US3303505A

US3303505A - Broadband linear slot antenna with impedance matching network

Info

Publication number: US3303505A
Application number: US685996A
Authority: US
Inventors: Wilfrd H Bacon; Byers Gordon; Katchky Max
Original assignee: Canadian Arsenals Ltd
Current assignee: Canadian Arsenals Ltd
Priority date: 1957-09-24
Filing date: 1957-09-24
Publication date: 1967-02-07
Anticipated expiration: 1984-02-07
Also published as: GB1219073A

Description

Fb. 7, 1967 w, BACON ET AL 3,303,505

BROADBAND LINEAR SLOT ANTENNA WITH IMPEDANCE MATCHING NETWORK Filed Sept. 24, 1957 3 Sheets-Sheet 1 Feb. 7, 1967 BACON ET AL 3,303,505

BROADBAND LINEAR SLOT ANTENNA WITH IMPEDANCE MATCHING NETWORK Filed Sept. 24, 195'? s Sheets-Sheet s United States Patent 3,303 505 BROADBAND LINEAR S LOT ANTENNA WITH IMPEDANCE MATCHING NETWORK Wilfred H. Bacon, Hugh Gordon Byers, and Max Katchky, all of Scarboro, Ontario, Canada, assignors to Canadian Arsenals Limited, Ottawa, Ontario, Canada,

a corporation of Canada Filed Sept. 24, 1957, Ser. No. 685,996 4 Claims. (Cl. 343-771) This invention relates to linear array antennae for use at microwave frequencies and more particularly to squintfree broadside linear array antennae.

A linear array is defined as one in which the beam pattern is formed by a series of radiating elements disposed along a common linear axis, the shape and direction of this beam pattern being controlled by the relative amplitude and phase of the currents in the elements of the array; the spacing of the elements along this axis; and the coverage diagram of the individual elements of the array. A broadside array is one in which the resultant beam pattern is normal to the axis of the array and a squint-free broadside array is one in which this normal condition is preserved over an appreciable range of frequencies. For a general description of the properties and characteristics of linear array antennae reference may be had to chapter 9, volume 12, of the Radiation Laboratory Series, Massachusetts Institute of Technology.

The preferred embodiment of the invention to be disclosed in this specification is a broadside linear array when used as a beacon antenna. Employed in this capacity, the antenna is used as part of an interrogation system working in conjunction with other primary search or allied radars. In this role, the transmitter associated with the interrogator system sends out a pulse on one frequency and receives a reply from the interrogated ob- 'ject, such as a ship, plane, tank, etc., on a different frequency. Since the system must transmit to, and receive from, the same object, although on different frequencies, the directivity of the antenna must be preserved over a band of frequencies, that is, no squint must be introduced by the difference in the two frequencies. Other types of antenna have been employed in this role but the linear array, or more particularly the broadside array, are to be preferred because of their peculiar property of being able to produce a beam of the required pattern from an antenna having an appreciable length but relatively small width and depth. This enables it to be mounted on the same support structure as the antenna of the primary radar, with little addition of weight and slight structural alteration.

The primary requirement of a broadside array when used in conjunction with a beacon system is that the array must retain its radiation properties over an appreciable band of frequencies, in general a band 20% about a given central frequency is considered desirable. The chief properties'which must be preserved over this frequency band are:

(a) The antenna azimuth pattern must change as little as possible.

(b) The vertical coverage must be quite broad and in general match that of the associated primary radar.

(c) The beam width of the antenna must be the minimum possible, compatible with the maximum length of the array, which in turn is dictated by the size and configuration of the associated primary radar.

(d) Any secondary or side lobes should be as low as possible with respect to the main beam, and must be down sufiiciently to permit the unambiguous transmission and reception of information.

(e) The impedance match of the antenna over the frequency range must be preserved so that the voltage standing wave ratio (VSWR) is held within reasonable limits.

The above conditions having been met, it is further required that the mechanical properties of the antenna be given consideration, with a view to reducing, as far as is practicable, the size and weight of the antenna, and also to render it as economically and simple produced as possible.

