Planar antenna array for point-to-point communications

Abstract: A planar antenna array for linearly polarized waves is proposed which offers a technique of improving the radiation pattern of the antenna by compensating for amplitude and phase imbalance due to coupling between adjacent lines in the feed network. This imbalance causes the radiation patterns to be severely distorted. The proposed configuration uses slots offset in certain parts of the array and then bonds the slots to an aperture/waveguide layer and produces an antenna with good gain, good match over a wide frequency band and good cross polar discrimination as well as providing an improvement in the overall radiation pattern of the antenna.

Description: The invention concerns antenna design, and more particularly, a planar antenna array for point-to-point communication which compensates for amplitude and phase imbalance in its feed network. In an antenna array using patch and microstrip antenna structure, amplitude and phase errors or discrepancies commonly occur from one radiating element or patch to the next in the array. For example, the feed network and radiating patches are typically carried on thin substrates such that the fields which are generated are not confined within the substrate but will radiate considerably. Thus, coupling between adjacent feedlines, adjacent patches, etc. can cause considerable amplitude and phase imbalances in the power distribution network. Such imbalances can result in undesirable radiating pattern characteristics. The present invention concerns a method and structure for compensating for such phase and/or amplitude imbalance in the feed network. A planar antenna array is described in EP-A-0 489 934. An antenna is constructed as a multilayer structure of a conductive backplane, a feed layer with a network of feed lines, and a slot layer, wherein slots are arranged above respective radiating ends of the feed network. A conductive grid layer forming resonator cells and a semi-transparent reflector layer are attached to the slot layer. The resonator cells are excited by energy coupled through the slots. Electromagnetic waves are emitted via the semi-transparent layer 11. The resonator cells have a size of a 4 λ x 4 λ and the slots feeding the resonators have a separation of 2 λ. DE-A-4014133 discloses a planar antenna array consisting of a backplane, a first spacing layer, a feed and radiating network layer, a second spacing layer and an emitter layer. Through holes in the emitter layer are formed in a specific shape in order to maximize the efficiency of receiving circular polarized waves. Pairs of adjacent radiating elements in the feed and radiating network layer may have a phase difference of 90° or 45°, respectively. In the case of a 90° phase difference, the respective through holes of corresponding pairs of through holes are slightly misaligned against each other in the vertical direction. A printed circuit antenna structure is disclosed in US-A-4 719 470. Antenna elements are spaced at regular intervals. Various configurations of radiating elements using micro strip lines are suggested. In particular, the ground plane is removed under a micro strip line to obtain a radiating element. It is an object of the present invention to provide an improved planar antenna array for point to point communications. This is achieved by the features of the independent claims. Further embodiments are subject-matter of the dependent claims. In the drawings: FIG. 1 is an exploded view of planar antenna array; FIG. 2 illustrates a modified slot design in accordance with the invention; FIG. 3 is a plot of a radiation pattern for a 16 x 16 prototype array; FIG. 4 shows a plot of a radiation pattern for a 16 x 16 array as in FIG. 4 wherein certain slots were offset in accordance with their amplitude and phase imbalance; FIG. 5 shows a plot of a cross polar discrimination pattern for a 16 x 16 array using offset slot design in accordance with the invention; FIG. 6 is a plot showing phase variation from aperture/waveguide numbers 250-256 of the array; FIG. 7 is a plot showing phase variation from aperture/waveguide numbers 170-176 of the array; FIG. 8 is a plot of the measured amplitude response across the frequency band for aperture number 151 of the array; and FIG. 9 is a plot of the radiation pattern for a compensated 16 x 16 array in accordance with the invention. In the description which follows, antenna array architecture (FIG. 1), and reasons for using variable slots within the aperture/waveguide are described. The usefulness of the invention is demonstrated by the measured radiation pattern (FIG. 3) from an initial prototype array known to have very poor amplitude and phase distribution within the feed network circuitry. Despite this very large amplitude and phase imbalance, FIG. 4 illustrates how the use of variable slots within a given aperture/waveguide in accordance with the invention resulted in improvements in the radiation pattern of the array. In a second prototype array, the design of variable slots within the aperture/waveguide in accordance with the invention resulted in even better phase and amplitude response as shown in FIG. 5 and FIG. 9. Referring to FIG. 1, an antenna array 10 has a ground plane 12 with the sides 14 turned up to act as a shield. A feed and radiating (patch) network 18 is constructed on microwave flex material 16 suspended above a foam layer 20 having a dielectric constant close to air. Electromagnetic coupling to a slot layer 22 and an aperture/waveguide plate or layer 24 is utilized to enhance the bandwidth of the