Microstrip patch antenna with increased bandwidth

Abstract

A microstrip antenna array including: a thin substrate; two or more microstrip radiating patches placed on a first side of the substrate, each radiating patch including: an input port; a radiating patch width (WRP) extending in a longitudinal direction; a radiating patch length (LRP) extending in a transverse direction, wherein the transverse direction is perpendicular to the longitudinal direction, and wherein the longitudinal and transverse directions are in the plane of the radiating patch; a radiating patch transverse axis (TRP) along the midpoint of the radiating patch width; and a radiating patch longitudinal axis along the midpoint of the radiating patch length, wherein the two or more radiating patches are spaced in the longitudinal direction such that the radiating patch longitudinal axis of each radiating patch is aligned along a common longitudinal axis (C); and one or more parasitic patches placed on the first side of the substrate.

Claims

1. A microstrip antenna array (200; 300; 400) comprising: a thin substrate (204); three or more microstrip radiating patches (202; 302; 402) placed on a first side (208) of the substrate (204), each radiating patch (202; 302; 402) comprising: an input port (210); a radiating patch width (W.sub.RP) extending in a longitudinal direction; a radiating patch length (L.sub.RP) extending in a transverse direction, wherein the transverse direction is perpendicular to the longitudinal direction, and wherein the longitudinal and transverse directions are in the plane of the radiating patch; a radiating patch transverse axis (T.sub.RP) along the midpoint of the radiating patch width; and a radiating patch longitudinal axis along the midpoint of the radiating patch length, wherein the two three or more radiating patches are spaced in the longitudinal direction such that the radiating patch longitudinal axis of each radiating patch is aligned along a common longitudinal axis (C); and two or more parasitic patches (212; 312; 412) placed on the first side (208) of the substrate (204), wherein there is at least one fewer parasitic patches than there are radiating patches, each parasitic patch comprising: a parasitic patch width (W.sub.PP) extending in the longitudinal direction; a parasitic patch length (L.sub.PP) extending in the transverse direction; a parasitic patch transverse axis (T.sub.pp) along the midpoint of the parasitic patch width; and a parasitic patch longitudinal axis along the midpoint of the parasitic patch length, wherein the two or more parasitic patches (212; 312; 412) are spaced in the longitudinal direction such that the parasitic patch longitudinal axis of each parasitic patch is aligned along the common longitudinal axis (C), wherein each parasitic patch is positioned between two radiating patches (202; 302; 402), and wherein the parasitic patch transverse axis (T.sub.PP) of each parasitic patch is positioned at the midpoint between the radiating patch transverse axes (T.sub.RP) of the two radiating patches either side of each parasitic patch, wherein the parasitic patch width and gaps between radiating patches G.sub.p are tuned to provide the certain strength of coupling k between radiating patches.

2. The array of claim 1, wherein the radiating patch input ports are positioned along the radiating patch transverse axis.

3. The array of claim 1, wherein the substrate has a thickness of 1.0 mm or less.

4. The array of claim 1, wherein the radiating patches are regularly spaced along the common longitudinal axis.

5. The array of claim 1, wherein the radiating patch transverse axes of adjacent radiating patches are separated by about a half wavelength of an input signal, wherein the wavelength of the signal is modified by the substrate.

6. The array of claim 1, wherein the parasitic patch length is about a half wavelength of an input signal, wherein the wavelength of the signal is modified by the substrate.

7. The array of claim 1, wherein at least one of the two or more parasitic patches is symmetric about the common longitudinal axis.

8. The array of claim 1, wherein at least one of the two or more parasitic patches is symmetric about its parasitic patch transverse axis.

9. The array of claim 1, wherein at least one of the three or more radiating patches is symmetric about its radiating patch transverse axis.

10. The array of claim 1, wherein at least one parasitic patch comprises at least one VIA.

11. The array of claim 10, wherein the VIA is positioned along the common longitudinal axis.

12. The array of claim 10, wherein the VIAs are positioned to divide the parasitic patch into two quarter wavelength λ.sub.d/4 resonant portions.

