US2994869A

US2994869A - Microwave antenna system

Info

Publication number: US2994869A
Application number: US163762A
Authority: US
Inventors: Orville C Woodyard
Original assignee: Individual
Current assignee: Individual
Priority date: 1950-05-23
Filing date: 1950-05-23
Publication date: 1961-08-01
Anticipated expiration: 1978-08-01

Description

Aug. 1, 1961 o. c. WOODYARD MICROWAVE ANTENNA SYSTEM 2 Sheets-Sheet 1 Filed May 23, 1950 PHASE a AMPLITUDE CONTROL HYBRID CIRCUIT W RADAR SYSTEM x -x o FIG.3

INVENTOR. ORVILLE C. WOODYARD Aug. 1, 1961 MICROWAVE ANTENNA SYSTEM Filed May 25, 1950 2 Sheets-Sheet 2 INENN: FTEUEI'GR? 27 I 9 2'.

I I4 20 P28 I9 ll l TRANSMITT DUPLEXER l t l-- l l8 I0 I I so 32 35 as TIMER DETECTOR 090. PH. DETECTOR LP. LE 39 AMPLIFIER AMPLIFIER AMPLITUDE PHASE DETECTOR 35 DETECTOR 4o TRACKING cmcun' FIG. 6

INDICATOR 1 V I/ AMPLITUDE A B c ANGLE F|G.7 o

F|G.8 I3 I MAGNETRON I 42 43 r INVENTOR.

45 44 ORVILLE' c. WOODYARD 32 080. 3'7

mi BY I FIG.9 W 7);. @944 United States Patent 2,994,869 MICROWAVE ANTENNA SYSTEM Orville C. Woodyard, Neptune, N.J., assignor to the United States of America as represented by the Secretary of the Army Filed May 23, 1950, Ser. No. 163,762 7 Claims. (Cl. 343-16) v (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereon. V This invention relates to a microwave antenna system for producing a highly directive radiation pattern of the type employed in radar systems. In particular it relates to microwave antenna systems employing a wave guide radiator or wave guide feed in a radiating system. Additionally, the invention relates to a microwave antenna system of the wave guide type for producing a radiation pattern of predetermined directivity suitably for use in a monopu-lse direction finding system.

Heretofore, in microwave radar systems where it is desired to provide a highly directive radiation pattern, wave guide radiators or more exactly antenna systems employing Wave guide radiating feed elements have been employed. For example, a familiar radar antenna structure is that of a wave guide feed directed to illuminate a parabolic reflector, the structure being arranged for orientation in azimuth and in elevation to direct a major lobe of radiation in a particular direction. The microwave energy is propagated from the mouth of a wave guide located at a focal point of the reflector. Within the guide this radiation is generally propagated in the fundamental mode and produces a radiation pattern of fixed directivity.

Inradar systems employing lobe switching, the wave guide feed is joined to the system so that the mouth of the guide, as directed toward the reflector, may be physically changed in position to alter the direction of the major lobe of radiation. Generally this physical displacement is accomplished mechanically at a high rate to cause a corresponding shift in the direction of the radiation at this rate. In other radar systems of the monopulse or simultaneous lobing type, to which the present invention is directed, the practice has been to employ two or more separate wave guide feeds directing the microwave energy at a reflector or lens. The circuit arrangements provide for simultaneously using the plurality of guides to determine the direction of an incoming signal. To determine the direction in one coordinate, say azimuth, at least two wave guide feed elements are used. To determine the direction in both azimuth or elevation, four or more wave guide feed. elements are employed.

The present invention contemplates in one of its embodiments the employment of a single wave guide feed instead of the multiplicity of guides or a single guide that is physically changed in position.

It is accordingly an object of the present invention to provide a microwave antenna system which avoids one or more of the disadvantages and limitations of prior art systems.

It is a further object of the present invention to provide a microwave antenna system comprising a duo-mode wave guide having a radiating open end to produce a radiation pattern having a major lobe of predetermined directivity.

It is an additional object of the present invention to provide in a radar system a microwave antenna system comprising a duo-mode wave guide having a radiating open end for determining the direction of arrival of the reflected wave energy.

