US3281843A

US3281843A - Electronically scanned antenna

Info

Publication number: US3281843A
Application number: US329075A
Authority: US
Inventors: Robert E Plummer
Original assignee: Electronic Specialty Co
Current assignee: Electronic Specialty Co
Priority date: 1963-12-09
Filing date: 1963-12-09
Publication date: 1966-10-25
Anticipated expiration: 1983-10-25

Description

Oct. 25, 1966 R. E. PLUMMER 3,281,843

ELECTRONICALLY SCANNED ANTENNA Filed Dec. 9. 1963 4 Sheets-Sheet l FIG. I FIG. 20

5; Ms flrraev gr Oct. 25, 1966 PLUMMER 3,281,843

ELECTRON I CALLY S CANNED ANTENNA Filed Dec. 9, 1963 4 Sheets-Sheet 2 Oct. 25, 1966 R. E. PLUMMER ELECTRONICALLY SCANNED ANTENNA 4 Sheets-Sheet 5 Filed Dec. 9, 1963 Y? 7&1

Mfg? 3; ii j aasqr E 4010444453 3,281,843 ELECTRONTCALLY SCANNED ANTENNA Robert E. Plummer, Granada Hills, Califi, assignor to Electronic Specialty Co., Los Angeles, Calif., 21 corporation of California Filed Dec. 9, 1963, Ser. No. 329,075 3 Claims. (Cl. 343-106) This invention relates to scanning antennas and more particularly to an antenna for electronically rotating a complex microwave energy pattern.

Microwave radiation patterns of complex shape are beamed from ground antennas as a navigational aid to aircraft. In one application when such a field pattern of radiation energy is rotated at 900 rpm, for example, a modulation occurs on signals received by a remote aircraft. The phase of this modulation when compared with a reference signal enables quite accurate measurement of the aircraft bearing relative to the ground station. Antennas currently used for this purpose generate the complex pattern shape through use of parasitic radiators mounted on dielectric cylinders which, when rotated about a central driven element, cause the modulation on signals to be received at some remote point. These antennas have reliability problems associated with the high speed mechanically rotated components and have limited the control of the elevational beam shape. The antennas require frequent maintenance repair of the bearings and the mechanical drive members, necessitating a large stock of spare parts and the staffing of installation sites with repair crews.

Briefly, the antenna and transmission system comprising the present invention is based upon the super-position of a plurality of electromagnetic energy propagation modes in a circular parallel plate transmission line radiator to provide a desired azimuth radiation pattern shape. This pattern may be rotated without the mechanical motion of any of the antenna components by rotating an exernally applied field about a material exhibiting birefringent characteristics in the antenna transmission line. In one form a ferromagnetic material or ferrite is used with a rotating magnetic field. This electronic scanning technique greatly increases the antenna reliability and reduces the maintenance requirements.

More particularly, in one embodiment electromagnetic energy of the TEM, TE and TE modes of transmission is combined in suitable proportions to produce the necessary cardioidal-shaped pattern with 9th-order scallops or ripples superimposed. By exciting the TE and TEgo modes from a feed system containing a field rotation device, these modes are caused to rotate in one-to-one correspondence with the field rotation in the feed system. The field rotation device is implemented with a ferrite material for generation of the required differential phase shift. The field rotation and hence the pattern rotation results from application of a rotating magnetic field to the ferrite material. Since a rotating magnetic field is a normal characteristic of the stator Winding of la synchronous motor, such a winding is formed, in one embodiment, about the waveguide containing the ferrite the result that the waveguide field will rotate at a speed dictated by the synchronous speed of the winding.

The proposed radial waveguide antenna provides a natural arrangement for generating the desired elevation atent pattern. The circular parallel plates can easily be flared to form a conical sectoral born for generating an elevation pattern whose shape and directivity will be governed by the horns vertical aperture and the flaring parameters. An important advantage of the proposed radial waveguide sectoral horn arrangement is that the azimuth and elevation radiation patterns are independently controlled by virtue of the separability of the horn aperture fields. Beam shaping techniques can be employed in the vertical aperture without influencing the azimuthal field distribution which is governed by the modal fields established by the feed system.

The radial waveguide approach to the design of a scanning ground antenna, in accordance with the present invention, stems from recognition of the fact that the combination of certain wave modes in such a circularly symmetric system will produce a resultant field pattern having the necessary amplitude variations as a function of azimuth angle to generate fundamental and ninth harmonic modulation components on the received signal at a far field point when the pattern is rotated at a prescribed rate.

