-interceptor for sending and receiving circularly polarized radiation and adapted for use with apparatus of the type shown in Figs. 5 and 6.

There is shown in Fig. l a portion of a cyclindrical wave guide form of a hollow cylindrical metallic pipe 2. Located within the pipe 2 is a slab or plate 3 of a solid dielectric material, preferably polystyrene. If it be now assumed that the incident wave transmitted in the wave guide 2 is so polarized that its electric vector corresponds with the direction of the arrow a, which is at an oblique angle with the plane parallel to the faces of the plate 3, it will be seen that the component waves having electric vectors oriented as indicated by the arrows b and c respectively, which are respectively in a plane parallel to the faces of the plate 3 and perpendicular to such plane, will be propagated with unequal velocity in that portion of the wave guide occupied by the plate 3. The difference in velocity arises from the fact that the plate 3 has relatively little effect upon an electric field directed perpendicularly to the surfaces of the plate whereas it has a relatively large eliect upon an alternating electric field in which the electric vector lies in a plane parallel to the surfaces of the plate 3. It should be pointed out here that the waves in the guide 2 are of the H1 mode, sometimes referred to as the TEM mode. Waves in this mode may be transmitted with plane, elliptical or circular polarization. The effect of the plate 3 may be further understood when it is considered that with respect to the component of the incident waves polarized in the direction 0, the plate lies entirely within the region of maximum electric field and therefore exerts a relatively large shortening effect upon the wave length of the said component in the wave guide, whereas with respect to the component oriented in the direction b, only a small portion of the plate 3 lies in the region of the maximum electric field strength, so that only a relatively small shortening efiect upon the wave length of the said component in the wave guide results.

As a result of the axial asymmetry of the plate 3 the components ,b and c of the incident radiation polarized in the direction a (referring in the direction of the electric vector as the direction of polarization, as is common in the electrical art, as is distinguished from the optical art, in which the direction of the magnetic vector is referred to as defining the plane of polarization) progressively fall out of phase as the wave is propagated along that portion of the wave guide occupied by the plate 3. The relative magnitude of the component in the direction of the plate 3 and perpendicular thereto is determined by the angle between the plate 3 and the plane of polarization of the incident wave, whereas the 7 relative phase shift between the components produced by the plate 3 is determined by the axial length of the plate 3 (assuming that the other dimensions are uniform) If the length of the plate 3 is so chosen that a 90 phase shift is produced, the incident wave will be entirely transformed into an elliptically polarized wave, which in the case that the angle between the polarization of the incident wave and the direction of the plate 3 is 45 will be a circularly polarized wave. The foregoing statement takes no account of losses in the dielectric 3, the assumption that these losses are negligible being safe in the case of polystyrene and other low loss dielectrics. In order to produce circular polarization with a dielectric such that some account must be taken of losses, the desired angle of the plate 3 to the plane of polarization of the incident wave may be adjusted to compensate for unequal attenuation of the components b and c so that after passing beyond the plate 3 these two components may have essentially equal amplitudes.

The discontinuity in the characteristics of the wave guide 2 produced at the transverse edges of the plate 3 would, if the plate 3 were provided in a simple rectangular shape, tend to cause reflection in the wave guide 2 thus setting up standing waves and reducing the amount of energy transmitted past the plate 3. In order to reduce these reflections and to improve the efl'iciency of power transfer, the transverse edges of the plate 3 are provided with notches 4 so that the transition between the empty guide 2 and the guide completely spanned by the plate 3 may be made in two steps of substantially equal relative magnitude separated approximately by a quarter-wave length so that the reflections from each of these steps may be of equal magnitude but opposite in phase. The calculation of the proper width and depth of the notches 4 is complicated by the fact that the quarter-wave length diifers for the components polarized respectively in the directions b and 0, but the difference is small enough so that a good approximation may be obtained without exact calculations, and the first approximation of the dimensions of the notch 4 may thereafter, if desired, be readily provided with minor corrections by experiment. We have found the following values for the dimensions of the plate 3 suitable for wave lengths of several centimeters or thereabouts when the plate 3 is made of polystyrene. The dimensions are given in terms which represent the free-space wave-length, which of course will be different from the wave length in the guide 2.