13. The array of claim 1, wherein one of the parasitic patches comprises two or more parasitic microstrip lines, the lines being spaced apart along the common longitudinal axis and between two radiating patches.

14. The array of claim 13, wherein the gap between the two or more parasitic microstrip lines is tuned to provide necessary coupling between them.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Certain embodiments of the disclosure will now be described by way of example only and with reference to the accompanying drawings in which:

(2) FIG. 1A is a top view of a prior art microstrip antenna array.

(3) FIG. 1B is a side view of a prior art microstrip antenna array.

(4) FIG. 2A is a top view of an example microstrip antenna array.

(5) FIG. 2B is a side view of an example microstrip antenna array.

(6) FIG. 3A is a top view of another example microstrip antenna array.

(7) FIG. 3B is a side view of another example microstrip antenna array.

(8) FIG. 4A is a top view of yet another example microstrip antenna array.

(9) FIG. 4B is a side view of yet another example microstrip antenna array.

(10) FIG. 5 shows S-parameters for the prior art antenna array and for each of the example antenna arrays.

(11) FIG. 6 shows a graph of the voltage standing wave ratio (VSWR) at the input of a radiating patch for each of the prior art and example antenna arrays.

(12) FIG. 7 shows a spherical polar coordinate system applied to a microstrip antenna array.

(13) FIGS. 8A and 8B are radiation patterns of the prior art patch antenna array and for each of the example arrays at angles of φ=0 and φ=90 based upon the coordinate system shown in FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

(14) A prior art patch antenna array 100 is presented in FIGS. 1A and 1B, where FIG. 1A shows a top view of the array 100 to display an arrangement of radiating patches 102 and FIG. 1B shows a side view of the array 100. The prior art patch antenna array 100 comprises a substrate 104 formed from a single layer of substrate material, a layer of conducting material forming a ground layer 106 on the bottom side of the substrate 104, and a plurality of radiating patches 102 on a top side 108 of the substrate 104. Each radiating patch 102 has an input port 110, a radiating patch width W.sub.RP extending in a longitudinal direction, and a radiating patch length L.sub.RP extending in a transverse direction. The each of the plurality of radiating patches 102 are spaced along a common longitudinal axis C and are oriented so that the input ports 110 for each of the radiating patches 102 are oriented in the same direction.

(15) Each radiating patch 102 also comprises a radiating patch transverse axis T along the midpoint of the radiating patch width W.sub.RP. Starting from the leftmost radiating patch in FIG. 1A and moving rightwards, the radiating patches 102 may be labelled RP1, RP2 . . . RPN, for N number of radiating patches 102. The distance between the transverse axes T of two adjacent patches 102, starting from the distance between RP1 and RP2 and moving rightwards may then be labelled S.sub.RP1, S.sub.RP2 . . . S.sub.RP(N-1).

(16) The mutual coupling between patches 102 is characterized either by the conductance matrix (G-matrix) or by the scattering matrix (S-matrix).

(17) The mutual conductance between two rectangular microstrip patches for the radiating patch arrangement is [1]:

(18) $G_{12}=\frac{2}{\pi }.Math.\sqrt{\frac{\mathrm{.Math.}}{\mu }}.Math.{\int }_0^{\pi }{\left[\frac{\sin \left(\frac{k_{0}W}{2}.Math.\cos \theta \right)}{\cos \theta }\right]}^2{\sin }^3\theta .Math.\cos \left(\frac{Z}{{\lambda }_0}2\pi .Math.\cos \theta \right)\left[1+J_0\left(\frac{L}{{\lambda }_0}2\pi \sin \theta \right)\right]d\theta$

(19) J.sub.0—the Bessel function of the first kind of order zero;

(20) Z—the center-to-center separation between the patches and equal to the array step S.sub.RP.

(21) W—the width of the radiating patch;

(22) L—the length of the radiating patch;

(23) λ.sub.0—is the wavelength in free space;

(24) ε—the permittivity of free space;

(25) μ—the permeability of free space.

