GB2232302A - Flat slot array antenna - Google Patents

Description

" 1 - Title: "Flat slot array antenna" The present invention relates to a

flat slot array antenna for communication, broadcasting and other purposesy and more particularly to the form and arrangement of power radiation slots provided on a radiating side of such an antenna.

Figure 21 shows a conventional slot array antenna which comprises a plurality of slots b equidistantly formed in a plate of a rectangular waveguide a. The electromagnetic wave propagates in the rectangular waveguide a in the mode TE 10 The electromagnetic power radiates from each slot b. Figure 22 shows a power density distribution in the waveguide of Figure 21.

Figure 23 shows another conventional antenna having a circular waveguide. The electromagnetic power is fed from a power feeder opening 11 formed in the center of a circular plate 13 and propagates in a space S formed by a pair of metallic circular plates 12 and 13 and an annular side plate 14. Slots 12al are coaxially disposed on the plate 12, each slot 12al having a cross shape in the same dimension. The power radiates from each slot 12al. Remaining power in the circular waveguide is absorbed in a terminal resistor 16. A circular polarised wave generator is attached to a circular power feeder waveguide 181 as power feeder means for radiating the power at equiphase.

Figures 24 and 25 show another conventional antenna having a different form and arrangement of slots. The electric power is fed to the circular waveguide through a power feeder 18 of a coaxial cable. In the antenna, as shown in Figure 25, the direction of a slot 12a is perpendicular to that of an adjacent slot 12a to form a pair of slots. Both slots of each pair are disposed at a distance of one fourth (Ag/4) of the wavelength Ag in the radial direction of the plate 12. The resultant electric field of the wave radiated from a pair of slots 12a becomes a circularly polarized wave. The pairs of slots 12a are spirally disposed on the plate 12 along a dash-dot line DS so that the wave composed by the whole slots 12a becomes the circularly polarised wave.

Figure 28 shows a further conventional antenna in which the waveguide space S is vertically divided into a lower waveguide space S1 and an upper waveguide space S2 by means of an intermediate metal plate 15. The terminal resistor 16 is provided in the center of the space S2. The electric power fed from the power feeder opening 11 propagates in the space S passing through the lower space S 1, an annular gap D' between the side plate 14 and - the intermediate plate 15 and the upper space S2. The power of equiphase radiates from the slots 12a.

However, there are problems in conventional antennas as follows.

_j In the antenna shown in Fig. 21, each slot b has the same coupling rate of the slot, which represents the rate of power radiating from the slot b, as the others. Consequently, the power density in the waveguide a exponentially reduces as shown in the graph of Fig. 22. As a result, the amplitude distribution on the antenna is irregular so that the side lobe becomes large and the antenna gain reduces.

In the circular waveguide, the internal electro- magnetic field density reduces with the distance r from the power feeder opening 11 as shown by a curve Po of Fig. 26. The internal electromagnetic fields couple with the power radiation slots 12a to be radiated from the slots 12a as the electromagnetic wave in the free space. A curve Pl of Fig. 26 represents the radiation characteristic thereof. Thus, as shown in Fig. 27, the aperture power distribution is irregular, so that the aperture efficiency reduces. In addition, the slots disposed adjacent the resonance wavelength affect the power feeder portion to produce a higher order mode.

In the antenna shown in Fig. 28, the power is guided to a central portion in the upper space S2 by the side plate 14. Consequently, the power density has a comparatively.flat characteristic as shown in Fig. 29, and a preferable power distribution is obtained as shown in Fig. 30. However, the power fed in the waveguide is reflected at the power feeding portion and the side plate 14.

Figure 31 shows an antenna in which a conical matching member 17 is mounted in the power feeder 18 and the side plate 14 is formed to have an inside wall of a V-shaped cross section, thereby preventing the power from reflecting. However, in such an antenna, it is difficult to manufacture the waveguide and manufacturing cost increases.

An object of the present invention is to provide a flat slot array antenna which may increase the slot efficiency by providing a desirable amplitude and phase distributions about the slot with simple construction.