Table 1 Inner diameter of cylindrical wave guide 0.741 Thickness of plate 3 0.10% Depth of notches 4 0.28%

Width of notches 4 0.22).

For a plate 3 adapted to produce a circularly polarized,

wave in the guide 2 when originally excited with a plane polarized wave of 45 incidence, as above set forth, the axial length (including the length of the notches 4) should, for the other dimensions given in Table I, be equal to 1.51%.

The plate 3 may be made to produce a 180 phase difference between the component having the electric vector b and the component having the electric vector 0, with the result that the transmitted wave will again be planepolarized but will have its plane of polarization shifted by an angle equal to twice the angle between the direction of the electric vector of the incident wave and the direction of the plate 3. Such a plate, in the case where the other dimensions are given by Table I, have been found to operate satisfactorily when the axial length, including the length of the notches 4, is 2.74A.

Fig. 2 shows another method for converting waves of plane polarization into elliptically or circularly polarized waves and vice versa. In this case a clamp 5 is brought to bear upon the pipe 2 so as to deform the pipe to give it an elliptical cross section. If the incident radiation is again represented by the arrow :1 the component represented by the arrows b and c will have different velocities of propagation and different wave lengths in that part of the pipe 2 which has an elliptical cross section as shown, because of the dependence of the wave length and velocity of propagation of waves of the mode here in question upon the width of the pipe in the transverse direction perpendicular to the direction of the electric vector, which width is different for the components b and 0 respectively. The clamp 5 may be provided to exerta clamping action over a substantial length of pipe, or a number of clamps may be employed, as illustrated, for instance, in Fig. 6. If desired, the pipe may be permanently deformed instead of constrained by clamps. The provision of clamps makes the deformation adjustable, however. The problem of reflection at the transition between circular and elliptical cross section is not serious in the arrangement of Fig. 2 because the natural resilience of the pipe may be relied on to provide a smooth taper between the portions of circular cross section and the portions of elliptical cross section. If the taper is sufficiently long, a condition not usually difficult to realize in practice provided the requirements regarding reflections are not too exacting, the reflections caused by the transition will for practical purposes neutralize each other.

Figs. 3 and 4 show a form of apparatus of the general type shown in Fig. 1 with an additional provision of a second plate of dielectric material 7, which is adapted to provide a fine adjustment for the apparatus. The plate 3 is preferably provided with approximately the same dimensions as those preferred in the case of Fig. 1. Consequently for wave lengths in the neighborhood of three centimeters the thickness of the plate 3 might be about A inch. The plate 7 is made much thinner and because of this fact it need not be provided with notches corresponding to the notches 4, and may instead be rectangular in form as shown in Fig. 4. For a wave length of about three centimeters the thickness of the plate 7 may be about inch. The desired effect to be produced by the combined action of the

plates

3 and 7 may then be controlled by varying the angular position of the plate 7. When the plate 7 is at right angles to the plate 3 it acts in opposition to the plate 3, while when the plate '7 is parallel to the plate 3 its effect is directly added to the effect of the plate 3.