(26) In the prior art array 100 shown in FIGS. 1A and 1B, the fields in the space between the elements are primarily transverse electric (TE) modes and there is not a strong dominant mode surface wave excitation. Therefore, there is reduced coupling between the elements. When the coupling is small, the resonant frequency of the patch radiator is close to the resonant frequency of uncoupled antennas f.sub.0.

(27) When the strength of coupling increases, two resonant frequencies f.sub.1 and f.sub.2 of coupled patches appear. The strength of coupling is described with the coupling coefficient k that can be computed from the following formula:

(28) $k=\frac{f_2^2-f_1^2}{f_2^2+f_1^2}$

(29) f.sub.1—the lower resonant frequency of coupled antennas;

(30) f.sub.2—the upper resonant frequency of coupled antennas.

(31) To improve the coupling between radiating patches a parasitic patch is used. Placing a resonance structure (the parasitic patch) between active radiating patches increases coupling between the radiating patches and provides mutual detuning of radiators. Active radiating patches are radiating patches that are being fed with a signal via the input port of the radiating patch.

(32) One example of a microstrip patch antenna array 200 having parasitic patches is shown in FIGS. 2A and 2B. The microstrip antenna array 200 comprises a thin substrate 204 and two or more microstrip radiating patches 202 placed on a first side 208 of the substrate 204. Each radiating patch 202 comprises an input port 210, a radiating patch width W.sub.RP extending in a longitudinal direction, and a radiating patch length L.sub.RP extending in a transverse direction, wherein the transverse direction is perpendicular to the longitudinal direction, and wherein the longitudinal and transverse directions are in the plane of the radiating patch 202. Each patch 202 also comprises a radiating patch transverse axis T.sub.RP along the midpoint of the radiating patch width W.sub.RP and a radiating patch longitudinal axis along the midpoint of the radiating patch length. The two or more radiating patches 202 are spaced in the longitudinal direction such that the radiating patch longitudinal axis of each radiating patch 202 is aligned along a common longitudinal axis C.

(33) The microstrip patch array 200 also comprises one or more parasitic patches 212 placed on the first side 208 of the substrate 204, wherein there are at least one fewer parasitic patches 212 than there are radiating patches 202. Each parasitic patch 212 comprises a parasitic patch width W.sub.PP extending in the longitudinal direction, a parasitic patch length L.sub.PP extending in the transverse direction, a parasitic patch transverse axis T.sub.PP along the midpoint of the parasitic patch width, and a parasitic patch longitudinal axis along the midpoint of the parasitic patch length. The one or more parasitic patches 212 are spaced in the longitudinal direction such that the parasitic patch longitudinal axis of each parasitic patch 212 is aligned along the common longitudinal axis C.

(34) Each parasitic patch 212 is positioned between two radiating patches 202 and the parasitic patch transverse axis T.sub.PP of each parasitic patch is positioned at the midpoint between the radiating patch transverse axes T.sub.RP of the two radiating patches 202 either side of each parasitic patch 212.

(35) The parasitic patch 212 has such dimensions so that to provide necessary coupling k between radiating patches 202. The length of parasitic patch L.sub.PP is approximately close to a half wavelength in substrate λ.sub.d at a central working frequency f.sub.0. The parasitic patch width W.sub.PP and gaps between radiating patches G.sub.P are tuned to provide the certain strength of coupling k between radiating patches 202.

(36) Another example of a microstrip patch antenna array 300 is shown in FIGS. 3A and 3B. The construction of the antenna array 300 is similar to that of the previous example in that the radiating patches 302 are the same and the parasitic patches 312 comprise a strip of conducting metal, each parasitic patch 312 being positioned between two radiating patches 302. That is, the length of parasitic patch L.sub.PP approximately is close to half wavelength in substrate λ.sub.d/2 at central working frequency f.sub.0. The width of parasitic patch W.sub.PP and gaps between radiating patches G.sub.P are tuned to provide the certain strength of coupling k between radiating patches.