Broadside arrays acting as beacon antennae and meeting to some extent the above requirements, are known, however, they all suffer from one or more deficiencies, the chief of which are:

(a) The power feed systems used to distribute power to the radiating elements in the correct proportions to give the required beam shape with low order side lobes, are all somewhat complex and required factory facilities not norm-ally available for field maintenance.

(b) The essential squint-free condition has not been met; that is, the beam pattern changes its principal direction as the frequency is altered. This is caused chiefly by a relative phase change in the microwave energy emitted by the individual radiating elements.

(0) It has not proved practical to maintain, within acceptable limits across the required frequency band, the impedance match of the antenna to the microwave transmitter and receiver. The principal impedance mismatch being introduced by the frequency selectivity of the individual radiating elements used.

A significant improvement in the above undesirable conditions is brought about in the antenna disclosed in this specification.

An object of this invention is to provide a radiating element for use in broadside array antennae whose impedance is held within acceptable limits over a prescribed frequency range.

Another object of this invention is to provide abroadside array antenna with a tapered power feed system which is simple and easy to make, and which can be readily maintained in field service.

A further object of the invention is to provide a broadside array antenna with a simple feeder system whereby the relative phase of the microwave energy emitted by the individual radiating elements in the broadside array is held constant over a broad frequency band.

The principles of the invention will be more readily understood from the following etailed description of the preferred embodiment, in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of the antenna,

FIG. 2 is a plan view of the top of the antenna,

FIG. 3 is a cross sectional view of one of the radiating elements along the line III-III of FIG. 2,

FIG. 4 is a front elevation of one of the radiating elements with the beam forming plates and power feed omitted,

FIG. 5 is a top plan view of one of the power feed sections with the upper conducting plate removed,

FIG. 6 is a perspective view of one of the power feed sections with the upper conducting plate shown in a raised position,

FIG. 7 shows diagrammatically the power distribution in a feed section, and

FIG. 8 shows diagrammatically the impedance network of a power feed section.

With reference to FIGS. 1 and 2 it will be seen that the antenna comprises a linear series of radiating elements 3, ten being shown for purposes of illustration though obviously the number used is dependent upon the performance characteristics required within the overriding limitation of the physical space available. Energy which is fed into each of these radiating elements is radiated through two horizontal slots 4 in each element, the

3 resulting horizontal beam pattern being a composite func tion of the number of slots and the power distribution along them and the vertical coverage being primarily controlled by the beam forming plates 1 which extend the full length of the array.

Power is fed to each of the radiating elements through two identical, dissymmetrical, feed sections and 6 which are coupled by cables 7 to each of the radiating elements and also to a T-junction 9, into which is fed the microwave energy from the associated transmitter.

Turning now to FIGS. 3 and 4 it will be seen that the radiating element is essentially a length of waveguide formed into a boxlike structure by plates which short circuit each end, and having two identical horizontal slots 4 symmetrically displaced on either side of the vertical centre line of the broad face of the waveguide section but displaced from the horizontal centre line through that face by approximately two thirds of the distance from this centre line to the top edge of the waveguide section.

The power coming from the feed section 6 through the cables 7 enters the radiating element through a connector 8 and is introduced into the waveguide section by a probe 10. The slots 4, due to their position on the broad face of the waveguide section, transfer this energy into free space.

The

power feed sections

5 and 6 are, as stated, identical but dissymmetrical and the following description of power section 5 will therefore apply with equal validity to power section 6.