The far field azimuth pattern shape required for proper operation of an airborne bearing measurement gear is an approximate cardioid pattern upon which is superimposed a ninth-order ripple of appropriate magnitude. 'Ihis composite complex pattern must in one application rotate at a rate of 900 revolutions per minute in order to impart the requisite fundamental modulation component on the received signal of 15 c.p.s. along with a ninth harmonic modulation component of c.p.s.

An object of the present invention is to provide for a highly desirable inertialess scanning arrangement for the azimuth pattern of radiation from a scanning ground antenna.

Another object is the provision of superimposed radial waveguide modes of energy propagation to generate a desired azimuth pattern shape.

Another object is the provision for inertiale-ss rotation of microwave energy propagation modes resulting in rotation of radiation patterns Without the conventional mechanical rotation of antenna components.

Another object is the provision of a rotating magnetic field about a triaxial transmission line having multi-modes of microwave energy propagation therein.

Another object is the provision of a ferrite material within a waveguide subjected to a rotating magnetic field for rotating the modes of energy transmission therethrough.

Another object is the provision of orthogonal modes of microwave transmission with amplitude changing means for rotating the resultant energy radiation pattern without rotation of antenna components.

Other objects will become more apparent as a description of the invention proceeds having reference to the drawings wherein:

FIGURE 1 is a perspective view showing parallel circular plates and appropriate coordinate system;

FIGURES 2a2e are diagrammatic illustrations of field intensity patterns of various energy propagation patterns;

FIGURE 3 is a cross-sectional view of a triaxial transmission line for feeding multi-mode energy to the antenna;

FIGURE 4 is an elevational sectional view of a triaxial line to radial waveguide transformer;

FIGURE 5 is a plan view of a transducer that permits simultaneous excitation of multi-mode energy into a triaxial transmission line;

FIGURE 6 is a sectional view taken along the line 66 of FIGURE 5 FIGURE 7 is a schematic illustration of a slotted array for TE mode excitation;

FIGURE 8 is a schematic diagram of an energy feed distribution for the slotted array;

FIGURE 9 is a schematic illustration of a dual-ring array of slots for implementation of orthogonal TE modes;

FIGURE 10 is a sectional view of one form of antenna;

FIGURE 11 is a partial perspective view of the mode rotator showing a single energy source and the orthogonal outlets of the system; and,

FIGURE :12 is a simplified block diagram of the power distribution system for the antenna.

A radial waveguide is basically a parallel plate transmission line Whose conducting surfaces are circular plates which are fed at or near their axis with a circularly symmetric feed system. FIGURE 1 shows the basic waveguide configuration with

plates

10, 12 and illustrating the appropriate coordinate system wherein line Z passes through the axis of the plates and line X is a reference line from which angular measurements may be taken. Such a waveguide is capable of supporting an infinite number of wave modes in general, which are mathematically describable as so-called cylindrical harmonics. Of particular interest is the fundamental mode which has the properties of a transverse electro-magnetic (TEM) wave and two specific transverse electric (TE) modes, the TE mode and the TE mode. The mode indices indicate a single cosinusoidal variation in the -coordinate in the first case and nine cosinusoidal variations with in the second case. The zero index means that there are no variations of the field in each mode in the direction parallel to the system axis (the Z coordinate). The propagation direction for all three waves is radially outward for transmission and inward for receiving.

FIGURES 2a-2c show field intensity diagrams for the three modes in polar coordinates. It is seen that the TEM mode has a constant field intensity as a function of azimuth angle while the TE mode and TE modes are multi-lobed. The TE mode has a figure-of-eight characteristic and is mathematically describable by the trigonometric functioncos where 45 is the angle from the horizontal reference line X. The TE mode is characterized by an eighteen-lobed rose petal pattern, as shown, and is describable by the function cos 9. All three modes are characterized by a cylindrical phase front and by propagation velocities that are functions of the radial waveguide physical parameters and the mode characteristic wave numbers.

FIGURES 2d and show the qualitative result of combining, first, the TEM and TE modes and, secondly, that combination with the T E mode. Mathematically these combinations are representable as follows:

The coefiicients A, B, C represent the amplitudes of the modes.