Fig. 5 shows the organization of an arrangement such as that shown in Fig. 1 into a system for transmitting and receiving radio waves. A transmitter located at 10 is coupled to a rectangular wave guide 11 which feeds through a suitable taper section of wave guide 12 into a cylindrical wave guide 13. A dielectric plate 14, arranged in the manner described in Fig. l and located in the cylindrical wave guide 13, serves to convert the plane-polarized waves excited by the transmitter 10 into circularly polarized waves. The circularly polarized waves are then radiated by means not shown, which may be a form of radiator-interceptor or antenna shown in Fig. 8 and described below. The resulting radiation may then be reflected by objects in the path of such radiation and the reflected radiation will likewise be circularly polarized since the reflection introduces only a change in phase. The reflected echo may then be intercepted by the radiator-interceptor, thus causing circularly polarized waves to travel to the left in the wave guide 13 of Fig. 5. The said waves are, by the action of the plate 14, converted into planepolarized waves but the resulting plane-polarized waves are polarized in a plane at right angles to the plane of polarization of the waves in the rectangular wave guide 11. This effect results from the fact that the waves have now passed twice through quarter-wave plate 14 so that the result is the same as if they had passed through a halfwave plate. The plate 14, as pointed out in connection with Fig. 1, should have its faces atan angle of 45 to the plane of polarization of the wave in the wave guide 11.

Because of the direction of its polarization, the received signal after passing through the plate 14 cannot be a ccepted by the wave guide 11, but proceeds instead into the rectangular wave guide 15, which is oriented so as to accept waves polarized in the direction of polarization of the received signal and to reject waves polarized in the manner of those produced in their Wave guide 11 by the transmitter to. The wave guide 15 leads to a receiver 16 which is operated by the received signal to provide information concerning the location of the objects producing the echo of the transmitted signal. The orientation of the rectangular wave guide 15 thus serves to protect the receiver against damage from overload which might otherwise result from the operation of the transmitter It). For some types of apparatus no other overload protection will be necessary.

Fig. 6 shows a form of radio transmission and reception system having a mode of operation generally similar to that of the system shown in Fig. 5 but employing a protective breakdown discharge device for additional protection of the receiver 16. The conversion of plane-polarized waves into circularly polarized Waves is accomplished, in the particular example illustrated, by the method shown in Fig. 2 instead of by the method illustrated in Fig. l. The clamps 5 and 5a operate in the manner indicated in Fig. 2 to deform the wave guide 2 into an elliptical pipe for a suitable distance. The amount of de formation and the distance between the clamps 5 and 5a is so coordinated that the components b and c (referring to Fig. 2) of the plane-polarized wave a are given a relative phase shift of 90 by passing through the squeeze section.

The pipe 17 which forms a junction with the pipe 2 between the clamp 5 and the taper section 12 is a cylindrical pipe of the same as the pipe 2 and leads through joints 1S and l? and a protective diaphragm 20, to a taper section 21 which feeds a rectangular wave guide 22, which is oriented at right angles to the rectangular wave guide 11 in such a manner as to accept waves at a polarity at right angles to that of the wave which the wave guide 11 is adapted to transmit. Although the difference in polarization at the junction of the pipes 2 and 17 between the transmitted and received signals would be adequate to protect even a sensitive receiver in a perfectly matched system, the possibility of part of the transmitted signal being reflected in some part of the system between the circular-polarizing element and the antenna or radiator makes it desirable to provide additional protection to the receiver, because such reflections would give rise to waves in the pipe 17 of a polarization appropriate for reaching the receiver 16 through the wave guide 22. It is very difficult to construct an antenna system which is so well matched to the rest of the system and to the surrounding space that substantial internal reflections do not occur.