The power feed section is required to accept microwave energy and to distribute this energy between the five radiating elements associated with this feed section in accordance with a definite ratio determined by mathematical analysis. This power division is done by a series of quarter wave transformers where the impedance transformations are accomplished by varying the diameter of a centre conductor positioned midway between two conducting plates, the combination thus acting as an unbalanced line of varying impedance. 7

Referring to FIGS. 5 and 6 the two parallel conducting plates 11 and 13 are separated by polystyrene spacers 12 which also serve to support the centre conductor structure and provide the mounting for the output connectors 21. Power entering the feed section through the righthand connector 21 is transmitted by the quarter wave section having centre conductor 14 to the first junction where it is divided between the quarter wave section having centre conductor 15 and conductor 23 in accordance with the required impedance matching, the power fed to the centre conductor 23 goes through connector 21 and then to the first radiating element to the left of the centre line of the antenna. The power donated to quarter wave section 15 at this first junction is in turn further divided between quarter wave section 16: and the next conductor 23 feeding the second element from the centre line on the antenna. This process of impedance transformation and coupling to one of the radiating elements is continued through

quarter wave sections

16, 17, 18, 19 and 20, section 20 being directly coupled to the connector 21 for the last outer radiating element. It will be noted that certain of the impedance transformations are accomplished by varying the diameter of the center conductor in two steps i.e. two quarter wave sections rather than one, This, as will be described later, is due to the necessity of keeping the impedance transformation in any one section below a certain limit.

As previously stated, the feed section is connected to the individual radiating elements by lengths of cable 7 having a common impedance. These cables are of varying length decreasing from the cables connecting the first power take-ofis to the first radiating elements on either side of the centre line of the antenna, down to the shortest cables which join the ends of the feed sections to the 4 outermost radiating elements. These cables are of such a length that the microwave energy reaching each radiating element is in phase with that reaching the other elements, that is to say, the electrical length of the path from the T section 9 to each of the radiating elements 3 is the same.

In understanding the theory of operation of the antenna it is necessary only to consider one half of the antenna since the structure is symmetrical about the normal to the array and the following description is, therefore, related to only one feed section feeding, in the embodiment described, five radiating elements, though of course the complete antenna would comprise two feed sections feeding ten radiating elements.

Since it is felt that a better understanding of the invention will result thereby, practical results are included in thefollowing description of the power feed section. This data is for a broadside array having ten radiating elements i.e. five elements associated with each feed section, and a theoretical horizontal beam pattern in which the side lobes are 34 db down from the main beam. These figures are for illustration purposes only and should not be construed as placing any restrictions on the scope or spirit of the invention.

As stated, the correct power taper is determined by mathematical analysis. This is done by obtaining a formula from known radiation theory for the power radiated by the half antenna in any direction between the normal to the array and its axis, substituting arbitrary constants for the value of the current fed to each of the individual radiating elements, placing this formula in a form suitable for comparison with a polynomial defining the curve which describes the theoretical beam pattern required and, by equating coefiicients, obtaining the correct current ratios required at the radiating element so as to produce this pattern. This method of comparing a generalized radiation pattern formula with the idealized pattern given by the (Tchebysheff) polynomial appropriate to the number of radiating elements used is common in the study of linear arrays, and a more detailed description will be found in any of the standard works of reference, including volume 12 of the M.I.T. Radiation Laboratory Series. Its inclusion here is therefore unnecessary and it will suifice to list the current distribution and hence the corresponding power distribution determined from a consideration of the general radiation formula and the associated polynomial for the five elements of the half array under consideration, as follows:

where 1;, I I I and I denote the calculated currents fed to the five elements of the half array, I being that for the innermost'element, I for the next, and so on to I for the outer element of the half array. The power taper required to give the correct azimuth beam pattern thus being known, the feed section is designed to divide the power it receives from the transmitter in accordance with the ratios worked out for the different elements. Its method of doing this uses three basic principles.

The first is that power fed to a Y-junction will divide between the two arms of the junction in inverse proportion to the impedances presented by the two arms. Considering a junction A having arms A and A power P fed to this junction will be divided between the two arms A and A in the ratio of 1 2:552 A2 AI where Z and Z are the impedances presented to the Y-junction by the two arms of the junction.

