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; principally, small bandwidths and low 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 British Patent 2067842). The mechanisms by which such parasitic patches are excited have not hitherto been well understood, however, so it has not proved possible to design optimum performance antennas comprising an array of patches of which some are parasitically fed.

Our co-pending PCT patent application published as WO 89/07838 provides, in one aspect, 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 an array of 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 a preferred embodiment of that invention, two sheets of substrates are employed, the feed network being printed on one and the patches on the other.

The use of parasitic elements to improve the directivity of microstrip patches has been described in the above referenced patent application. However, the use of these arrangements to provide optimum bandwidth has not been described. Objects of the present invention are to provide an antenna having a bandwidth suitable for microwave or millimetre wave video signal reception, capable of being made at a relatively low cost.

According to the present invention there is therefore provided an antenna comprising a planar array of aligned groups of rectangular microstrip patches disposed upon a substrate, each group comprising a central, fed, patch connected at a radiative edge thereof to a coplanar feedline, and a pair of patches coupled parasitically one to each of the non-radiative edges of the fed patch.

The groups will hereinafter be referred to as ' subarrays' .

The term "array" is used in this specification to mean an arrangement of NxM elements, or groups or subarrays, as the context requires, where N and M are integers greater than one.

Because the feed lines are upon the same substrate as the patches, only a single sheet of substrate and a single printing operation is needed, thus reducing the antenna cost.

It was found that simply adding parasitic elements according to the dimensions derived for the embodiment in the above referenced patent application resulted in two discrete resonances rather than a single broad band because the parasitic elements were too far separated in resonant frequency from the directly fed element. This effect is due to the feed line altering the resonant frequency . of the central patch, and acting as if to extend its length. It might be thought that lengthening the parasitic elements relative to the fed element would therefore bring the two

resonances together. In fact, this is not found. Several further embodiments of the invention are therefore addressed to the problem of realising an antenna of this type with a single broadband resonance. In a first embodiment, the spacing between adjacent edges of the fed and parasitic patches within a group is 15-17% of the width w of the fed patch, and the parastic patches are typically 6% shorter than the fed patch.

Preferably, in this embodiment, the parasitic patches are shorter than the non-radiative edges of the fed patch by an amount sufficient to cause the antenna to exhibit a substantially broadband resonance rather than individual fed patch and parasitic-patch resonances.

To maintain sufficient radiation efficiency, the subarr y width must be kept high and yet to avoid ' grating lobes' in the radiation pattern, the distance between subarray centres must be kept well below the operating wavelength. Also, the feed lines may need to pass between the radiating elements so that the area available for the patch radiators is further limited. With normal substrates, these constraints are very much in conflict, and it is found that, in this first embodiment, the subarray groups are too wide to fulfil the latter constraints (although this is not vital in all applications).

A second embodiment therefore provides a surface-fed parasitic array antenna wherein the parasitic patches are approximately equal in length to th.e fed patch, and are longitudinally displaced so that they are only adjacent the fed patch along a portion of their length, the displacement being sufficient to cause the antenna to exhibit a substantially broadband resonance rather than individual fed-patch and parasitic-patch resonances.

This embodiment satisfies the width criteria above. However, the effective length of the subarrays has been increased (in a preferred embodiment by around 60%), so making the arrangement of feed lines on adjacent rows of subarrays more difficult in some applications.

In a third embodiment, there is provided a surface-fed parasitic array antenna in which the width of the parasitic patches is 40-60% of their length and they are spaced from the fed patch by a spacing s between 15-35% of their length, the spacing s being substantially less than the spacing between adjacent groups in a row. This embodiment is particularly advantageous as a micro or millimetre wave (12-100 GHz) domestic TV reception antenna.

In a fourth embodiment, each elemental group further comprises a pair of third patches, each disposed between and spaced from the first and each second patch, the width of each third patch being relatively narrow and of a length related to the resonant frequency of the antenna.

The invention will now be described by way of example only, with reference to the accompanying drawings, in which;

Figure 1 shows schematically a generalised antenna array group according, to the invention;

Figure 2 shows schematically an antenna array group according to a first embodiment of the invention;

Figure 3 shows schematically an antenna array group according to a second embodiment of the invention;

Figure 4 shows schematically an antenna array group according to a third embodiment of the invention;

Figure 5 shows schematically an antenna feed according to a different aspect of the invention;

Figure 6 shows schematically an antenna including the embodiments of Figures 4 and 5; and

Figure 7 shows schematically an antenna array group according to a fourth embodiment of the invention.

Referring to Figure 1, an array element in an array antenna comprises a first printed patch 1 having a resonant length L,, centrally fed at a radiative edge from a printed feed line 2, both being disposed upon a dielectric substrate. Flanking the non-radiative edges of the first patch 1 and spaced therefrom at a distance S _j are a pair of parasitic patches- 3a, 3b, coupled parasitically thereto.

In this example, the substrate is 1.57mm thick Tachonics TLY-5 with a relative permittivity ε _r of 2.2, and the

resonant length L _j is approximately λ/2_Jε - in this example 20mm for the frequency range 4.4 to 5 GHz (the thickness of substrate will also be varied proportionately to the operating wavelength).

The width _j of the central patch of the subarrays was maintained at 18.5mm throughout the work, since patches with a lower aspect ratio (W/L) than 0.925 have unacσeptably high feed impedances, and aspect ratios nearer to 1.0 result in a poor boresight crosspolar radiation level. This patch is fed using a 50 ohm miσrostripline 2 with a quarter wavelength transformer 2a to match to the (experimentally determined) impedance for a single patch at the centre of the radiative edge of 225 ohms.