It requires only a relatively small reflection within the system to produce a disturbance at the receiver input having many times the amplitude of the usual received signal. For this reason the protective diaphragm is provided in the arrangement of Fig. 6. The protective diaphragm 20 is shown in elevation in Fig. 7 which is a cross section along the line 77 of Fig. 6, looking downwards at the location 7-7. The diaphragm 20, as shown, comprises a partition across the pipe wave guide 17, closing off said wave guide except for a cross-shaped aperture which may b regarded as made up of crossed slits, each slit being in the shape of a dumbbell. The slits are designed so that each will be resonant at the frequency of operation. Because of the relative perpendicularity of the slits and because of their orientation parallel to the sides of the rectangular wave guides 11 and 22, one of the slits is adapted to be excited by waves 'of the polarization of those transmitted by the Wave guide 11 and another of the slits is adapted to be excited by waves of the polarization of those which the wave guide 22 is adapted to transmit. In consequence, when the transmitter 10 is energized, the slit adapted to be excited by the wave of the polarization which the wave guide 11 is adapted to transmit will be excited and a breakdown will take place across it, the breakdown being concentuated towards the center of the diaphragm 20 because of the higher voltages occurring near the centerportion of the slit. The said breakdown will detune the slit which is not excited by the waves in the wave guide 11 and provide ionization in the neighborhood of the center of said slit, so that waves reflected at places in the system between the clamp 5a and the antenna and having a polarization, when they readh the wave guide 17 adapted for transmission in the wave guide 22 will be substantially stopped by the diaphragm and prevented from reaching the receiver 16 in sufiicient intensity to cause damage thereto. When the transmitter 10 is not in operation an echo is received by theantenna of thissystem, there will be no breakdown at any part of the diaphragm 2i) and the slit aligned with the wave guide 22 will permit the received signal to proceed to the wave guide 22 and the receiver 16 with little, if any, attenuation. The diaphragm 20 should be located at approximately a halfwave length from the junction of the guides 17 and 2 so as to produce a minimum of interference with the transmission of energy along the guide 2 during transmitter operation when a discharge detunes the slits of the diaphragm.

Fig. 3 shows a form of radiator-interceptor or antenna suitable for use with systems such as Figs. 5 and 6 which transmit and receive circularly polarized radiation. For some purposes it may be sufficient for radiation and interception of circularly polarized waves to provide simply an open termination of the wave guide 2 without any additional apparatus except possibly an iris diaphragm near the end of the wave guide 2 for improving the impedance match. In order that a concentrated beam may be emitted and in order that reception of the echoes may be directionally sensitive, thus eliminating interference from other directions, a radiating and intercepting system such as that shown in Fig. 8 may, however, be advantageous. in Fig. 8 is shown a cylindrical Wave guide 24, which may be an extension of the wave guide 2 or may be a wave guide connected to the wave guide 2 through suitable bends and rotating joints. The wave guide 24 is open at its right hand extremity. A parabolic reflector 25 is mounted upon the wave guide 24 coaxially there'- with and with its focus situated a small distance in front of the open end of the wave guide 24. I A reflecting metallic plate 26 is located also a small distance in front of the open end of the wave guide 24, preferably at a distance somewhat more than a quarter-wave length, which may be as much as a half-wave length. The reflecting plate 26, which may take the form of a disk, is held in place by means not shown, made of insulating material such as polystyrene and mounted on the end of the pipe 24.

Another suitable form of radiating and intercepting system might be simply a parabolic reflector, such as the reflector 25, fed at its focus by the open end of a cylindrical wave guide facing toward the vertex of the parabolic reflector. Such a system, however, is subject to difiiculties when it is desired to provide for rotation or rapid alteration of the orientation of the system because of the difficulty of mechanically rotating the feed wave guide as well as the parabolic reflector.

in order to make apparatus according to the present invention operate satisfactorily over the widest possible frequency band it may be desirable to provide the plate 3 or some equivalent structure in a form providing the largest possible relative phase shift per unit axial length between the two components of the incident radiation. The increase of the relative phase shift per unit axial length between the components may be expected to be limited in practice because of the possibility of permitting modes other than the desired mode of oscillation for one or the other of the components in question. This limit may be avoided to some extent working with wave guide dimensions closer to the critical dimensions for transmission of the frequency in question, but if this is done frequency-sensitivity may yet fail to be avoided, for in order to maintain the desired high relative phase shift between the components of the waves, it may be necessary to work so close to the said critical dimensions as to introduce an increase of frequency-sensitivity and to increase excessively the attenuation of one component, as well as to introduce the necessity for considerable precision in the dimensions of the wave guides and the configuration of the wave guide cross section and precautions against thermal expansion effects and the like. Dielectric plates corresponding in sections to the plate 3 of Fig. 1 may be used which have a cross-sectional shape other than the rectangular cross section of the plate of Fig. l. Various shapes may be devised in order to obtain a maximum effect upon the wave length of one component and a minimum etfect upon the wave length of the other component polarized at right angles to the first component. If desired, instead of polystyrene, materials of higher dielectric constants, such as Mycalex or even rutile may be used for the dielectric plate 3.