In accordance with the'present invention there is pro- Patented Aug. 1, 1961 vided a microwave antenna system comprising a duomode wave guide having a radiating open end. The guide has dimensions which are chosen to propagate wave energy simultaneously in its fundamental and second order modes, for example, a rectangular wave guide having dimensions chosen to propagate wave energy in the TE and TE modes. Means are also provided for proportioning the relative amplitude and phase of wave energy propagated in the two modes to produce a radiation pattern having a major lobe of predetermined directivity.

Also in accordance with the present invention there is provided in a radar system an antenna structure comprising a duo-mode wave guide radiator. The guide has dimensions chosen to propagate wave energy simultaneously in its fundamental mode and its second order mode, for example, a rectangular wave guide having dimensions chosen to translate wave energy simultaneously in the TE and TE modes. Means are provided for utilizing the guide to radiate the wave energy in the fundamental mode. Further means are provided for utilizing the guide to receive a reflected part of the energy in both of the modes and additional means are provided for combining and detecting the received energy and for utilizing the detected energy to indicate the direction of arrival of the reflected energy.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing and its scope will be pointed-out in the appended claims.

Referring to the drawings, FIG. 1 is a drawing, partly in block diagram and partly schematic, illustrating. a fundamental embodiment of the present invention; FIG. 2 is a diagram corresponding to FIG. 1 illustrating the fundamental and second order modes propagated in the guide and the directive radiation pattern which obtains; FIG. 3A and B are respectively graphs and corresponding vector diagrams illustrating the wave energy distribution in amplitude and in phase at the mouth of the wave guide of FIG. 1 for the purpose of explaining the operation; FIG. 4 is a diagram illustrating a modified form of the invention; FIG. 5 illustrates a preferred form of wave guide radiator for use in the present invention; FIG. 6 is a block diagram illustrating a monopulse or simultaneous lobing radar system in accordance with a preferred arrangement of the present invention; FIGS. 7 and 8 are diagrams illustrating the operation of the system of FIG. 6; and FIG. 9 is a diagram illustrating the embodiment of a part of the FIG. 6 arrangement in wave guide form.

Considering now FIG. 1 of the drawing, the arrangement illustrated is that of a microwave antenna system comprising a rectangular duo-mode wave guide 10 having an open end 11; the end or mouth of the guide being directed toward a reflector or lens system 12. A radar system is indicated by unit 13 and ordinarily comprises a multiplicity of electronic units and elements for transmitting pulses of microwave energy and for receiving reflected echo pulses. Unit 13 is coupled to wave guide radiator 10 via a first coaxial line or path, 14, for translating the fundamental mode and via second path comprising a coaxial path 15, a phase and amplitude control unit '16 and a coaxial path 17 for translating the second order mode. The coaxial path 14 is coupled to guide 10 by means of a probe element 18, while the coaxial path 17 is coupled to guide 10 by means of the

probes

19 and 20 extending into the guide. The length of the coaxial path between :19 and 20 is a half wave length so that the excitation of wave energy produced by these probes is of opposite polarity. This arrangement is conventionalfor introducing or deriving the second order TE mode and is shown for convenience but it will be understood that any of the known equivalents may be employed.

Before considering the operation of the system certain simple relations affecting the propagation in a rectangular guide should be noted, viz:

(a) The cut-elf wave length A i related to the width a and height I) of the guide by the equation:

AFFFY where m and n are mode index numbers. From this relation it follows that to pass the TB mode the width a must be greater than A/ 2, where is the free space wave length. If the guide is also to pass the TE mode the Width a must be greater than )t.

(b) The phase velocity of wave energy in a guide is greater than the free space velocity and differs for each mode. The wavelength in the guide is correspondingly increased and for the TE and TE modes is defined respectively by the relations:

a As an example of a practical case, the free space wave length of the microwave energy is 3.2 cm. and the guide dimensions are chosen with a=4 cm. and b=l cm. in order to propagate both the fundamental and second order TE modes. For these dimensions the guide wave lengths are hgl=3.5 cm. and Ag2=5.33 cm.