array. A radome cover 26 attaches to the ground plane 12 and covers the above-described elements. Because the feed and patch layer is designed on a thin substrate suspended on an "air" dielectric, the fields are not confined within the substrate and as a consequence will radiate considerably. With the element spacing restricted due to grating lobe consideration, coupling between adjacent lines causes severe amplitude and phase imbalance in the power distribution network and as a consequence will result in very poor pattern characteristics. In addition, radiation from discontinuities will also contribute. FIG. 2 illustrates the principles of the invention, wherein at least some slots are offset within the aperture/waveguide in order to equalize the amplitude and phase imbalance due to coupling between adjacent lines. The slots are moved in accordance with their amplitude and phase distribution. The size and/or shape of each slot can also be changed to achieve the desired result. That is, any or all of slot shape, size and position can be changed to compensate for the feed network amplitude and phase imbalance due to coupling between adjacent lines. The feed and aperture/waveguide remain fixed. Size, shape and/or positional change in the slots is all that is required to compensate for this imbalance. In FIG. 2, the structure of FIG. 1 is viewed through a 2 x 2 array or sub-set of the apertures 30 in the aperture layer or plate 24. In FIG. 2, the respective apertures 30 are designated by reference numerals 32, 34, 36 and 38. In this regard, FIG. 2 is a somewhat diagrammatic view, in that it shows only the respective apertures 32, 34, corresponding slots in the slot layer 22, and corresponding parts of the feed network and radiating patches of the layer 18 of FIG. 1. In this regard, a portion of the feed network is designated in FIG. 2 by the reference numeral 40. Respective radiating patches 42, 44, 46 and 48 are illustrated in connection with the corresponding apertures 32, 34, etc. Also, the corresponding slots of the slot layer 22 are designated by reference numerals 52, 54, 56 and 58. It will be seen with respect to the slots 52, 56 and 58 that these have been offset to different relative positions relative to their corresponding radiating elements 42, 44, etc. and their respective aligned apertures 32, 34, etc. With respect to the slot 54, the size of this slot has been changed in accordance with the invention. The size and positional changes of the slots are to compensate for imbalance in the network, as mentioned above. The slot layer 22 and the aperture/waveguide layer 24 are bonded together to create a very thin composite layer that results in good gain for the array, good return loss and good cross polar discrimination. Bonded in this way, the layer of slots can be kept flat and aligned accurately to the apertures/waveguide. This eliminates tolerancing problems that can be acute at millimeter-wave (mm-wave) frequencies. This also eliminates the need to equalize the amplitude and phase in the feed network; specifically, with space being a key restriction, compensation of amplitude and phase in the feed network would be quite difficult. Hence the bonding of the slot circuit to the aperture/waveguide, together with offsetting (certain) slots to compensate for the amplitude and phase imbalance resulting from coupling between adjacent lines provides an effective mechanism for compensation. In the following, a complete example of an antenna structure of the present invention is given. The ground plane 12 and the aperture plate 24 may be constructed of aluminum, with the aperture plate being about 2.5 mm thick. The foam layer 20 is an extruded polyethylene foam with a thickness of 1.5 mm. A suitable foam is available from Advanced Materials Ltd. of Newhall, Naas, County Kildare, Ireland, under the designation AMLTE2001.5 White. The feed network or circuit 18 on the layer 16 is formed or etched in a copper layer carried on the dielectric substrate. In the illustrated embodiment, this is an 18 micron copper layer on a 50 micron substrate, available for, example, from Dupont under the designation Pyralux AP8525. The slot layer 22 may be formed by etching apparent appropriate slots of the appropriate size, shape and position relative to the radiating elements of the feed circuit and the apertures 30, on a copper covered dielectric substrate. In the illustrated embodiment, a 35 micron copper layer is used on a 50 micron substrate of polyester. An additional polarizer layer, formed on a sheet of polyester 75 micron substrate with 35 micron copper coating, (not shown) may also be used, if desired, to operate with the antenna between the aperture layer 30 and the inside of the radome cover 26, rotated 45° from the principal planes. The radome 26 may be constructed of a dielectric material such as one sold under the trademark LUSTRAN ABS. This material is polyacylontrile-butudience-styrene (ABS), also sold under trademarks: CYCOLAC, NOVODUR, and LUSTRAN is available from RONFALIN. In one embodiment, all of the slots are of the same dimensions with the relative offset of slots being used to accomplish the desired corrections. In this embodiment, the slot dimensions have a width of 2.8 mm, a length of 6 mm and a corner radius of 1 mm. The slot layer is bonded to the aperture layer by spraying the aperture layer with an adhesive such as 3M spraymount, available from 3M UK, 3M House Brackenell, Burks, UK RG121JU. The measured H-plane co-polar radiation patterns of the initial prototype antenna are shown in FIGS. 3 and 4. FIG. 3 shows a 16 x 16 array