What is claimed is:

1. Apparatus for transmitting and receiving highfrequency radio pulses comprising, a transmitter and a receiver and having a single radiator-interceptor arrangement for transmitting and receiving, said apparatus including a substantially cylindrical wave guide, a first rectangular wave guide connecting said transmitter and said cylindrical wave guide, a second rectangular wave guide connecting said receiver and said cylindrical wave guide, said rectangular wave guides being adapted respectively to transmit waves having respectively mutually perpendicular planes of polarization and to attenuate waves of a polarization perpendicular to that of the waves adapted to be transmitted respectively in said rectangular wave guides, and means associated with said cylindrical wave guide between the radiator-interceptor and the junctions of said cylindrical wave guide with said rectangular wave guides adapted to convert plane-polarized waves excited by said transmitter into circularly polarized waves and also adapted to convert circularly polarized waves excited by said radiator-interceptor into plane-polarized waves of a polarization adapted to be transmitted through said'second rectangular wave guide to said receiver.

2. Apparatus for transmitting and receiving highfrequency radio pulses comprising, a transmitter and a receiver and a radiator-interceptor adapted to operate with both said transmitter and receiver, said apparatus including a substantially cylindrical wave guide having a branched termination, a first rectangular wave guide connecting said transmitter to one branch of said termination of said cylindrical wave guide, a second rectangular wave guide connecting said receiver to a second branch of said cylindrical wave guide, means located in said second branch including crossed resonating structures adapted to provide an electrical breakdown discharge during operation of said transmitter and adapted to permit transmission of received signals towards said second rectangular guide during nonoperation of said transmitter, and means in the unbranched portion of said cylindrical wave guide adapted to convert plane-polarized waves excited by said transmitter into circularly polarized Waves and also adapted to convert circularly polarized waves of a polarizat'ion excited by said radiator-interceptor into planepolarized waves of a polarization adapted to be transmitted through said second rectangular wave guide to said receiver.

3. Radio echo detection apparatus comprising a transmitter, a receiver and a common antenna, a cylindrical wave guide connected to said antenna, a first rectangular wave guide connecting said transmitter and said cylindrical wave guide, a second rectangular wave guide connecting said receiver and said cylindrical wave guide, said rectangular wave guides being arranged respectively to transmit waves having respectively mutually perpendicular planes of polarization and to attenuate waves of a polarization perpendicular to that of the waves adapted to be transmitted respectively in said rectangular wave guides, and means associated with said cylindrical wave guide between said antenna and the junction of said first rectangular wave guide with said cylindrical wave guide for converting from linear to circular polarization waves excited by said transmitter, and for converting circularly polarized waves excited by said antenna into linearly polarized waves of a polarization adapted to be transmitted through said second rectangular wave guide to said recclver.

4. The combination in accordance with claim 3 wherein said last-mentioned means comprise a -degree phaseshift section.

5. The combination in accordance with claim 3 wherein said last-mentioned means comprise a plate of dielectric material disposed axially of said cylindrical wave guide in a diametral plane thereof.

6. The combination in accordance with claim 3 where I in said last-mentioned means comprise a plate of dielectric material disposed axially of said cylindrical wave guide in a diametral plane thereof, said plate being dimensioned to provide a large wave length modification to the component of incident waves which is parallel to the planeof said plate and a relatively small wave length modification to the component of the incident waves oriented perpendicularly to the plane of said plate.

7. The combination in accordance with claim 3 wherein said last-mentioned means comprise a plate of dielectric material disposed axially of said cylindrical guide in a diametral plane thereof, said plate being of a width equal to the internal diameter of said cylindrical wave guide and of a thickness substantially less than the width.