Consider now the operation of the system of FIG. 1 with reference to FIGS. 2 and 3 of the drawings. In FIG. 2 the guide 10 is again illustrated showing the positions of

probes

18, 19 and 20 in view A. View B is a standard .type of illustration of the potential or electric field distribution of mircrowave energy at the mouth of the guide for the fundamental TE mode wherein the potential distribution is indicated by vertical vectors which are concentrated at the center. Referring to FIG. 3, graph A, this potential distribution across the mouth of the guide is indicated by curve M which shows that the electric intensity is maximum at the center of the guide and diminishes sinusoidally to zero at the edges of the guide.

For the second order mode view C of FIG. 2 shows the TE distribution of the electric potential across the mouth of the guide and again referring to graph A of FIG. 3 this distribution is illustrated for two different values of intensity, by curve NN' for a low intensity of the second order mode and by curve OO for a greater intensity of second order mode. It will be noted that the portion of the curve N is shown in solid line to indicate, say, positive polarity of the potential relative to the portion N shown in dotted line to indicate negative polarity. Similarly O and 0' indicate positive and negative intensities.

The field distribution across the mouth of the guide at distances, x, from the center, 0, may be written as:

e =E cos ITJC/ a cos wt for the TE mode (4) and as:

e =KE sin 21rx/a cos (w.+) for the TE mode (5) where a=width of the guide E=rnaximurn field intensity K=constant w=27rf=angular frequency =phase difference Referring again to the wave guide of FIG. 1, it will 7 2,994,869 n 7 W V be noted that the location of the probe 18 for propagating the fundamental mode TB in the rectangular guide is in the center line of the guide and will preferably be located a distance from the closed rear wall of the guide by a distance labeled L3. This distance L3 will generally be chosen to be a quarter wavelength, Agl/4, in order to help match impedances of the guide and the coaxial line 14.

Similarly it will be noted that the probes l9 and 20 for propagating the second order mode are located at positions which are each substantially half way between the center line of the guide and a side wall. These probes are located a distance L4 from the rear wall of the guide which is also substantially a quarter wavelength, )tg2/4.

It will be clear that, since lgl is less than l\g2, the location of probe 18 is closer to the rear wall of the guide than the locations of probes 19 and 2b. The distances of the probes from the mouth 11 labeled L and L are also dif ferent and it will be evident that the waves will arrive at the mouth of the guide with a phase difference as in dicated in the equations. However, unit 16 may be adjusted to control the amplitude and phase of the second order mode and in accordance with the invention the phase adjustment will be made such that the phase difference measured at the mouth of the guide is ninety electrical degrees. The amplitude, as will presently be explained, will be chosen to produce a predetermined directivity of the major lobe as measured in a plane normal to the electric vector.

The radiation pattern is shown in FIG. 2 as a single major lobe with two small minor lobes and is a somewhat idealized illustration but one which substantially shows the operation. Shown in solid line the lobe 21 is normal to the mouth of the guide and represents the radiation pattern that would be produced by exciting the guide with only the fundamental mode for transmission or utilizing only the fundamental mode in reception. In other words the directivity is along the center line or axis of the guide when only the fundamental mode is employed. For this condition the phase of wave energy at all points across the mouth 11 is the same. This condition is indicated by the vertical vectors in the diagram B of FIG. 3. Here points x x etc., equally spaced across the guide mouth, have been chosen to picfront in the plane of the mouth 11 to propagate the major part of the energy normal thereto. It is also well known that if the amplitude is tapered so that the radiation is a maximum at the center and diminishes gradually to zero at the edges then the minor lobes of the radiation pattern are reduced although the major directive lobe may be somewhat broadened.

If the phase distribution varies linearly across the mouth 11 the wave front will be tilted to direct the ma or lobe at any angle 0. For radiation directed at an angle 0, a good distribution of amplitude and phase at the mouth 11 would be one of sinusoidal amplitude to reduce minor lobes and of linear phase to determine the directivity. This distribution can be defined by the equation:

11-2: :04 e E'eos a cos (wt+; (6)

Where I is the total phase shift from the center 0 to an edge of the guide.

The wave energy received at a distant point in a direction of angle 0 from an element of aperture may be written as and a maximum of radiation will be received at that point when, for all elements,

:0 2am a/-2-+ T (308 0- 0 Accordingly for linear phase distribution a total phase shift of is the amount required to tilt the major lobe in the direction 0. Such a tilted lobe 21. is shown in dotted line in FIG. 2.