According to the present invention, there is provided a flat slot array antenna having a waveguide with a space hav,ing a rectangular sectional shape and a power feed opening, the waveguide having a plurality of wave radiation slots formed in one of the metallic plates forming the waveguide. The size of the slot and the distance between the slots are progressively changed toward a terminal end of power propagation in the space of the waveguide.

In an embodiment of the present invention, the slot length is progressively increased toward the terminal end in a range without exceeding a resonance length of the slot, and the distance between the slots is progressively reduced toward the terminal end.

In another embodiment of the. invention, the slot length is progressively reduced toward the terminal end in a range without exceeding a resonance length of the slot, and the distance between the slots is progressively increased toward the terminal end.

These and other objects and features of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings, wherein:-

Figure 1 is a perspective view showing a flat slot array antenna according to the present invention; Figure 2 shows an arrangement of power radiation slots of the antenna; Figure 3 is an illustration showing a propagation mode; Figure 4 is a graph showing the relationship between the length of the slot and the impedance of the slot; Figure 5 is a graph showing the relationship between the length of the slot and the coupling rate of the slot; Figure 6 is a graph showing the relationship between the length of the slot and the slow-wave ratio of the slot; Figure 7 is a perspective view showing a first modification of the antenna of Figure 1; Figure 8 is a perspective view showing a second modification; Fig. 9 is a perspective view showing a second embodiment of the present invention, a part of which is in section; Fig. 10 is a schematic perspective view showing an arrangement of the slots of the second embodiment; Fig. 11 is a graph showing a power density distribution in a waveguide space of the antenna; Fig. 12 is a perspective view showing a third embodiment, a part of which is in section; Fig. 13 is a schematic plan view of the third embodiment; Fig. 14 is a schematic illustration showing the flow of the electric power in the waveguide space; Fig. 15 is a graph showing an aperture power distribution; Figs. 16a and 16b are graphs showing impedance characteristics of the slot of the third embodiment Fig. 17a is a graph showing the relationship between the length of the slot and the coupling rate of the slot of the third embodiment; Fig. 17b is a graph showing a slow-wave ratio; Fig. 18 is a schematic plan view showing a modification of the third embodiment; Fig. 19 is a sectional perspective view showing a fourth embodiment.

Figs. 20a and 20b are sectional views showing a fifth embodiment; 6 - Fig. 20c is a sectional view showing a sixth embodiment; Fig. 21 is a perspective view showing a conventional slot array antenna; Fig. 22 is a graph showing a power density distribution in a space of the conventional antenna; Fig. 23 is a sectional perspective view showing a second conventional antenna; Fig. 24 is a sectional perspective view showing a third conventional antenna; Fig. 25 is a schematic plan view of the third conventional antenna; Fig. 26 is a graph showing a power density distribution of the third conventional antenna; Fig. 27 is a graph showing a power distribution thereof; Fig. 28 is a sectional perspective view showing a fourth conventional antenna; Fig. 29 is a graph showing a power density distribution of the fourth conventional antenna; Fig. 30 is a graph showing a power distribution thereof; and Fig. 31 is a sectional perspective view showing a fifth conventional antenna.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to Fig. 1 showing a first embodiment of the present invention, a slot array antenna according to 7 the present invention comprises a rectangular waveguide having a power feed opening 3a formed at an inlet side thereof, and a horn waveguide 4 connected to the rectangular waveguide at the power feed opening 3a. The rectangular waveguide comprises opposite rectangular metallic plates 1 and 2, and metal side plates 3 secured to the three sides of each plate 1 (2) to form a rectangular waveguide space S having a rectangular sectional shape. The metallic plate 1 has a plurality of electric power radiation slots la, arranged in a matrix. On the inside of the end side plate 3 of the rectangular waveguide, a terminal resistor 6 is provided. The horn waveguide 4 has a lens antenna 5a of dielectric therein.