A further consideration in element design is that of separation. Group centre-to-centre distances D in a rectangular array should be limited to about 0.9 wavelengths at mid-band if grating lobes are to be avoided over about 10% bandwidth. In other words, the sum of the widths of the fed and parasitic patches, plus the two spacings, plus the intergroup spacing S ₂ , must be below 0.9λ; i. e. W,i + 2W2, + 2S,1 + S2, < 0.9λ.

Since _t = 0.925L, and L _j - λ/3 for this type of substrate, if W ₂ = W

3 x 0.925 x λ/3 + 2S _j + S ₂ < 0.9λ, 0.925λ + 2S _j + S ₂ < 0.9λ, which cannot be solved.

So, simply using three equal width patches in line cannot satisfy the criteria for avoiding grating lobes over a 10% bandwidth (although this structure is useful in narrower bandwidth antennas).

There is also the problem that, with the surface-fed arrangement, if the parasitic patches 3a, 3b have the same shape as the fed patch 1, the antenna shows two poorly matched discrete resonances ^* rather than a single broad band match, .one being the fed patch resonance which is shifted to a lower frequency by the feedline.

Referring to Figure 2, this latter problem is overcome in a first embodiment of the invention in which the lengths L, of the parasitic patches 3a, 3b are made shorter than that (L. ) of the fed patch, preferably by around 6% resulted in a good broad band match, giving a 10 dB return loss bandwidth of 7.5%. However, the width of this subarray element is 0.92 free space wavelengths and so is too great to give a simple rectangular array with wide bandwidth.

It is found that reducing the parasitic patch width W. to overcome this problem results in the reappearance of the two resonances, and reducing the spacing S _j reduces the bandwidth.

Referring to Figure 3, in a second embodiment a single lumped resonance can be produced by offsetting the parasitic patches 3a, 3b longitudinally from the fed patch by an amount X.

At low values of .X, a good match is only achieved to a lower resonance (although the return loss shows some evidence of an upper resonance). As X is increased, the match improves, and at X = 12mm, or 0.6L _j the lOdB return loss bandwidth was found to be 6.8%. At this value of offset, it found that reducing L ₂ to 19.5mm increases the bandwidth to 7.5%. This 3 element group has a total width Y of 50.5mm, equivalent to 0.77 λ.at 4.6 GHz. The element length, however, is increased significantly, making a coplanar feed layout difficult to realise, since the space between adj cent rows of groups is usually largely filled with the feed network.

Referring to Figure 4, we have found that a particularly preferred antenna for wide (* ^» 1'0%) bandwidth applications such as multichannel video distribution comprises groups in which the central patch is - ^» 92% of the non-radiative edge, and the sum of SJ+ S-0.75L _t ; with a substrate of dielectric ε _r = 2.2, this keeps the total element width sufficiently narrow to prevent grating lobes. With S _j = 0.15L _j the lOdB return loss bandwidth is 7.4%, and the return loss is above 12dB across the band.

With S, = 0.20L _j the bandwidth is 7.8%, but the resonances are beginning to separate. With S _j = 0.29L _j a usefully broad bandwidth is still obtained. In an example of such an antenna, suitable for 29GHz operation, dimensions are as follows: - L _j = 3.275mm L, = L,

S _j = 0.945mm W _j = 3.025mm W ₂ = 1.485mm D = 9.473mm

(spacing between centres of adjacent groups) ε, = 2.2, substrate thickness = 0.254mm

In order to form an array antenna whilst avoiding grating lobes in the radiation pattern, the groups must be sufficiently small to allow the feed network to pass between them. When a corporate feed network is used, it is necessary to employ a number of T-splits, and in general, a transformer (e. g. a 1/4 wavelength transformer) is needed to match the feedline to the patches. In the prior art, the transformer is coupled at 90" to the cross arm of the T-split. However, when employing parasitic patch groups, there is insufficient space for the transformer. Preferably the transformer is formed from a substantially straight section of the feedline. This avoids the excessive radiation which would result from a bend in a high impedance section of feedline.

Referring to Figure 5, in a preferred embodiment of the invention, the feedline 2 is retroflexed, or curved back, in a ϋ-shape, so that the T-split 4 lies nearer the patches than the outer end of the transformer 2a.

Referring to Figure 6, an array antenna for use at 29GHz comprises a square array of the groups of the embodiment of Figure 4, fed according to the embodiment of Figure 5; it will be seen that there is very little spare space for the feed network. The antenna is fed from the central point, via an aperture in the substrate.

Referring to Figure 7, in a further embodiment of the invention it is found that in a wide-spaced group of the type shown in Figure 4, it is possible to employ a further pair of parasitic strip patches 5a, 5b located within the gap. It is found that the overall resonant frequency of the group depends upon the length L ₃ of the strip patches 5a, 5b, which has the advantage that a single antenna design can be used for different frequencies, merely by redesigning the strip length.

Antennas in accordance with the embodiments above can have better gain, better bandwidth and narrower H-plane bea widths than single-patch antennas, and better cross-polarization over much of their angular range. Additionally, the complexity of the feed network and associated splitter losses are reduced compared to a single patch antenna with the same gain.

Whilst in the foregoing the invention has been discussed in terms of a transmitter, it is of course equally (indeed preferably) applicable to receiver antennas; references to feeds and feed lines will be generally understood to include this.

The following table sets out both general and particular criteria for dimensions of microstrip parasitic patch antennas of the preferred embodiments.