The described amplitude-phase distribution can be approached closely for small angles a of tilt from the axis of the guide (e.g., large angles 0) by introducing the second order mode. Consider the potential distribution across the mouth of the wave guide due to the fundamental mode to be as indicated by curve M in FIG. 3 and that a certain amount of second order mode is supplied to the guide by adjustment of the amplitude in unit 16 of FIG. 1 and that this amount is represented by the curve N, N. If we further consider that an adjustment is made in unit 16 to control the phase of the second order mode energy supplied to

probes

19, 20 so that the wave, when propagated in the guide, will reach the mouth of the guide 11 in phase quadrature with that energy supplied and propagated in fundamental mode, then the wave energy at the mouth of the guide is the vector sum of the fundamental and second order modes.

That is, the phase of the second order mode is in time quadrature and hence corresponding points on curves M and N, N must be added in quadrature. 'This is shown by the vectors for positions x x and x in B of FIG. 3. The component of N for position x is therefore the small vector 50 indicated as lagging 90 from the reference vector and the addition produces a resultant vector labeled to indicate that the resultant vector lags the reference vector by that amount. For the position x the component of curve N is a larger vector, as illustrated, lagging 90 and its. addition to the reference vector is indicated as a resultant vector labeled 20 to indicate that the resultant lag is 20 from the reference vector. For position x the component of curve N to be added in quadrature is smaller but the reference vector to which it is added is also smaller so that the resultant vector is as labeled, 28. The component and resultant vectors have not been drawn for the left hand side of the diagram but it will be evident that since these components of curve N are of opposite polarity to those of curve N the resultant vectors for negative distances x from the center of the mouth of the guide will lead the reference vectors. In other words, the resultant vectors for points x on the left side of the guide will be conjugate to those on the right side. If similar vectors were drawn for all positions x along the guide, it will be evident that the angle of lead or lag relative to the center 0 is substantially linear. The example has illustrated that for the distance x the lag is 10, for the distance x which is twice x the lag of 20 and for the position of x;; which is three times the distance x the lag is 28 which is nearly that of the ideal of- 30". In other words the injection of second order mode components corresponding in amplitude to the potential distribution N, N' and arriving at the mouth of the guide in phase quadrature to the wave energy of fundamental mode produces substantially a linear phase distribution of wave energy across the mouth of the wave guide and where near the side walls of the guide this phase distribution begins to fail,-the amplitude of the energy is so relatively small that the failure of linearity 6 of phase in this region is negligible. It therefore follows from graphical description that injecting a small component of second order mode will produce a tilt of the major lobe of the directional pattern as indicated for example by the lobe 21' of FIG. 2.

If now we consider the injection of wave energy of the second order mode in larger amplitude as shown by the curve 0, O of FIG. 3 and consider that the adjustment of 16 has been so made that again the wave energy arrives at the mouth of the guide in phase quadrature to the energy arriving in the fundamental mode, then the vector addition as illustrated in B of FIG. 3 indicates that for positions x x and x the phase lag is respectively 20, 36, and 44 which is still not too far from the ideal phasing of 20, 40 and For the posit-ions x x x similar but leading conjugate vectors will be produced. It will be evident, however, that the departure from the linear phase distribution across the mouth of the guide is greater for larger injections of the second order mode so that while the major lobe will be turned or tilted through a larger angle we may expect that the lobe will be somewhat distorted and that minor lobes of greater intensity may appear. It follows, therefore, that directional control of the radiated major lobe pattern can be accomplished by employing components of the fundamental and second order modes for angles a that do not depart too far from the axis of the guide. For most purposes, particularly in a radar system for angle or range tracking where use of the antenna system of the present invention is contemplated, the degree of control it provides is entirely adequate.

The description of operation has been chiefly from the transmitting point of view in showing that the introduc: tion of a second order mode component will produce a substantially linear phase characteristic at the mouth or open radiating end of the guide. However, as in all antenna radiating systems, the reciprocity theorem operates so that an incoming wave incident at the open end 11 will be received with similar directivity.