Electric power propagates in the horn waveguide 4, with phase fronts being coaxial with an ideal origin. The power is converted to a plane wave when passing through the lens antenna 5a. Thus, the power is fed to the rectangular waveguide in the form of the plane wave.

In the space S, a slow-wave device 5b such as dielectric is provided for suppressing the generation of the grating lobe.

Referring to Fig. 2, in order to uniform the power distribution, the length of the slot la in each row is progressively longer toward the terminal end of the antenna. Further, the distances Syl, SY2P SY3 between the rows become progressively smaller toward the end.

The electric power fed from the horn waveguide 4 propagates in the waveguide space in the basic mode as shown in Fig. 3 and radiates from the slots la in the free space. In the figure, a reference E designates the line of electric force and M designates the line of magnetic force. The waveguide shown in Fig. 3 is illustrated regardless of the actual size of respective parts thereof to clearly show the mode. In fact, since the ratio of the width to the height of the waveguide is tens to one, the waveguide is a very thin with a large width. Tens of slots can be provided in the lateral direction in one mode accordingly.

In order to uniformly radiate the electric power from all of the slots in n rows and to radiate the whole electric power completely without loss, if the whole electric power is Po, the quantity of the power radiated from the slots in each row is Po/n. Therefore, the coupling rate at each row of the slot should be determined so that the radiated quantity of power may be Po/n at every row.

If the coupling rate.of the slots in a kth row is ak and the internal electric power after passing through the slots in the kth row is Pk, a I P a = a 2 P t = a 3 P 2 =a K PK-1 = - - =P o/n Further, P 1 /n) P P 2 /n) P 9 - 0 0 P K = ( 1 -- k/n) P Consequently, a 1 = 1 /n a 2= P o/nP i = 1 /(n- 1 a K = 1 /(n-k+ 1) Thus, the coupling rate ak at the kth row is 1 / (n - k + 1) Fig. 4 shows the relationship between the length of the slot and impedance about the one-half wavelengh at a constant frequency. If the length of the slot la increases, resistance R and reactance X increase respectively. The reactance X largely reduces in the vicinity of the one-half wavelength so that the impedance becomes a resonance state. The reactance X further reduces as the length of the slot increases. When the reactance X reaches a peak, the reactance X increases again. Meanwhile, the resistance R reduces.

Fig. 5 shows the relationship between the length of the slot and the coupling rate a dependent on the impedance. The coupling rate a has a peak value when the length of the slot is in the vicinity of the onehalf wavelength. Thus, the length of the slot la for obtaining a desired coupling rate can be determined from the graph.

The first embodiment uses the slot having a length shorter than the onehalf wavelength. The slots in every row is formed to increase the lengths thereof such as ú1 (first row), ú2,..., Lk,..., as shown in Figs. 2 and 5, so that the coupling rates of the slots may be such that the first row becomes al, the second row is O2 and the kth row is ak so as to uniform the radiated power. If almost entire power is radiated from the slots and the influence of the reflection of the power on a terminal wall in the waveguide can be neglected, the terminal resistor 16 is unnecessary.

The phase of the electromagnetic wave radiated from the slots advances or retards with respect to the phase in the waveguide space in accordance with the reactance X shown in Fig. 4. Fig. 6 shows a slow-wave ratio of the wavelength X9 in the waveguide space to the wavelength X in the free space in consideration of the retard phase and the advance phase. The slow-wave ratio C reduces when the length of the slot la is smaller than the one-half wavelength. The ratio C largely increases about one-half wavelength and reduces again as the length of the slot further increases. If the distance between the rows of the slots is adjusted in accordance with the slow-wave ratio C, the electromagnetic wave of equiphase is radiated from the slots in each row. In the embodiment, the distance between the rows is progressively reduced toward the terminal end of the waveguide.

For example, if the width of the waveguide is 30 cm and the length is 50 cm, the efficiency is 70% and the 11 - gain is about 33.2 dBi at 12 GHz.

If the slot having a longer length than the one-half wavelength is used, the length of the slot is progressively reduced and the distance between the rows is gradually increased toward the terminal end.

