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[ EN ]
Microstrip antenna

This invention relates to microstrip antennas comprising a plurality of patches on a substrate.
Microstrip ' patch antennas are resonant radiating structures which can be printed on circuit boards. By feeding a number of these elements arranged on a planar surface, in such a way that their excitations are all in phase, a reasonably high gain antenna can be obtained that occupies a very small volume by virtue of being flat. Microstrip antennas do have some limitations however that reduce their practical usefulness.
1) Microstrip patches are resonant structures with a small bandwidth of operation, typically 2.5 - 5°/o.
Communication bandwidths are usually larger than this. Satellite receive antennas for instance should ideally work from 10.7-12.75 GHz, which requires a bandwidth of 17.5°/o.
2) The patches in isolation have low gain, typically 6-8 dBi. This leads to a large number of elements being needed to produce useful gains. A satellite receive antenna for instance should have a gain of around 40 dBi, implying the use of thousands of elements.
However, the loss in the power splitting networks required to feed the elements increases as the array 5 increases in size so leading to ' an upper limit in achievable gain.
It is known to improve the bandwidth of a rectangular patch by adding, in proximity thereto, further patches which are fed parasitically therefrom (as for example in C British Patent 2067842). In that patent, the edges of the parasitic patches are capacitatively coupled to the radiative edges of the fed patch. The mechanisms by which such parasitic patches are excited have not hitherto been well understood or described, however, so it has not proved possible to design optimum performance antennas comprising an array of patches of which some are parasitically fed.
In particular, one proposal has been to fabricate arrays of spaced patches, only some of which are fed using a constant inter-patch spacing.
According to the invention, there is provided an antenna comprising a plurality of substantially rectangular patches energisable at a resonant frequency each having an opposed pair of first edges and an opposed pair of second edges corresponding in length to the resonant frequency, disposed upon a substrate, characterised in that the patches are so arranged as to form a plurality of elemental groups, each such group comprising a first patch adapted to be fed from a feed line and a pair of second patches each adjacent to and spaced from one of the second edges of the first patch, the second patches being adapted to be fed only parasitically from the first, the groups being spaced apart on the substrate in an array, such that the spacing between patches of adjacent groups substantially exceeds the spacing between patches within a group.
In another aspect, the invention provides an antenna comprising a plurality of elemental groups disposed in an array upon a substrate, each group comprising a central patch adapted to be fed from a feed line and four parasitic patches adapted to be parasitically fed from the central patch, disposed around the central patch so as to form a cross, wherein the elemental groups are arranged with their cross axes parallel one to another, the array comprising a plurality of lines of groups spaced along the line by a distance P less than twice the wavelength λ corresponding to the resonant frequency of the antenna, alternate lines being displaced by P/2 so that the effective spacing in at least one antenna plane is less than λ.
Preferably, a feed network comprising a plurality of feed lines is disposed upon one face of a second substrate, aligned parallel with the first so that a feed line lies adjacent a feed point of each central patch, and there is provided between the two substrates a ground plane, including apertures between each such feed point and the adjacent feedline, so as to allow the patch to be fed therefrom.
Other preferred embodiments of the inventions are as recited in the claims appended hereto.
The invention will now be described by way of example only, with reference to the accompanying drawings, in which;
Figure 1 is a front elevation of a sub-array group forming part of an antenna according to a first embodiment of the invention;
Figure 2 is an exploded isometric view showing a cross section through the antenna of Figure 1;
Figure 3 shows a sub-array group forming part of an antenna according to a second embodiment of the invention;

Figure 4 shows a first array arrangement of an antenna according to the embodiment of Figure 4;
Figure 5 shows a second array" arrangement of an antenna according to the embodiment of Figure 4.
Referring to Figure 1, a sub-array group for use in a microstrip array antenna comprises a central, fed, rectangular patch 1 having a pair of edges of resonant length L chosen, in known manner, to be L = λ/2 ζ_ (where λ in the following is 64.82 mm) flanked at either of these edges by a pair of identical parasitic patches 3a, 3b, all upon a substrate layer 4.
Referring to Figure 2, one preferred method of feeding the central patch 1 is to provide, under the ground plane layer 5, a second substrate layer 6 (which may be of the same material as the first layer 4) upon the outer side of which the feed line 2 for that patch is printed, forming a combining network with the feedlines of neighbouring patches. The ground plane layer 5 is traversed by a coupling slot or aperture 7 between the feeding point of the fed patch 1 and the feed line 2, so as to allow the patch 1 to couple to the feed line 2.
In the following, the first, resonant-length, edges will be referred to as 'non-radiative edges', and the second pair of edges as 'radiative edges', for convenience.

Experimental evidence shows that, in this arrangement, a) parasitic excitation is proportional to patch width w. Thus, for maximum parasitic excitation, the width w of all patches must be made large. It cannot, however, be made equal to the length L or else the non-radiative edges will start to radiate and give rise to unwanted cross-polar radiation so, for a bandwidth of, say

10°/ , the width must not be within 95-105o/Q of the length.
b) parasitic excitation is, to a good approximation, an exponential decay function of patch separation. For high excitation, therefore, patch separations should be kept low.
c) parasitic phase is a function of patch separation. For large separations, above about 0.08λ (in this case, 5mm), the phase difference between the central and parasitic patches is proportional to separation; below this the phase difference is always greater than this relation would predict.