Equation (1) is recognized as a quasi-cardioid shaped function-a true cardioid pattern results if A=B. The second equation is, of course, the result of adding the TE mode of amplitude C to the cardioid field configuration. If the composite pattern represented by Equation 2 were rotated at some continuous rate, the amplitudes A, B and C would be found to be directly related to the fundamental and ninth harmonic modulation indices for the signal received at a distant point. It should be noted that the radio-frequency time variation has been omitted in the above equations but should be understood to be a multiplying factor on all terms.

Having shown that a combination of three specific radial guide modes can result in the desired resultant field configuration, it is now desirable to illustrate suitable techniques for mode excitation using conventional transmission lines. The methods for exciting the TEM and TE modes are quite straightforward while the TE feed technique is relatively more complex. From a practical standpoint it is desirable to feed the radial guide, such as shown in FIGURE 1, through only one of its circular plates to avoid crossing the radiating aperture with a transmission line and, thus, disturb the circular symmetry of the system.

The TEM and TE radial guide modes can be excited from a triaxial transmission line 14 wherein two coaxial line modes are separately transported in the manner illustrated by the cross-sectional view in FIGURE 3. Here a coaxial TEM mode exists in the inner coaxial system between

conductors

16 and 18, while the coaxial TE mode travels in the outer coaxial system between

conductors

18 and 20.

FIGURE 4 shows a possible method for launching the energies in these two modes into the radial waveguide where they are transformed into the TEM and TE modes respectively. A TEM mode transformer 22 interconnects conductor 16 with plate 10 while TE mode transformer and isolation choke 24 terminate conductor 18 and extend midway between

plates

10 and 12 to transform the TE mode into the desired TE mode.

FIGURES 5 and 6 show a mode transducer which will allow the simultaneous excitation of the coaxial TEM and TE modes into the triaxial transmission line 14. The

orthogonal probes

26, 28, 30 and 32 entering the outer coaxial region between

conductors

18 and 20 are excited by means of unbalanced-tobalanced line transformers (balun) to ensure TE mode purity and symmetry. This technique launches two orthogonal modes with excellent isolation. It will be seen later that the system requires the launching of a pair of orthogonal coaxial TE modes from a pair of energy sources since mode purity is of extreme importance in minimizing cross-talk.

The method for exciting the radial waveguide TE mode employs a system of eighteen discrete slots, 3468, arranged in a ring configuration such that the slots couple into one of the circular plates of the radial waveguide structure (as can be seen at 6 in FIGURES 9 and 10). The slot coupling is similar to the slot in the rectangular wave-guide as explained at pages 298-291 in the book entitled Microwave Antenna Theory and Design by S. Silver, published by McGraw-Hill in 1949. This circular array of slots is concentric with the radial waveguide axis. FIGURE 7 illustrates the ring array and the feed arrangement for a typical pair of

adjacent slots

34, 36. It is seen that adjacent slots receive energy through a device 70 which ensures equal amplitude excitation with electrical degrees of phase difference. This device is a 180 hybrid, commonly known as a 4-port magic T. A signal fed into input D appears at each output but with a 180 phase difference. If the several slots are all fed in this Way with a reversal of phase from slot to slot as one traverses about the ring, the necessary conditions exist for the generation of the cos 9 functional behavior about the radial guide. Consequently, with suitable care in the physical design of the launching region in the vicinity of the ring, the TE mode will be generated by the ring array.

FIGURE 8 is a schematic diagram of the feed distribution system to the 18 slots. It is seen that each of the two outputs 72, 74 of the hybrid 70 branch nine ways through use of a tree comprised of four 3-way balanced power dividers. One output 72 from hybrid 70, shown as a solid line, feeds dividers 7 6, 78, 80 through divider 82. Similarly, hybrid output 74, shown as a dash line, feeds

dividers

84, 86, 88 through divider 90. The nine outputs of one tree are interlaced among the nine terminals of the sec,

nd tree so that the desired phase reversals occur from slot to slot about the ring.

' The rotation of the complex field pattern obtained by combination of the three radial guide modes is based on the fact that rotation of the TE and TE modes in the radial guide is suflicient to rotate the entire pattern. To develop the rotation scheme, it is useful to write the azimuthal dependence of the two modes in a way that describes the desired angular rotation. These equations take the following form:

E =B cos (qS-Q!) (3) E =C cos (9-m) (4) Where Q=angular rate of mode rotation t=time azimuthal angle B and C=mode amplitude-s If the above equations are expanded by the well-known trigonometric identity for the cosine of the difference of two angles, one obtains:

E =B(cos cos nt-l-sin sin Gt) (5) E :C(cos 9 cos tlt+sin 9 sin or) (6) Equation 5 is easily interpreted to mean that, if two orthogonal TE modes (cos and sin exist simultaneously in the radial waveguide, and, if their amplitudes are caused to vary with time according to the functions cos Q2 and sin tlt respectively, the result will be a single TE mode which rotates continuously at angular rate Q.