Thus, for reciprocal receiving operation with the same directional pattern, in the FIG. 1 arrangement the unit 16 serves to proportion the energy translated in second order mode to unit 13 and to adjust the phase.- In practice no actual unit 16 need be employed but instead a single coaxial path from 13 to the

probes

19 and 20 might be employed having a length relative to the path 14 chosen to produce the desired relative phase and having a characteristic impedance chosen to proportion the amplitudes of the modes. Suitable matching sections will of course be employed where needed in a manner well known in the art.

Referring now to FIG. 4 there is illustrated in block diagram an alternative arrangement of the present invention which employs only two probes which are 10 cated in the guide 10 at positions 19 and 20'. These positions are spaced from the walls of the guide as in FIG. 1 for the positions of

probes

19 and 20 but are to' be here employed to establish both fundamental and second order modes. Therefore their location relative to the closed end of the guide is roughly an average quarter wavelength; that is the distances L will be an average of Agl/4 and AgZ/ 4. The coupling to the probes 19' and 20' is via the two

paths

14 and 15 corresponding to the similarly labeled paths of FIG. 1 and thence through a hybrid circuit 22. Hybrid circuits are now well known in the art and generally for microwave operation will be in the form of a ring guide circuit or a magic-T guide circuit of the types shown, for example, in US. Patent to' W. A. Tyrrell No. 2,445,895 issued July 27, 1948. The hybrid circuit 22 here indicated by block diagram is characterized by having two

balanced positions

23 and 24 where the

paths

14 and 15 connect and two

side positions

25 and 26 where connections are made to the probe locations 19' and 20 respectively. Considering the operation from the transmitting point of view, energy supplied via connection 14 to position 23'will be translated from

side positions

25, 26 to the probe locations 19' and 20' in time phase but will not be translated to position 24 and back into line since 23 and 24 are balanced or conjugate positions. Similarly wave energy supplied via connection 15 to position 24 will be translated from

side positions

25 and 26 in opposite phase to probe locations 19' and but will not be translated to the balanced position 23 and back into line 14.

Because of reciprocity of operation discussed above, it will be evident that, in receiving, a wave incident at the mouth of the guide will produce a component of fundamental mode which will be propagated down the guide to elements 19' and 20 and a component of second order mode which will be propagated at a different velocity down the guide to these same elements, the relative amplitude of these components corresponding to the direction of arrival of the incident wave. The wave of fundamental mode travelling in the guide, is of equal intensity at these probe locations and in time phase and therefore passes to the hybrid circuit at positions and 26 and leaves the circuit at position 23 via path 14 but does not produce any output at the balanced position 24. The second order mode, however, travelling down the guide provides equal amplitudes but opposite phase of wave energy at the probe positions 19' and 20' and this energy which also enters the hybrid circuit at

positions

25 and 26, being of opposite polarity or phase at these two positions, leaves the hybrid circuit at position 24 by con nection 15 but does not produce any output at position 23. The arrangement therefore operates in a manner entirely similar to that described for the arrangement of FIG. 1 in propagating both fundamental and second order modes of wave energy in the guide 10 but some compromise has been introduced to reduce the arrangement to two probes. Referring now to FIG. 5 an improved arrangement is shown for independently translating wave energy of the fundamental and second order modes to and from a rectangular duo-mode wave guide. The arrangement insofar as it relates to efliciently and independently coupling between wave guides of single mode and a duomode guide is not a part of the present invention. Its use in a duo-mode guide antenna system in accordance with the present invention is, however, preferable to that of the FIGS. 1 and 4 arrangements since it provides a better matching of impedences for the two paths between the wave energy apparatus 13 and the guide 10. Here the standard section of single mode wave guide 14' corresponds to the feed 14 of FIG. 1, the standard section of wave guide 17 corresponds to the coaxial feed line 17 of FIG. 1 and the section 10 corresponds to the duomode guide radiator 10 of FIG. 1. It will be evident that the arrangement is a form of magic-T hybrid circuit for coupling to a duo-mode guide. and 17' as joined to the duo-mode section 10 correspond to the hybrid circuit 22 of FIG. 4.