Further, if the distance between the rows is increased at a predetermined rate, the directivity of the wave inclines toward the terminal end. If the distance is reduced, the directivity inclines in the opposite direction. Thus, the directivity can be easily and desirably inclined.

Referring to Figs. 7 and 8 showing first and second modifications of the first embodiment of the present invention, a rectangular feeder waveguide 10 having feeding openings 9 on a metallic plate thereof is attached to the rectangular waveguide as power feeder means. Other constructions are the same as the first embodiment. Thus, the uniformity of the internal power is increased and the distribution efficiency is improved. In addition, the antenna is compact in size.

In the firsts modification, the antenna may be symmetrically formed about the power feeder 10 as shown by the dash-dot lines of Fig. 7, as is formed in the second modification of Fig. 8. In such a construction, even if the frequency changes to change the directivity, the resultant directivities of both sides are constant. Thus, a stable characteristic can be obtained in a wide A range.

As power feeder means, a microstrip line may be employed for energizing a plurality of posts or slots.

In the antenna of the embodiment, although the power is radiated at Hplane, E-plane may be used for radiating the power.

Referring to Fig. 9 showing the second embodiment, a circular slot array antenna comprises a metallic circular radiation plate 1 having a plurality of slots la coaxially or spirally disposed therein, a metallic circular plate 2 provided opposite to the plate 1, and a metallic annular side plate 3 secured between the outer peripheral portions of the circular plates 1 and 2 form a waveguide space S. A power feeder 7 comprising a coaxial cable is mounted in a power feeder opening 2a formed in the center of the plate 2.

An intermediate metal plate 8 is provided in the waveguide space S, with a space between the plate 8 and the side plate 3 for feeding the power. The waveguide space S is vertically divided into a lower waveguide space S1 and an upper waveguide space S2.

The length of the slots is progressively reduced toward the center of the waveguide in order to obtain a uniform aperture power distribution.

The power Pf fed from the power feeder opening 2a propagates in the space S passing through the lower space S1, an annular gap D between the side plate 3 and the intermediate plate 8 and the upper space S2.

The power of equiphase radiates from the slots la.

Describing the slot array arrangement of the second embodiment with reference to Figs. 10 and 11, n circles of slots are disposed at regular intervals d. Therefore, the radius of the outermost circle is represented as nd. If the power density before radiating from the slots in the kth slot circle from the outermost circle (first. circle) is Q k-O' the power density after the radiation from the slots in the kth circle is Q k+O' the initial power density is Qo, and the power radiated from each slot is C, the power densit before passing the slots in the first circle is represented as Q1-0 Q0 The power density after passing the first slot circle is Qi+0 QO - C The power density before passing the second slot circle is X nd C) n d (n-1)d (n-1)d The power density after passing the second slot circle is Q 2+0 =Q 2-0 - C=(Q,- C) nd -C (n-1)d If the power density becomes zero after passing the slots in the nth circle, a following equation is obtained.

[{(Q 0 -C) nd (n-1)d,-C]...)3 d -C) 1d -C (n-l)d '-'(n-2)d 2d d 0 14 - 4 nQo -In + ( n -1) + ( n -2) + C= 2 Q o n+ 1 + 3 + 2 + 11 c = 0 Thus, the power radiated from each slot is 2 Q 0 n+ 1 If the coupling rate ak of the kth slot circle is determined so that the product of the coupling rate ak multiplied by the power density Q k-O fed to the kth slot circle may be the radiated power C, the aperture amplitude distribution of the plate 1 becomes uniform. Since the power density Q k-O is Q K-O =( E 1 (Q, -C) nd -C, (n -1) d -C 1)In-(k-2)1d (n-1)d (n-Z)d In-(k-1)1d.

n-Q" n + n-1 +...+n-(k-2))C n-(k-1) n -(k -1) n-(k-1) n-(k-1) n -(k 1) {nQO - (k 2 the coupling rate is a K = c 'Q K-0 ( n -k + 1) C nQ,(k-l)(2n-k+2) C 2 Thus, the length of the slot la is determined based on the coupling rate ak and the distance between the slot circles is adjusted, so that the electromagnetic waves having equiphase and the same amplitude radiates from the slots.