Correspondingly, Equation 6 has a similar interpretationthe cos 9 and sin 94 terms represent a pair of orthogonal ninth order modes in the radial guide which, if modulated according to the respective cos 9! and sin Qt prescription, give rise to a resultant single TEgo mode which rotates at the angular rate S2.

The result is that, if the orthogonal modes for both the TE and TE cases receive excitation from a common modulator, the two modes will rotate in synchronism at the angular rate imposed by the modulation device.

The so-called modulator required to produce the cos Qt and sin 9! amplitude functions involved in moderotation is describeable in simplest terms as a rotating halfwave plate. Such a device is readily realizable in a section of circular waveguide propagating the circular guide TE mode by introducing a medium into the guide which has the property that an electric field component oriented in one angular direction within the medium progresses through the medium unperturbed while a field oriented perpendicular to the first suffers a phase retard tion (or advance) of 180 relative to the first or orthogonal leld component. One way to achieve this result is to place a thin slab of dielectric material into the waveguide. Here the fields incident perpendicular and parallel to the slab emerge with 180 differential R.F. phase shift. The result is that, as the half-wave plate rotates the output field undergoes rotation also. In fact, the output electric field vector rotates at exactly two times the angular rate of the half-wave plate. Thus, a stationary TE mode in circular waveguide can be rotated by introducing a rotating half-wave plate. If now the output of this half-wave plate region is sampled by two orthogonal probes, the probe voltage amplitudes will have a cos a and sin a relationship to the amplitude of the field at the input to the device. Here a is the angle of the half-wave plate relative to the input polarization vector. If the plate rotates at angular rate Q/ 2 then the output voltages will vary as cos Qt and sin or as desired (two-to-one factor as noted previously). For example, if the plate rotates at 450 r.p.m., the field polarization at the output of the plate region will rotate at 900 r.p.m.

From the foregoing it may be noted that the radial waveguide TE and TE modes can be continuously rotated if each is generated by a set of orthogonal modes of identical character whose amplitudes vary as sine and cosine functions of time. It has also been shown that the sine and cosine time variations of the excitation voltages can be realized through use of orthogonal probes at the output of a rotating half-wave plate in circular waveguide excited by a stationary TE mode.

It is well known in the waveguide art that a ferrite material placed within a circular waveguide and subjected to a transverse D.C. magnetic field becomes birefringent. With suitable tailoring of the ferrite physical dimensions under these conditions it is possible to obtain a half-wave plate behavoir wherein the angular orientation of the plate depends upon the orientation of the drive field. If the drive field can be caused to rotate continuously about the waveguide, the result will be a continuously rotating TE mode at the output of the device with rotation rate equal to twice that of the drive field. The magnetic field of the stator winding of a synchronous motor has the desired properties; namely, the field is transverse to the motor axis and it rotates at a synchronous speed dictated by the line frequency and the character of the winding. Consequently, if a suitable winding is arranged about the circular waveguide containing the ferrite, the TE mode in the guide will rotate at two times the synchronous speed characterizing the winding. For the preferred embodiment the winding must be tailored to produce 450 r.p.m. synchronous speed from a 60 c.p.s. line frequency.

One additional aspect of the present invention is the implementation of the orthogonal pair of TE modes. For the TE mode pair the excitation was provided by launching two perpendicular TE modes in the outer coaxial region of the triaxial transmission line as explained with reference to FIGURES 5 and 6. A single TE mode was shown to be excited by a ring array of slots as shown in FIGURE 7. The orthogonal TE mode can be similarly launched by use of a second ring a-rray rotated 10 in azimuth relative to the first. This will result in a sin 9 distribution if the first ring yields a cos 9 distribution. The two rings would then receive excitation from the co sine and sine output probes of the ferrite field rotator. This dual ring array 96 is illustrated in FIGURES 9 and 10.