Referring now to FIG. 6 there is illustrated a preferred embodiment of the invention in a radar system wherein the antenna structure comprises a rectangular duo-mode wave guide radiator having dimensions chosen to translate Wave energy simultaneously in the fundamental TE mode and in the second order TE mode together with means for utilizing the guide to radiate the Wave energy in the fundamental mode. The arrangement includes means for utilizing the guide to receive a reflected part of the energy in both of the modes together wtih means for combining and detecting the received energy and utilizing the detected energy to indicate the direction of arrival of the reflected energy. In the diagram elements corresponding to those illustrated in FIG. 1 are similarly labeled. Thus 27 is an antenna structure which includes the duo-mode guide 10 having a radiating open end 11 directed toward the reflector 12. The guide is equipped with a probe 18 as in FIG. 1 for The guide sections 14 coupling the fundamental mode and probes 19 and 20 connected by a half Wavelength path as in FIG. 1 for coupling the second order mode. The antenna structure 27 is arranged to be oriented in azimuth and in elevation by means of a control mechanism indicated generally by block unit 28 and a control connection 29. Unit 28 may be an arrangement partly mechanical and partly electrical such as a servo control link and therefore has been indicated in dotted line. The transmitter 13 is here assumed to be a radar microwave pulse transmitter which is controlled via a timer unit 31} to transmit narrow pulses of microwave energy at a chosen repetition rate determined by the timer. The wave energy output of 13 is coupled to duplexer 31, one output of which is coupled via path 14 to probe 18. Another output of 31 is coupled to the detector 32 of a receiving channel. Duplexer 31 may be of any well known form, its function being to translate the wave energy from 13 to probe 18 while preventing the energy from entering detector 32. Conversely unit 31 operates to translate received echo signals from probe 18 to detector 32 and prevents the received energy from entering transmitter 13. Wave energy from oscillator 33 is also coupled to detector 32. The output of detector 32 is coupled to IF. amplifier 34 and one output of 34 is coupled to an amplitude detector 35. The output of 35 is coupled to an indicator 36 for indicating the reflected wave. Indicator 36 may be, for example, an oscilloscope for indicating range. A second channel connecting to

probes

19 and 20 for receiving the second order mode is comprised of detector 37 to which an output of oscillator 33 is coupled via a phase control unit 38. The output of detector 37 is coupled to LP. amplifier 39 which in turn is coupled to phase detector 40. An output of LP. amplifier 34 is also coupled to phase detector 40. The output of phase detector 40 is coupled to a tracking circuit unit 41. The control potential output of tracking circuit 41 is supplied by the connection 29 to control unit 28 to orient the antenna structure 27 to track the received echo signal.

The unit 41 may also comprise range circuits for tracking the chosen target in range and to exclude echo signals from targets of other range. For this purpose synchronous operation with the pulse repetition rate is provided by the coupling connections from timer 39. The output from unit 41 to indicator 36 provides a suitable time base synchronous with the pulsing rate for displaying the signal output of detector 35.

Considering now the operation of the system, pulses of microwave energy from transmitter 13 enter duplexer 31 where the energy is prevented from reaching detector 32 and is directly translated via path 14 to probe 18 to energize the wave guide radiator 10. The transmitted energy is therefore established within guide 10 entirely in fundamental mode and so travels to the mouth 11 of the guide to be radiated via reflector 12. The energy propagated in the guide is balanced with respect to

probes

19 and 20 which select and translate only wave energy of second order mode and therefore cannot reach the input of detector 37. The arrangement thus far described comprises means for utilizing the guide 10 to radiate energy in the fundamental mode.

Consider now the operation of the system in receiving when wave energy from a particular reflecting object is received by the antenna structure and enters the mouth 11 of guide 10. This wave energy, if it arrives from a direction which is not normal to the mouth of the guide, produces components of fundamental mode and of second order mode. The component of fundamental mode is picked up by the probe at location 18 and translated via connection 14 to duplexer 31 where it is prevented from reaching transmitter 13 but is translated to detecor 32. In the detector 32, to which wave energy from oscillator 33 is also coupled, the received signal is converted to intermediate frequency which is amplified by unit 34. The LP. energy from 34 is supplied to amplitude detec- 9. tor-35 where it is detected to provide a video signal. The video signal output 35 is supplied to indicator 36 to indicate this received signal.