The deviation of the phase caused by the inequality of the length of the slot is corrected by adjusting the t 1 distance between the slot circles. Since a desired coupling rate can be provided, a desired aperture distribution such as a binominal distribution, Taylor distribution, and Dolph-Chebyshev distribution is obtained, whereby an antenna having high performance can be provided.

In the case that the slots la are spirally disposed on the circular plate 1, the internal electromagnetic power P per unit area on a circle at the radius r is expressed as P = Po / (2wr x h) where Po is the whole power fed to the waveguide and h is the distance in the waveguide space.

The radiated electric power Pr at the position of the radius r is Pr = a x P = a x Po / (2wr x h) Therefore, the radiated power Prn from the slot of the nth circle is Prn = an x Pn and Pn = (1 - a n-1) X P n-1 x r n-1 / r n (r n: the distance between the slot of the nth slot circle and the center of the waveguide) Although the slot la has a shorter length than the one-half waveguide, a slot having a longer length can be used in the second embodiment.

Referring to Fig. 12 showing the third embodiment, the circular slot antenna of the third embodiment has a slow-wave means in the waveguide space S. The slow-wave X means comprises a first layer 19 made of foam polystyrene and a second layer 20 made of polyethylene provided under the first layer 19.

As the slow-wave means, foam plastics such as foam -polyethylene and foam polypropylene, and a corrugated circuit may be used. If the slots la are formed within one wavelength, the slow-wave means is not provided, but an insulation is provided between the plates 1 and 2 to maintain the space.

Referring to Fig. 13, in order to obtain a desired aperture power distribution, the dimension (width or length) of the slot la is progressively increased toward the outer periphery of the waveguide. The distance between the slots disposed on a radius is progressively reduced toward the periphery (Sr 1 > Sr 2 > Sr 3 >...).

Fig. 14 shows the internal electromagnetic power in the waveguide. If the radius r of a circle passing a slot is r>X g, the internal power P per unit area reduces with an increase of the radius r. The internal electromagnetic power P per unit area on a circle at the radius r is expressed as P = Po / (2nr x h) where Po is the whole power fed to the waveguide and.h is the distance in the waveguide space.

The radiated electric power Pr at the position of the radius r is Pr = a x P = a x Po / (27Tr x h) The coupling rate a is determined in accordance with the 17 - 4 length of the slot corresponding to the wavelength X in the free space, the dielectric constant er of the resin used for the slow-wave means and the distance h in the waveguide space.

Therefore, the radiated power Prn from the slot of the nth circle from the center is Prn = a n x Pn and Pn = (1 - a n-l) x P n-l x r n-l / r n Figs. 16a and 16b show the relationship between the length of the slot and the impedance about the one-half wavelength at a predetermined frequency. If the other parameters are constant, the relationship between the length of the slot and the coupling rate a has a characteristic similar to the real part of the impedance as shown in Fig. 17a.

It will be seen from graphs that the coupling rate a reduces as the length of the slot deviates from the resonance length (in the vicinity of one-half wavelength). Since the length SL of the slot in every circle is gradually increased toward the periphery (SL 1 < SL 2 <...) so that the coupling rate a may increase (a 1 < a 2 < a 3 <...) and the aperture power distribution may be Pr 1 = Pr 2 =..., the aperture power distribution is uniformed as shown in Fig. 15. For example, if the diameter is about 20 Xo and the width of the slot is 0.04 Xo, the length SL is 0.3 Xo < SL < resonance length 0.46 Xo Since the impedance shown in Fig. 16b has an k imaginary component, the phases of the power Pn and the radiated power Prn are advanced or retarded about the resonance length. Accordingly, the slow-wave ratio changes irregularly as shown in Fig. 17b.