On embodiment of the antenna in accordance with the present invention, consists of a circular parallel plate system terminating in a biconical horn. FIGURE 10 is a cross-sectional view of such an antenna showing the salient members required for electrical performance and illustrating certain basic structural elements considered appropriate for supporting this rather unusual shape.

Support for the entire antenna system may be provided by a section of cylindrical pipe 98, approximately 15 inches long. This pipe section is provided with a flange at each end. At the lower extremity the flange constitutes the mounting base for the assembly while the upper flange :mounts directly to and supports the lower plate 100 of the radial waveguide system. A system of radial support arms 102, extend outwardly from the cylindrical section 98, providing support for the extremities of the radial waveguide and for the lower half 104 of the biconical horn. Thin aluminum sheets are used to cover this lower support system to provide protection for the R.-F. components and the R.F. cabling harness.

As indicated previously, the proposed method for achieving directivity and general control of the elevation beam pattern is basically through sectoral flaring of the radial waveguide. This approach results in a type of biconical horn whose cone angles and aperture dimension are governed by the beam uptilt angle desired and the directivity required for a given gain specification.

It must be emphasized that the diameter of the overall antenna system will be dictated by the sectoral horn angles and the vertical aperture dimension requirements in conjunction with the dimensional requirements of the ring arrays employed for excitation of the TE modes. A maximum diameter of 90 inches is estimated to be necessary to realize the required 5 db gain above an isotropic radiator at 1.0 krnc. FIGURE 10 shows an approximate overall antenna configuration taking into account the required gain and the ring array diameter required at the L-Band frequencies involved. The biconical horn is seen to be directed upward at some finite angle with respect to the horizontal such that the beam direction is also pointed upward from the horizon. The precise values of the cone angles can best be determined by experimentation to properly account for diffraction effects.

The beam pointing angle above the horizon is easily controlled by adjustment of the biconical horn parameters. Control of side-lobes and general beam shape can be achieved in this system if needed by the introduction of conical

metallic plates

92, 94 in the biconical horn aperture in the manner indicated in FIGURE 10. These metallic plates cause a division of the aperture field in a manner that is predictable according to the angular relationships f the metallic plates. These plates cause the field to be divided in a prescribed manner so that a voltage taper can be imposed in the E-plane of the aperture resulting in side-lobe control in the far-field radiation pattern.

Primary support for the upper radial Waveguide plate 112, and upper portion 114, of the horn must be provided by the radome 116, or dust cover at the horn aperture. This radome extends around the periphery of the system and is supported at its lower edge by attachment to the lower radial trusses. High strength-to-weight ratio in the radome is essential to realize the necessary structural properties while keeping weight to a minimum. The radome preferably is a sandwich structure consisting of thin laminated fiber glass skins with a honeycomb core. Such an arrangement has superior electrical properties and, simultaneously, produces an excellent structural element with far less weight than a continuous sheet of comparable strength. Radome weight preferably should be kept under about 75 pounds even for this quite large diameter structure. A secondary support for the upper assembly of the antenna can be provided in one of several possible ways. A desirable approach would be through use of a system of vertical dielectric rods arranged in a ring about the system axis.

In FIGURE 10, a shallow conical hat 118, surmounts the entire antenna structure. This element is provided to ensure run-off of water, snow, hail, etc. It can, accordingly, be constructed of very thin metallic sheets supported by a simple and lightweight frame extending from the upper edge of the horn and from the top plate of the radial guide at the internal support points.

Provision for lifting the antenna assembly during installation can be made at the lower edge of the horn where the radial trusses 102, emanating from the cylindrical base are accessible for connecting of a lifting harness.

While the biconical horn antenna in FIGURE is shown for illustrative'purposes it is to be understood that other types of antennas may be used. For example, a parabolic torus with an offset radial Waveguide feed type configuration may be preferable for certain applications.

Referring now to FIGURE 11 there is shown one form of mode rotating apparatus. Here there is shown a ferrite field rotator 120 consisting of an outer conductor 121 and an inner conductor 123. R.-F. energy is fed to input 122 and

terminals

124 and 126 form the output. Within the spacing between the outer conductor 121 and inner conductor 123 and intermediate the input and

output terminals

122, 124, 126 is positioned a ferrite pencil 130. Between the single mode input 122 and the

orthogonal output terminals

124, 126 and outside conductor 121 is positioned a stator winding 128 to .generate a rotating magnetic field about the ferrite pencil 130 positioned within the line.