8. The combination in accordance with claim 3 wherein said last-mentioned means comprise a plate of dielectric material disposed axially of said cylindrical guide in a diametral plane thereof, said plate being of a width equal to the internal diameter of said cylindrical wave guide and of a thickness substantially less than the width, said plates having notches in the axial extremities thereof of a depth of approximately one-quarter of the wave length of oscillations excited by said transmitter.

9. The combination in accordance with claim 3 wherein said last-mentioned means comprise apparatus for modifying the cross section of a portion of the length of said cylindrical wave guide to produce a relative phase shift between two mutually perpendicular components of the waves excited in said cylindrical wave guide.

10. The combination in accordance with claim 3 wherein said. last-mentioned means comprise a clamp secured to the exterior of said cylindrical Wave guide arranged to deform a portion of the length of said cylindrical wave guide from circular to elliptical cross section.

11. Radio echo detection apparatus comprising, a transmitter, a receiver and a common antenna, a cylindrical wave guide connected to said antenna, a first rectangular wave guide connecting said transmitter to said cylindrical wave guide, a second rectangular wave guide connecting said receiver to said cylindrical wave guide, said first and second rectangular wave guides being oriented at right angles to each other, and a 90-degree phase-shift section disposed between said antenna and the junction of said first rectangular wave guide with said cylindrical wave guide.

12. The combinationin accordance with claim 11 and a protective device interposed. between said cylindrical wave guide and said receiver.

13. The combination in accordance with claim 11 and a conducting barrier havingan aperture therein resonant at the frequency of said transmitter interposed between said cylindrical wave guide and said receiver.

14. The combination in accordance with claim 11 and a protective breakdown device interposed between said cylindrical wave guide and said receiver, said breakdown device being operative to isolate said receiver from said cylindrical wave guide during operation of said transmitter and to permit transmission of signals from said antenna to said receiver during nonoperation of said transmitter.

15. The combination in accordance with claim 11 and a protective breakdown device interposed between said cylindrical wave guide and said receiver, said breakdown device comprising a transverse conducting barrier having a pair of crossed apertures therein each resonant at the frequency of said transmitter.

16. Radio echo detection apparatus comprising, a transmitter, a receiver and an antenna, a first cylindrical wave guide connected at one end to said antenna, a first rectangular wave guide connected to said transmitter, a first transition section of wave guide joining said first rectangular wave guide to the other end of said cylindrical wave guide, a second cylindrical wave guide joined at right angles to said first cylindrical wave guide, a second rectangular wave guide connected to said receiver, a second transition section of wave guide joining said second rectangular wave guide and said second cylindrical wave guide, corresponding transverse axes of said first and second rectangular wave guides being oriented perpendicularly to each other, means associated with said first cylindrical wave guide between said antenna and the junction of said second cylindrical wave guide for producing a -degree phase shift of the plane of polarization of energy within said first cylindrical wave guide, and a protective breakdown device positioned in said second cylindrical wave guide. a

17. Apparatus in accordance with claim 16 wherein said phase-shifting means comprise clamping means arranged to deform said first cylindrical wave guide from a circular to an elliptical cross section.

18. Apparatus in accordance with claim 16 wherein said phase-shifting means comprise a plate of dielectric material disposed axially of said first cylindrical wave guide in a diametral plane thereof.

19. Apparatus in accordance with claim 16 wherein said phase-shifting means comprise a plate of dielectric material disposed axially of said first cylindrical wave guide in a diametral plane thereof, and said breakdown device comprises a conducting barrier having an aperture therein resonant at the frequency of operation of said transmitter positioned transversely of said second cylindrical wave guide.

20. A switching system comprising, a main section of cylindrical wave guide, a first branch of rectangular wave guide forming an axial extension of said main section, a second branch of rectangular wave guide joined to said main section, corresponding transverse axes of said first and second rectangular wave guides being oriented perpendicularly to each other, and a phase-shift section associated with said main section.

No references cited.