In order to correct the difference of phases, the distance between the circles of the slots is reduced (Sr 1 > Sr 2 > Sr 3 >...) so as to provide equiphase wave. Thus, the resultant electromagnetic field of equiphase is formed, thereby providing an antenna having a high efficiency. In the embodiment, the same effect as the previous embodiments is achieved.

Fig. 18 shows a modification of the third embodiment in which the length of the slot is larger than the resonance length. In the modification, the length of the slot in every circle is reduced toward the periphery. However, the slots are disposed so as to increase the coupling rate a toward the outer periphery of the waveguide. The slots of the outermost circle has the same lengths as that of the third embodiment, that is the resonance length at the operating frequency in the space S.

The phases of the electric power propagated in the space S and the radiated power Pr change in such a way as Sr 4 <Sr 5 <Sr 6 <... which is in inverse of the antenna of the third embodiment. Thus, the slots are disposed correspondingly. The same effect as-the third embodiment can be obtained.

Fig. 19 shows the fourth embodiment where a cylindrical power feeder waveguide 7' is mounted 1 adjacent the power feeder opening 2a in place of the power feeder 7. Other constructions are the same as the third embodiment.

The power in the mode of TE 01 or TM 01 is fed to the feeder waveguide V.. The embodiment may use two types of the slot arrangements described in the third embodiment.

The type of the antenna shown in Fig. 23 may also be arranged in accordance with the present invention so as to provide a desirable aperture distribution.

Fig. 20a shows the fifth embodiment. A circular antenna has a conical matching member 21 made of metallic material having a taper surface at an angle of 45. The conical matching member 21 is secured to the underside of the plate 1. The top of the matching member 21 is faced to the power feeder opening 2a. The power feeder 7 of the coaxial cable comprises an outer conductor 7a connected to the power feeder opening 2a and an inner conductor 7b connected to the top of the 20 matching member 21.

Fig. 20b shows a modification of the antenna of the fifth embodiment provided with a power feeder waveguide 71 in place of the coaxial cable 7. The matching member 21 is located on the axis of the feeder waveguide 7' for suppressing the reflection of the power at the power feeder portion.

Fig. 20c shows the sixth embodiment in which the power feeder 7 of the coaxial cable is used. The 1 matching is achieved by adjusting the length L of a probe portion and the diameter Do of the inner conductor 7b. The same effect as the fifth embodiment is obtained by the sixth embodiment.

In accordance with the present invention, the length of the slot and the distance between the rows of the slots on the antenna are arranged to obtain a desired aperture power distribution. Thus, the antenna has a desired characteristic, high efficiency and simple construction.

While the invention has been described in conjunction with preferred specific embodiments thereof, it will be understood that this description is intended to illustrate and not limit the scope of the invention, which is defined by the following claims.

1

Claims (6)

1. A flat slot array antenna having a waveguide with a space having a rectangular sectional shape and a power feed opening, the waveguide having a plurality of wave radiation slots formed in one of the metallic plates forming the waveguide, characterized in that the size of the slot and the distance between the slots are progressively changed toward a terminal end of power propagation in the space of the waveguide.

2. The antenna according to claim 1 wherein the length of the slot is progressively increased toward the terminal end in a range without exceeding a resonance length of the slot, and the distance between the slots is progressively reduced toward the terminal end.

3. The antenna according to claim 1 wherein the length of the slot is progressively reduced toward the terminal end in a range without exceeding a resonance length of the slot, and the distance between the slots is progressively increased toward the terminal end.

4. The antenna according to claim 1 wherein the resonance length is in the vicinity of a one-half wavelength.

5. A flat slot array antenna substantially as hereinbefort described with reference to and as shown in any one or more of Figures 1, 2, 7 to 10, 129 132 189 19 and 20 of the accompanying drawings.

6. Any novel feature or combination of features described herein.

Published 1990atThePatent Office. State House. 6671 High Holborn, London WCIR 4TP.Parther copies maybe obtainedfrom The Patent Office Sales Branch, St Mary Cray, Orpington. Kent BR5 3P.D. Printed by Multiplex techniques ltd, St Mary Cray. Kent, Con. 1/87 1