The component of second order. mode is picked up by the

probes

19 and 20 and conveyed via the connection 17 to the input of detector 37. Wave energy from beating oscillator 33 is also supplied to detector 37 in phase determined by the adjustment of unit 38. The output of 37 is therefore wave energy of the same intermediate frequency as that translated by I.F. amplifier 34 but of predetermined phase as determined by the setting of phase control unit 38. The LP. energy output from unit 34 and LP. energy output of unit 39 are both supplied to the phase detector 40. Phase detectors of a variety of forms are known in the art so that no particular form of phase detector need be described here. The book Waveforms, vol. 19 of the Radiation Laboratories Series-McGnaw-Hill Book 00., pages 511-513 illustrates and described a number of suitable phase detector circuits.

The operation of the phase detector may be visualized by reference to the vector diagrams of FIG. 7. The LF.

wave energy from unit 34 constitutes a voltage of reference. phase and is so indicated in FIG. 7 by the vertical vector at A; The wave energy output of intermediate frequency from I.F. amplifier 39 is set by phase control unit 38 to be in quadnature phase. This is illustrated at B,of FIG. 7 where a resultant vector is shown which is the sum of the reference vertical component from unit 34 and the horizontal quadrature component from unit 39. The phase of the resultant vector relative to the reference vector is measured by the phase detector 40 which produces a voltage of amplitude and polarity corresponding to the magnitude and sign of the phase angle. If the incoming wave energy is arriving from an angle which is further from the direction line of the antenna the quadrature component will be greater and the vector diagram C of FIG. 7 illustrates this condition where the resultant vector has a greater phase angle. If the incoming wave is from an opposite direction the quadrature component will be reversed in sign and the resultant vector will have an opposite phase angle. It will be evident therefore that the phase detector 40 has a characteristic such as that shown in FIG. 8. FIG. 8 indicates that when the wave energy is arriving in the direction line of the antenna the output of phase detector 40 is zero as shown at O in the diagram but that for negative angles, a, of arrival the signal output is a voltage of positive polarity and for positive angles of arrival the voltage is of negative polarity. It will be clear therefore that this characteristic may be employed to provide angle tracking in a manner well known in the 'art. Thus if the incoming signal is arriving from a positive angle a from the direction line of the antenna, negative voltage will be supplied via the connection 29 to control unit 28 which then operates to turn the antenna in a direction to make this voltage zero. If the incoming signal is from a negative angle a the voltage supplied by connection 29 will be positive to turn the antenna structure in the opposite direction so that the antenna structure will automatically seek and center itself so that its direction line will coincide with the direction of arrival of the incoming signal.

FIG. 9 is a redrawing of the microwave portion of FIG. 6 to illustrate in schematic form the microwave guide assembly. The duo-mode radiator 10' and connecting

guides

14 and 17 are of the preferred form shown in FIG. 5. The transmitter 13 is labeled to indicate the use of magnetron and the duplexer 31 is here comprised of the anti-transmit-receive (A.T.R.) unit 31 and the transmit-receive (T-R) unit 31" commonly employed in microwave guide apparatus. The

detectors

32 and 37 are indicated as crystal detectors located respectively in

resonant cavities

42 and 44. Oscillator 33 which may be of the klystron type is coupled to cavity 42 via a coupling loop 43 and to cavity 44 via a coupling 10 loop 45. The line 46 connecting oscillator 33 to cavity 44 may be of a chosen length to provide suitable phasing and so becomes the equivalent of the phase adjusting unit 38 of FIG. 6. I

The arrangements shown and described above disclose a directive antenna system for controlling and utilizing directivity in one dimension, say, the horizontal or the vertical. For controlling and utilizing directivity in two dimensions a pair of duo-mode guides may be employed or a single guide can be chosen and arranged for operation with the fundamental and two other second order modes in accordance with the principles of the invention as described for one dimension.