prototype with no slot offsets. FIG. 4 shows the 16 x 16 prototype with selected ones of the slots offset in accordance with their amplitude and phase imbalance. Despite the very large amplitude and phase error inherent in the circuit, the effectiveness of offsetting certain selected slots was apparent as shown by the improvement in the radiation patterns shown in FIGS. 3 and 4. To examine the phase and amplitude of the array, one array having straight (i.e., not offset) slots and another array having offset slots in certain parts of the array were used. Each antenna was probed and the amplitude and phase of each aperture/waveguide was recorded. Based on these measurements, we have found that controlling the amplitude and phase distribution through movement of the slot relative to the aperture/waveguide can be achieved without undue difficulty. FIGS. 6 and 7 show the phase response after probing a number of apertures/waveguides in the 16 x 16 array. With no offset of the slots ("straight slots"), the phase appears quite variable. This was predictable as the network was designed to be very simple and coupling between adjacent lines and nearby surroundings in the array was inevitable. On the periphery of the antenna, the phase variation is expected to be minimal since discontinuities from the immediate surroundings can be neglected and any error is due to inadequate compensation in the feed circuit. FIG. 6 shows the discrete phase measurement for aperture/waveguide numbers 250-256 counting from left to right starting at the top left hand comer. That is, these are the last 7 elements in the 16 x 16 array. FIG. 7 show the discrete phase measurements for aperture numbers 170-176 in the array. As can be seen, the phase varies considerably from one aperture to the next. From the plot, we find that the maximum phase variation is reduced from, on average, 40° to 15°, by offsetting at least selected slots. Amplitude variation within the array can also be controlled. Again as in the phase response, amplitude response also varies from one aperture/waveguide to the next. The amplitude response is quite flat around the periphery of the array but gets worse towards the center of the array. In certain aperture/waveguides, a large loss in power at certain frequencies (particularly at the high end of the band) occurs. Referring to FIG. 8, the results show a sudden fall in one of the apertures at the top of the operating band. This is probably due to coupling to the nearby feed lines. By changing the size and/or shape of the slot within the aperture, the result is improved considerably as shown by the trace marked "Modified Slot". When using uniform slots, the phase errors were found to be on average 40° out between one slot and the next. Offsetting certain slots to compensate for this error gave a flatter response throughout the array. The resultant radiation pattern is shown in FIG. 9. As can be seen by comparison to FIGS. 3 and 4, the offset slots bonded to an aperture/waveguide layer show remarkable improvements. This antenna design also show very good cross polarization discrimination with good match across the band. FIG. 5 shows a typical measured cross polarization discrimination of a 16 x 16 array using offset slots, in accordance with the invention, bonded to an aperture/waveguide layer. The radiation patterns of FIGS. 3-5 and 9 are drawn against European Telecommunications Standard ETS300 833 Class1, Class2 and Class3. Thus, offsetting the slots as described above has the effect of compensating for phase imbalance, and to an extent, amplitude imbalance. If the feed network does not show a large unexpected loss in power due to coupling from surrounding lines, the slot offset alone provides enough compensation. However, when a large or unexpected loss is encountered, the slot size and/or size and shape can also be changed to compensate for this loss in accordance with the present invention. The offset of a given slot can be determined from an equation based on the measured phase imbalance or phase offset of a given aperture. Using an approximation that one wavelength is equivalent to 360°, and the difference in phase between offset and non-offset slots in the prototype array, a conversion can be calculated from degrees to millimeters using a formula derived generally as follows. Given: I wavelength (λ) = 360 degrees In a dielectric medium, λ /√ε r = 360 λ = free space wavelength Given: At a midband frequency of 38.5 GHz, 1λ = (3 x 10 8 )/(38.5 x 10 9 ) = 7.792 mm Polyester slot circuit material properties: Dielectric constant ε r = 3.3 Therefore: λ = 360(√ε r ) 7.792 = 360 (√3.3) 1 mm = 360(1.8166)/(7.792) = 83.929° For example, if it is determined (from a simulator) that the phase error is approximately 17°, the distance that the slots in error needed to offset is approximately 17/83.929 = 0.2 mm, etc. Of course, this result would vary for other frequencies. The benefit of varying the slots within a fixed aperture/waveguide to control the amplitude and phase response of the antenna can be demonstrated herein through both probing (e.g., by probing each aperture with a dipole) and radiation pattern measurements. By showing the radiation pattern of an array where the amplitude and phase in the feed circuit is known to be very poor, we have demonstrated a pattern improvement in the modified slot design. When the array was compensated so that the amplitude and phase errors were as minimal as possible, the patterns improved considerably. The technique provides a very simple method of controlling the amplitude and phase distribution throughout the array.