From these results a simple expression for parasitic element excitation was derived, having the form:

Excitation = awe ^

where w, s and d are parasitic patch width, separation of parasitic patch edge from fed patch edge, and separation of patch centres respectively. Using the derived a, b and c values any H-plane parasitically coupled linear array can be modelled. In a first example, a sub-array is formed from 3 elements having a resonant length L of 20 mm, each 18.5mm (w = 0.925L) wide and with a separation of 2mm on a 1.57 mm thick PTFE substrate layer 4 having a relative permittivity ζ_ equal to 2.22. Its predicted directivity was 9.43 dB; the subsequent measured result showed a directivity of 9.33 dB. A second example has 14 mm wide patches (w = 0.70L), where the separation is

3mm; again, agreement between prediction and measurement is good.
From the foregoing, the criteria disclosed herein governing the choice of patch separation lead to the choice of a small patch separation relative to the operating wavelength used. The criteria governing inter-element spacing of a microstrip array are related to the wavelength rather differently, however, and favour inter-element distances of on the order of and below, λ. It has been found that providing further parasitic patches beyond those flanking the fed patch is counterproductive and severely reduces the antenna performance, so it is important that the edge to edge spacing between parasitic patches of adjacent sub-arrays is significantly greater than interpatch spacing within each sub-array.
It is also possible to parasitically excite patches from the radiative edges of a fed patch. The coupling mechanism here is different, however (apparently, predominantly reactive), and in general is very much more sensitive to the interpatch separation. It is found that adding parasitic patches at the non-radiative edges stabilises this sensitivity, however, so that practical antennas can be formed in the cross configuration shown in Figure 3 with a pair of parasitic patches 3c, 3d at the radiative edges of fed patch 1, and a pair of patches 3a, 3b at the non-radiative edges thereof. The five-element cross has a larger effective area than the three-element subarray, and hence a better gain and bandwidth.
Since the sub-arrays occupy a large area, it would be difficult to provide a feed network on the same surface of the substrate, so the feed mechanism for the fed patches in this case is preferably that of Figure 2, with the feed network 2 printed on the other side of a second substrate layer 6 coupled to the fed patches 1 via slots 7 in the ground plane 5.
The spacing of the sub-arrays is not straightforward, but is governed by several criteria. On one hand, as is stated above, the spacing between parasitic patches of adjacent sub-arrays must be significantly greater than the spacing within the sub-arrays. On the other hand, it is desirable to keep the minimum distance between lines of the array to below λ, so as to prevent the array acting as a diffraction grating and producing 'grating lobes' in the radiation pattern. These constraints are very much in conflict, since (depending on relative permittivity of the substrate) each patch can be up to λ/2 in length, and only slightly less in width; sub-array groups of three patches can thus each be over 1.5λ long.
Referring to Figure 4, one solution is to accept the occurrence of grating lobes but ensure that they do not occur in the major planes of the antenna (ie parallel or perpendicular to its cross axes). In Figure 4, the arrangement is a square lattice of parameter P = 1.8λ, with a motif comprising a sub-array group at the corners of the lattice cells and a sub-array group at the centres thereof; it may alternatively be regarded as a square lattice of parameter 0.9λ with alternate cell corners vacant. Here-, since the minimum distance between corresponding diagonal lines of sub-array groups is more than λ, grating lobes will appear in the radiation pattern of the antenna. But since in both major planes of the antenna the distance between adjacent lines of sub-arrays is only 0.9λ and these lines are staggered by P/2, the grating effects cancel and no grating lobes appear in these planes; 0.9λ is selected so as to maximise the distance apart of sub-array groups, without generating grating lobes.
Referring now to Figure 5, it is possible to achieve an array giving no grating lobes, although with maximum patch width w =_ 93°/0 L the spacing between parasitic patches of adjacent sub-array groups is reduced to what is effectively the minimum workable value of about 2S. This is achieved, as shown, by providing sub-arrays in lines spaced apart at P = λ (which is close to the minimum achievable), but arranging the lines in a staggered configuration so that the diagonal centre-to-centre distance between sub-arrays is just under λ and thus no grating lobes occur.
In the embodiments shown in Figures 4 and 5, L = 20mm, W = 18.5mm, and the substrate is 1.57mm PTFE (£_ = 2.22).
Antennas according to the invention thus have several advantages.
Since a single feed point is required for each parasitic sub-array rather than for each element, there is a reduction in feed complexity, and thus manufacture is simplified and power splitter loss reduced. Similarly, phase shifting and diplexing can also occur at sub-array level leading to a saving in hardware. Parasitic sub-arrays give significant improvement in directivity and bandwidth over single elements, but a drawback in the use of parasitic sub-arrays is that the directivity obtained is marginally "lower than that obtained from a similar corporate fed array due to the limited amount of phase control that can be obtained from this type of parasitic coupling between microstrip radiating elements.
Hitherto, the sub-array groups have been discussed in terms of symmetrical pairs of parasitic patches (3a, 3b), (3c,3d) flanking a fed patch 1.
It is of course possible to provide instead an asymmetrical pair of patches (having different widths or separations), or even only a single parasitic patch. In this case the beam produced will be 'squinted', instead of propagating perpendicular to the patch; such antennas find application in, for example, satellite reception since a satellite will usually be at an elevation angle (30 in the UK, for example) to the horizontal whereas a printed antenna is preferably mounted flat on a wall.
Whilst in the foregoing the invention has been discussed in terms of a transmitter, it is of course equally applicable to receiver antennas; references to feeds and feed lines will be generally understood to include this.