(37) The parasitic patches 312 shown in FIG. 4 also comprise two VIAs 314 in each patch 312. The VIAs 314 are an electrical connection between the conducting metal portion of the parasitic patch 312 and the ground plane, passing though the substrate. The VIAs 314 are positioned within the area of the conducting metal portion of the parasitic patch 312 and along the common longitudinal axis C. The VIAs 314 are placed along the parasitic patch longitudinal axis and divide the conducting metal portion of the parasitic patch 312 into two quarter wavelength λ.sub.d/4 resonant portions 316. The quarter wavelength λ.sub.d/4 portions 316 are coupled together through the VIAs 314. This coupling creates an additional resonance frequency f.sub.3. The distance between VIAs 314 and their diameters is tuned to provide necessary coupling between the two quarter wavelength λ.sub.d/4 resonance portions 316.

(38) Yet another example of a microstrip patch antenna array 400 is shown in FIGS. 4A and 4B. In this example, the radiating patches 402 are the same as in the previous two examples. The parasitic patch 412 in this example comprises two parasitic microstrip lines 414 are placed between the radiating patches 402. The length of parasitic microstrip lines L.sub.PML approximately is close to a half wavelength of the signal in substrate λ.sub.d/2 at the central working frequency f.sub.0. Each parasitic microstrip line has a width W.sub.PML. The gaps between parasitic microstrip lines and radiating patches G.sub.P are tuned to provide the certain strength of coupling k between radiating patches 402. The parasitic microstrip lines 414 are coupled together through the gap G.sub.PML. This coupling creates an additional resonance frequency f.sub.3. The gap between parasitic microstrip lines G.sub.PML is tuned to provide necessary coupling between them.

(39) The S-parameters for the prior art antenna array and for each of the examples are shown in FIG. 5. S-parameters characterize the mutual coupling between radiating patches, and the S.sub.21 parameter indicates power loss or gain at the output of the system as compared to the energy put into the system.

(40) FIG. 6 shows a graph of the voltage standing wave ratio (VSWR) at the input of a radiating patch for each of the prior art and the above example antenna arrays. At a VSWR of, 10% of the input power is reflected and this is a level at which the antenna may be considered to be impedance matched with the input feedline. At this value, it can be clearly seen from the graph that the bandwidth for each of the example patch arrays is significantly wider than that of the prior art array.

(41) FIG. 7 shows a spherical polar coordinate system, where the x-axis is collinear with the common longitudinal axis, the y-axis is parallel to the transverse direction, and the z-axis is in a direction upwards from the substrate and antenna and is perpendicular to the conducting plane. The origin of the coordinate axis is at the midpoint between two radiating patches.

(42) FIGS. 8A and 8B are radiation patterns of the prior art patch antenna array and for each of the example arrays at angles of φ=0 and φ=90 based upon the coordinate system shown in FIG. 7. The mutual coupling between the radiating patches and the parasitic patches causes a slight distortion of the radiating characteristic of radiating patch G(θ) and reduces the gain of the radiating patch no higher than 1.5 dB, which is appropriate for many applications.

(43) In some embodiments two adjacent radiating patches with a parasitic patch between them may be united by a common feeding network, hence forming them into one interconnected structure. In this case the input ports of the two adjacent radiating patches can be connected together and joined to the common feeding network. The feeding network can be configured to provide a necessary amplitude and phase distribution for signals exiting the radiating patches. Such a structure alleviates a distortion of the radiating characteristic, which is caused by the mutual coupling between the radiating patches, so that there is almost no reduction in the gain (lower than 0.5 dB). With this type of antenna, with two radiating patches having a common feeding network, the parasitic patch may be any of the types described previously. This antenna may be used as a single independent antenna with increased bandwidth or as a part (subarray) of a larger antenna array, with multiple pairs of radiating patches each pair having interconnected input ports. In an antenna array consisted of such subarrays, there may be a parasitic patch between two adjacent subarrays or it may be eliminated.