Claims: A planar antenna for point-to-point communications, comprising: a conductive backplane (12) having a planar conductive surface; a generally planar feed and radiating network layer (16) parallel to and spaced above said backplane surface; a generally planar slot layer (22) parallel to and adjacent said feed and radiating network layer (16); and a planar aperture layer (24) parallel and adjacent to said slot layer (22), wherein said feed and radiating network layer (16) has a plurality of radiating elements (42, 44, 46, 48) arranged in an array, wherein said aperture layer (24) has a plurality of apertures (32, 34, 36, 38) arranged in an array and respectively aligned with corresponding ones of said radiating elements (42, 44, 46, 48), and wherein said slot layer (22) has a plurality of slots (52, 54, 56, 58) respectively aligned with corresponding ones of said radiating elements (42, 44, 46, 48) and corresponding ones of said apertures (32, 34, 36, 38), characterized in that individual slots (52, 54, 56, 58) are configured and arranged relative to respective radiating elements (42, 44, 46, 48) so as to compensate for at least one of amplitude errors and phase errors of said respective radiating elements (42, 44, 46, 48). A planar antenna in accordance with claim 1, wherein said aperture layer (24) is bonded to said slot layer (22). A planar antenna in accordance with claim 1 or 2, wherein individual slots (52, 54, 56, 58) are configured and arranged so as to compensate for amplitude and phase error by selecting at least one of slot size, slot shape and slot position of one or more of said slots relative to said respective radiating elements. A planar antenna in accordance with any of claims 1 to 3, further including a dielectric layer (20) having a dielectric constant close to air, said dielectric layer (20) being interposed between said backplane (12) and said feed and radiating network layer (16). A planar antenna in accordance with any of claims 1 to 4, further including a radome (26) overlying said aperture layer (24), said slot layer (22) and said feed and radiating network layer (16). A method of compensating errors in a radiating array (10) for point-to-point communications, said array (10) including a conductive backplane (14) having a planar conductive surface, a generally planar feed and radiating network layer (16) parallel to and spaced above said backplane surface, a generally planar slot layer (22) parallel to and adjacent said feed and radiating network layer (16), and a planar aperture layer (24) parallel and adjacent said slot layer (22), said method comprising bonding said aperture layer to said slot layer, wherein said feed and radiating network layer (16) has a plurality of radiating elements (42, 44, 46, 48) arranged in an array, said aperture layer (24) has a plurality of apertures (32, 34, 36, 38) arranged in an array and respectively aligned with corresponding ones of said radiating elements (42, 44, 46, 48), and said slot layer (22) has a plurality of slots (52, 54, 56, 58) respectively aligned with corresponding ones of said radiating elements (42, 44, 46, 48) and corresponding ones of said apertures (32, 34, 36, 38), characterized by the step of: configuring and arranging individual slots (52, 54, 56, 58) relative to respective radiating elements (42, 44, 46, 48) so as to compensate for at least one of amplitude errors and phase errors of said respective radiating elements (42, 44, 46, 48). A method of compensating errors in a radiating array (10) in accordance with claim 6, further comprising the step of bonding said aperture layer (24) to said slot layer (22). A method of compensating errors in a radiating array (10) in accordance with claim 6 or 7, wherein said step of configuring and arranging individual slots (52, 54, 56, 58) includes the step of selecting at least one of slot size, slot shape and slot position of one or more of said slots relative to said respective radiating elements. A method of compensating errors in a radiating array (10) in accordance with claim 6, further including interposing a dielectric layer (20) having a dielectric constant close to air between said backplane (12) and said feed and radiating network layer (16). A method of compensating errors in a radiating array (10) in accordance with claim 6, further including providing a radome (26) overlying said aperture layer (24), said slot layer (22) and said feed and radiating network layer (16).

Inventors: HO JIMMY, HO, JIMMY
Applicants: ANDREW CORP, ANDREW AG
Application year: 2001
Main classification: H01Q21
IPC codes: H01Q21, H01Q1, H01Q13, H01P11