FIGURE 12 is a simplified schematic diagram of the R.-F. power distribution system for the radial waveguide antenna. It is seen that energy from the transmitter/ receiver (duplexer) 132 is divided two ways through power divider 134 with an unequal power division-the predominant energy going to the TEM wave excitation 136. An approximate power division ratio of 2:1 is visualized at this point corresponding to the modulation indices of about 20% each for the fundamental and ninth harmonic modulation components.

The power divider 134 with approximately 33% of the total power (about 135 watts average, 6750 watts peak) is coupled into input 122 of the ferrite field rotator 120 whose output terminals 124, 126 (orthogonal probes) pick up energy of a magnitude that is dependent upon the instantaneous magnetic field orientation. Each of the output probes of this device couples to a two-way power divider 138, with approximately evenpower s lit. One side of each power divider (142, 144) couple to the orthogonal probes of the TE coaxial mode transducer to feed TE wave excitation 146. The other side (148, of each of the two

power dividers

140, 142 couples into the difference arm of a coaxial magic T 70 in FIGURE 8 whose output arms feed alternately to the slots of a ring array, thus assuring that all slots in a ring receive equal amplitude and alternating phases to provide for the TE excitation 152 in FIGURE 12. The cos Qt arm of the field rotator 126 couples to the cos 9 ring while the sin Qt arm of the rotator feeds the sin 94 ring in accordance with the prescription of the equation for E earlier mentioned.

It will be obvious to those skilled in the art that various changes may be made in this device without departing from the spirit of the invention and therefore the invention is not limited to what is shown in the drawings and described in the specification but only as indicated in the appended claims.

What is claimed is:

1. Means for generating a rotating microwave pattern comprising:

a triaxial transmission line for propagating a TEM mode between a pair of conductors and a TE mode between another pair of conductors of said line;

a pair of spaced waveguide plates;

an aperture in one of said plates;

said triaxial transmission line having an outer conductor terminating at said aperture and having its inner conductor extending through said aperture and terminating at said other plate and an intermediate conductor terminating between said plates;

said intermediate conductor having transformer means between said plates for changing the TE mode to a TE mode; and

means for rotating said TE mode within said triaxial line.

2. Means for generating a rotating complex microwave pattern comprising:

a triaxial conductor adapted for TEM transmission between a pair of conductors and TE transmission between another pair of conductors;

probes connected to said triaxial conductor for providing a pair of orthogonal TE modes of microwave energy transmission from energy sources;

antenna means connected to said triaxial conductor including a pair of spaced plates between which electromagnetic energy is radiated;

one of said plates having a dual ring of slots for orthogonally propagating a TE mode;

transformer means between said plates for changing the TE mode to a TE mode; and

means for rotating the resultant complex microwave pattern by rotating the TE and TEgo modes by rotation of a magnetic field around said triaxial conductor.

3. Means for generating a rotating complex microwave pattern comprising:

References Cited by the Examiner UNITED STATES PATENTS Bollinger 343783 X Fox 333-21 Herscovici et a1. 343--773 X Hatkin 343783 X Carson 343--106 X Bowman 343777 10 CHESTER L. JUSTUS, Primary Examiner.

H. C. WAMSLEY, Assistant Examiner.

Claims

1. MEANS FOR GENERATING A ROTATING MICROWAVE PATTERN COMPRISING: A TRIAXIAL TRANSMISSION LINE FOR PROPAGATING A TEM MODE BETWEEN A PAIR OF CONDUCTORS AND A TE11 MODE BETWEEN ANOTHER PAIR OF CONDUCTORS OF SAID LINE; A PAIR OF SPACED WAVEGUIDE PLATES; AN APERTURE IN ONE OF SAID PLATES; SAID TRIAXIAL TRANSMISSION LINE HAVING AN OUTER CONDUCTOR TERMINATING AT SAID APERTURE AND HAVING ITS INNER CONDUCTOR EXTENDING THROUGH SAID APERTURE AND TERMINATING AT SAID OTHER PLATE AND AN INTERMEDIATE CONDUCTOR TERMINATING BETWEEN SAID PLATES; SAID INTERMEDIATE CONDUCTOR HAVING TRANSFORMER MEANS BETWEEN SAID PLATES FOR CHANGING THE TE11 MODE TO A TE10 MODE; AND MEANS FOR ROTATING SAID TE11 MODE WITHIN SAID TRIAXIAL LINE.