The second principle concerns impedance transformation by quarter wave transformers. A quarter wave transformer is a section of transmission line or waveguide having a length equal to one quarter of the wave length of the frequency to be propagated and it can be shown that for such a quarter wave length of transmission line terminated in an impedance Z,, the impedance looking into the quarter wave length so terminated is given by where Z is the characteristic impedance of the quarter wave length line and Z, is the impedance looking into this line. Conversely where the required terminating and input impedances are known it will be seen that the characteristic impedance of the quarter wave length of line required to bring about this transformation is given by Z /Z,Z,

Where the impedance transformation is a large one it is found that the characteristic impedance of the line would reach an undesirably high value if only one quarter wave section were used. This is due to the fact that, as will be seen from the next section, the higher the impedance the narrower the centre conductor in a parallel plate system becomes. This makes for difficult manufacture and lack of mechanical rigidity. In addition an unusually high impedance transformation results in a more rapid deterioration in impedance match as the frequency is changed. For these reasons two quarter wave sections are used wherever the characteristic impedance required would be high using only one quarter wave section. In this case, denoting the characteristic impedances of the two quarter Wave sections by Z and 2 respectively, then 2,, the input impedance looking into the two quarter wave sections terminated by an impedance Z is given by The third basic principle governs the means of obtaining the characteristic impedance required. This is done by controlling the relative dimension of a centre conductor in between two parailel conducting plates when the characteristic impedance is given by where Z is the characteristic impedance of the line, K is the dielectric constant of the material used, which could be any non-conducting medium, such as polystyrene, though in this case since simplicity and ease of manufacture were desired, air was used,

It is the distance between the two parallel conducting :plates, and

d is the size of the centre conductor.

Thus, for a fixed value of h the characteristic impedance may be raised by reducing d, that is reducing the size of the centre conductor. However, for high values of characteristic impedance, the centre conductor may become so small that it becomes undesirable from a mechanical point of view, in which case, as previously stated, the impedance transformation is made in two steps, using quarter wave sections with lower characteristic impedances.

In FIG. 7 is shown a diagrammatic representation of the power entering and leaving the feed section. It will be seen that it consists of a series of Y-junctions A, B, C, D, at each of which power entering the junction is fed into two arms, one arm at every junction being connected to one of the radiating elements, the other arm transmitting power for the remaining elements to be fed. Thus power P entering the feed system at E is divided at junction D into power P which proceeds on to junction C, and power P feeding the first radiating element in the ratio where Z and Z are the impedances presented to the junction D by its two arms.

Power P is, in turn, further divided into P and P controlled by impedances Z and Z presented by its arms to junction C, and so on down the line to the last junction A where the power is divided between the re- .maining two radiating elements. The known factors here ,are PAI, PAQ, P32, peg and P132 have been determined from the previously mentioned mathematical analysis. Since P and P are known, then is also known and hence P can be found. Also known are the values of the impedances of thearms of the various junctions connecting directly to the radiating elements, namely Z Z Z and Z These are all identical and are of known value, namely the characteristic impedance of the matched cable and radiating element.

Considering then firstly junction A, P and P are known and also Z Thus since the required power division is known the required impedance division is known and 2 can be found.

Considering junction B, P is known, Z is known and P can be found by adding P and P and hence Z can be found, and so on for the remaining junctions C and D. It is thus possible to determine the power ratio required between the two arms at each of the junctions and hence the two impedances required to accomplish this power division. The following table gives these values for the adduced example of a five element array:

TABLE Junction A B C D Power in Arm 1 PA1=L00 PB1=PA=4-98 PCi=PB= 6-O0 P131=Pc=36.51

Power in Arm 2 PAr=3.98 Pni=1l.02 PC2=20.51 P =27.59

Power entering junction PA=PA1 PB=P Bl PBZ PC=PCi Poz PD=PD1 Pm Ratio:

Power in Arm 2 P 12 11.02 2 20.51 13 27.59

Power in Arm 1 1251.00 Pm 4.9a Pcr 16.00 Pm 3fi.5l

Which equals:

Impedance of Ann 1 ZAI ZB1 c1 Z D 1 Impedance of Ann 2 Zn zrf Zcf Zm Since Z =Z =Z =Z =Z (50 ohms) the common impedance of a radiating element and its associated feed cable, Z Z Z and Z can be found The next requirement is that each of the junctions has the correct impedances presented to it. Referring to FIG. 8, the required value for Z the impedance presented by its arm 1 to junction A, has been calculated, but this does not correspond to the terminating impedance of arm A which is Z the common impedance of a radiating element. It is therefore necessary to transform impedance Z to Z This is done by two quarter wave transformers having characteristic impedances Z and Z calculated in the manner previously described using the known values of terminating and input impedances required. Looking next into junctionA from junction B, junction A has an impedance Z given by l/Z is equal to 1/Z +l/Z Since Z and 2 are known, Z A can be calculated. Again it does not, however, equal the required impedance Z which must be presented at the junction B to ensure the correct power division at this junction. Impedance Z must therefore be transformed by two quarter wave sections Z0131 and Z to impedance Z This process is continued for each of the junctions with the output impedance of junction B, namely Z beingtransformed to Z Z to Z etc. a final transformation being on the output impedance of junction D, namely Z which has to be transformed by the quarter wave section having characteristic impedance Z into impedance Z rwhich is the characteristic impedance of the feeder cable feeding the power feed section.

The following table gives the impedance transformations required and the characteristic impedance-s necessary to accomplish them for the example cited:

radiatingelement concerned is made aqual by adjusting the length of the connecting cable to each element, though in certain circumstances it could prove of value to have the electrical paths of unequal length, such as when an unequal inter-element phase relationship is required. Hence the microwave energy supplied to all radiating elements'has a common phase and this uniphase condition is preserved at all frequencies.

The remaining conditions required of the array are, that the radiating element accept all of the power denoted to its by the feed section and transmit this power into free space through the horizontal slots cut in it; that the radiating element presents to the connecting, cable the correct impedance match for this cable, and that each radiating element is not affected by adjacent radiating elements, since this would disrupt the power distribution and hence the azimuth beam pattern.

The basic theory associated with the element is that of a slot cut in a wall of a section of waveguide parallel to, but displaced from, the centre line of the wall. Under these condition the slot acts as a radiator and if displaced a suitable distance from the centre line will provide unity normalized conductance looking along the waveguide towards the slot. When this condition is achieved, substantially all the power in the waveguide is radiated by the slot with-out reflection. Where more than one slot is used, then the conductance must be adjusted so that for each element its normalized value is inversely proportional to the number of elements used, that is l/n where n is the number of slots. The radiating elements of the array have two such slots and these are spaced one half guide wavelength apart so that they are effectively in parallel and the waveguide section is short circuited one quarter guide wavelength outwardly beyond each slot so that the admittance in parallel with each slot is zero. It may at first sight appear that each element should have only one slot and this could be done, but, by symmetrically placing the probe feed it is possible to employ two slots and still TABLE Section A 11-13 13-0 0-1) 13-13 ZAIZA2 Bi Bz oi oi ZmZm u Z Terminating Impedance 1:. 50 A ZA1+ZA2 B 8 +Zm v c ZCI+ZC2 D zm+zm Input Impedance ZA1=199 Zn1=110.5 Zci=64 Zm=38 ZE=50 Characteristic Impedance:

Section 1 Zon1= v zm zo Zom= 'V BI ZA Z0c1= 'VZClZB Zor 1= VZmZc ZOE1= VZDZE 1199 .50 V110.5 .4O V64.34.4 13828.1 V21.6.50

Section 2 Zonz= w ZmZo Zonz= /Zn1ZA The quarter wave transformers used in the [feed section are, of course, to some degree frequency selective and provide their optimum transformation only at the design frequency. Deviations from this frequency introduce an impedance mismatch with a corresponding reflection of some power and some disruption of the distribution of power to the associated radiating elements. This effect is negligible however for slight deviations and does not cause a serious deterioration in the performance of the feed section within a frequency band 10% on either side of the design frequency.