The invention has been illustrated for operation with the TB and T13 modes in a rectangular guide but it will be clear to those skilled in the art that other related modes may be used and other than rectangular guides may also be employed. For example it will be evident that a round wave guide operating in the TE and TE modes may be so employed.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A microwave antenna system comprising a duomode wave guide having a radiating open end, said guide having dimensions chosen to propagate wave energy simultaneously in the fundamental and second order modes and means operatively associated with said guide for proportioning the relative amplitude and phase of wave energy propagated in said two modes to produce a radiation pattern having a major lobe of predetermined directivity.

2. A microwave antenna system comprising a rectangular duo-mode wave guide having a radiating open end, said guide having dimensions chosen to propagate wave energy simultaneously in the TE and TE modes and means operatively associated with said guide for proportioning the relative amplitude and phase of wave energy propagated in said two modes to produce a radiation pattern having a major lobe of predetermined directivity.

3. A microwave antenna system for producing a radiation pattern having a major lobe of predetermined directivity comprising a duo-mode wave guide having a radiating open end, said guide having dimensions chosen to propagate wave energy simultaneously in the fundamental and second order mode, means for separately translating the wave energy of said modes to or from said guide over two different paths and means for adjusting the relative amplitude and phase of the wave energy translated over said paths.

4. A microwave antenna system for producing a radiation pattern having a major lobe of predetermined directivity comprising a rectangular duo-mode wave guide having a radiating open end, said guide having dimensions chosen to propagate wave energy simultaneously in the TE and TEQU mode, means for separately translating the wave energy of said modes to or from said guide over two diiferent paths and means for adjusting the relative amplitude and phase of the wave energy translated over said paths.

5. A microwave antenna system for use with wave energy apparatus to produce a radiation pattern having a major lobe of predetermined directivity comprising a duo-mode wave guide having a radiating open end, said guide having dimensions chosen to propagate Wave energy simultaneously in the fundamental and second order modes, means coupling between said guide and said apparatus comprising means for separately translating the wave energy of said modes between said apparatus and said guide over two different paths and means included in one of said paths for adjusting the relative amplitude and phase of the wave energy translated over said paths.

6. A microwave antenna system for use with wave energy apparatus to produce a radiation pattern having a major lobe of predetermined directivity comprising a duo-mode wave guide having a radiating open end, said guide having dimensions chosen to propagate wave energy simultaneously in the TE and TE modes, means coupling between said guide and said apparatus comprising means for separately translating the wave energy of said modes between said apparatus and said guide over two different paths and means included in one of said paths for adjusting the relative amplitude and phase of the wave energy translated over said paths.

7. In a radar system an antenna structure comprising a duo-mode wave guide radiator, said guide having dimensions chosen to translate wave energy simultaneously in the fundamental and second order modes, means for utilizing said guide to radiate energy in said funda mental mode, means for utilizing said guide to receive a reflected part of said energy in both of said modes, means for translating the wave energy received in said modes separately over two different paths, means for effectively adjusting the relative phase of the output energy from said paths and means for combining and phase detecting said energy to produce a potential indicating the direction of arrival of said reflected energy.

8. In a radar system an antenna structure comprising a duo-mode wave guide radiator, said guide having dimensions chosen to translate wave energy simultaneously in the TE and TE modes, means for utilizing said guide to radiate energy in said TE mode, means for utilizing said guide to receive a reflected part of said energy in both of said modes, means for translating the wave energy received in said modes separately over two different paths, means for efiectively adjusting the relative phase of the output energy from said paths and means for combining and phase detecting said energy to produce a potential indicating the direction of arrival of said reflected energy.

9. A microwave antenna system comprising a duomode wave guide radiator having a radiating open end, said guide having dimensions chosen to propagate wave energy simultaneously in the fundamental and the second order modes and means operatively associated with said guide for utilizing the relative amplitude and phase of wave energy propagated in said two modes to control the directivity of the antenna.

10. A microwave antenna system comprising a rectangular duo-mode wave guide radiator having a radiating open end, said guide having dimensions chosen to propagate wave energy simultaneously in the T15 and TE modes and means operatively associated with said guide for utilizing the relative amplitude and phase of wave energy propagated in said two modes to control the directivity of the antenna.

References Cited in the file of this patent UNITED STATES PATENTS Montgomery Mar. 18,

OTHER REFERENCES Reprint from Proc. IRE, vol. 37, No. 9, September 1949, p. 1031.