Thus the power feed section supplies the correct power to each of its associated radiating elements and, as previously stated, the electrical length of the path taken by this power \from the input to the feed section to the individual preserve the in phase condition; this has the decided advantage of halving the number of feed points required. Since the two slots are symmetrically placed they are thus in parallel with the feed probe and must therefore be so displaced from the centre line as to present a normalized conductance of /2 to the probe. The radiating element is therefore a section of waveguide, one guide wavelength long with two horizontal slots one half wavelength apart and each one quarter wavelength from the short circuit plates at the end of the waveguide section and both displaced upon the centre line of the waveguide such a distance that the two slots in parallel present unity normalized conductance. The slots themselves are quite wide so as to preserve their conductance match over an appreciable frequency range, and

since, as is general, the perimeter of each slot is made one guide wavelength, the length of each slot is somewhat less than one half of the guide wavelength.

The probe which couples the energy into the waveguide setcion is a piece of tubing having a broad band characteristic and 'is of such a length that it matches the normalized conductance of the two slots seen in parallel from the probe to the feeder cable. This length though calculated approximately is finally determined by experiment.

Of the three conditions required of the radiating elements therefore, the unity conductance factor is preserved over an appreciable range of frequencies and the voltage standing wave ratio of a radiating element which represents the amount of power reflected from the element was found to be very good over the 20% frequency band required in the example given above. The impedance matching condition is determined by the probe and due to the broad band characteristics of the slot and the probe it is also held within acceptable limits over the 20% band.

The requirement that minimum mutual impedance shall exist between slots is one of the characteristics of this type of radiator since between a slot end to end with another parallel slot there i little or no mutual impedance.

The anntenna is therefore a broadside linear array for use at microwave frequencies, and is made up of a series of radiating elements disposed along a linear axis. Each of these radiating elements is a section of waveguide, resonant at a given design frequency, and short circuited at each end. Power is radiated from the element by slots, parallel to the linear axis of the array, acting as radiators, cut into that wall (or walls, since, if desire-d, the antenna could radiate in more than one direction) of the section which is contiguous with those like walls of the other elements which together form the radiating face, or faces, of the array. Though the element would radiate if only one slot were used in a radiating wall of the section, it is desirable to have two slots, end to end and working in conjunction, in any radiating wall since this reduces the number of feed points. The slots (or slot) are displaced [from the centre line of the wall parallel to their linear axis by such a distance that their normalized conductance viewed along the waveguide is unity, which is the condition required to give maximum transfer of energy from the waveguide section to free space. The slots are spaced one half design frequency guide wavelength apart and the waveguide short circuits are placed one quarter guide wavelength outwardly beyond each slot, so that the slots are in parallel, and the admittance in parallel with the slots due to the short circuit is zero.

The waveguide section is excited by a probe placed symmetrically with respect to the slots, in one face of the section; this probe is designed to provide a correct match from the radiating element to the feeder cable supplying power to it.

Power is conveyed to each of the radiating elements by a system of feeder cables whose lengths are adjusted to control the phase of the energy reaching the element associated with each cable, in general this phase being the same for all elements.

These cables connect the radiating elements to a power distribution system which includes one or more feed sections, whose function is to accept power from a microwave transmitter and distribute this power amongst the radiating elements associated with the section in a predetermined proportion so that each element gets that power necessary to give the array its desired azimuth beam pattern. This is done in the feed section by a series of quarter wave transformers, whose impedance transformation control the power division at a series of Y-junctions, the characteristic impedance of each transformer section being governed by the relative dimension of a centre conductor, between two parallel plates -in a dielectric medium, which together with the centre conductor constitute an unbalanced transmission line.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A broadside linear array antenna for use at microwave frequencies comprising, in combination, a series of radiating elements disposed along a linear axis, each of said radiating elements being a section of waveguide short circuited at each end, resonant at the prescribed design frequency, which has, on at least one one of its walls, which with the like wall on adjacent radiating elements forms a radiating face of the array, at least one radiator slot parallel to the linear axis of the array and placed from the axial centre line of said wall by a distance such that the normalized conductance of the total number of slots is unity; an impedance matched power distribution system for supplying microwave energy to said radiating elements including at least one power feed section capable of accepting microwave energy from a source and distributing this microwave energy by consecutive impedance transformations in predetermined proportions amongst a series of power take offs connecting to the radiating elements associated with the feed section, said impedance transformations being accomplished by a series of quarter wave transformers whose characteristic impedance is controlled by the relative dimension of a centre conductor between two parallel conducting plates in a dielectric medium; and an appropriate number of feeder cables connecting said power take-off to said associated radiating elements, the length of each said feeder cables being adjusted to control the relative phase of the microwave energy reaching said radiating elements.

2. A broadside linear array antenna for use at microwave frequencies as defined in claim 1 wherein the quarter wave transformers of said power feed section have common parallel plates a fixed distance apart, and obtain their required characteristic impedance by the suitable dimensioning of the diameter of a round centre conductor fixed midway between the two parallel plates, and aid dielectric medium is air.

3. A broadside linear array antenna for use at microwave frequencies as defined in claim 1, wherein said radiating element has two radiator slots in said wall of said waveguide section displaced from the axial centre line of said wall by a distance such that the normallized conductance of each slot is /2.

4. A broadside linear array antenna for use at microwave frequencies as defined in claim 3 wherein said radiator slots are spaced one half of a guide wavelength apart and the waveguide section is short circuited one quarter guide wavelength outwardly beyond either slot.

References Cited by the Examiner UNITED STATES PATENTS 2,602,856 7/1952 Rumsey 333--35 X FOREIGN PATENTS 905,384 3/1954 Germany.

ELI LIEBERMAN, Primary Examiner.

CHESTER L. JUSTUS, Examiner.

R. E. BERGER, Assistant Examiner.

Claims

1. A BROADSIDE LINEAR ARRAY ANTENNA FOR USE AT MICROWAVE FREQUENCIES COMPRISING, IN COMBINATION, A SERIES OF RADIATING ELEMENTS DISPOSED ALONG A LINEAR AXIS, EACH OF SAID RADIATING ELEMENTS BEING A SECTION OF WAVEGUIDE SHORT CIRCUITED AT EACH END, RESONANT AT THE PRESCRIBED DESIGN FREQUENCY, WHICH HAS, ON AT LEAST ONE ONE OF ITS WALLS, WHICH WITH THE LIKE WALL ON ADJACENT RADIATING ELEMENTS FORMS A RADIATING FACE OF THE ARRAY, AT LEAST ONE RADIATOR SLOT PARALLEL TO THE LINEAR AXIS OF THE ARRAY AND PLACED FROM THE AXIAL CENTRE LINE OF SAID WALL BY A DISTANCE SUCH THAT THE NORMALIZED CONDUCTANCE OF THE TOTAL NUMBER OF SLOTS IS UNITY; AN IMPEDANCE MATCHED POWER DISTRIBUTION SYSTEM FOR SUPPLYING MICROWAVE ENERGY TO SAID RADIATING ELEMENTS INCLUDING AT LEAST ONE POWER FEED SECTION CAPABLE OF ACCEPTING MICROWAVE ENERGY FROM A SOURCE AND DISTRIBUTING THIS MICROWAVE ENERGY BY CONSECUTIVE IMPEDANCE TRANSFORMATIONS IN PREDETERMINED PROPORTIONS AMONGST A SERIES OF POWER TAKE OFFS CONNECTING TO THE RADIATING ELEMENTS ASSOCIATED WITH THE FEED SECTION, SAID IMPEDANCE TRANSFORMATIONS BEING ACCOMPLISHED BY A SERIES OF QUARTER WAVE TRANSFORMERS WHOSE CHARACTERISTIC IMPEDANCE IS CONTROLLED BY THE RELATIVE DIMENSION OF A CENTRE CONDUCTOR BETWEEN TWO PARALLEL CONDUCTING PLATES IN A DIELECTRIC MEDIUM; AND AN APPROPRIATE NUMBER OF FEEDER CABLES CONNECTING SAID POWER TAKE-OFFS TO SAID ASSOCIATED RADIATING ELEMENTS, THE LENGTH OF EACH SAID FEEDER CABLES BEING ADJUSTED TO CONTROL THE RELATIVE PHASE OF THE MICROWAVE ENERGY REACHING SAID RADIATING ELEMENTS.