CIRCULARLY POLARIZED ANTENNA

Abstract

An elliptically or circularly polarized microstrip antenna. The antenna includes a substrate, a conductor arranged on a first side of the substrate, and a ground plane. The conductor includes a first antenna extending generally in a first direction, and a second antenna extending generally in a second direction, wherein the second direction is orthogonal to the first direction. The second antenna is connected to the ground plane via one or more electrical components including one or more resistors, one or more inductors and/or one or more capacitors.

Claims

1. A microstrip antenna configured to emit and/or receive elliptically or circularly polarized radiation, the microstrip antenna comprising: a substrate; a conductor arranged on a first side of the substrate; and a ground plane; wherein the conductor comprises a first antenna extending generally in a first direction, and a second antenna extending generally in a second direction, wherein the second direction is orthogonal to the first direction; and wherein the second antenna is connected to the ground plane via one or more electrical components comprising one or more resistors, one or more inductors and/or one or more capacitors.

2. The antenna of claim 1, wherein the first antenna comprises a first monopole section extending along the first direction, and the second antenna comprises a second monopole section extending along the second direction.

3. The antenna of claim 1, wherein: the first antenna comprises an inverted L-antenna or an inverted F-antenna; and/or the second antenna comprises an inverted L-antenna or an inverted F-antenna.

4. The antenna of claim 1, wherein the first antenna comprises an inverted F-antenna.

5. The antenna of claim 1, wherein: the first antenna comprises a first monopole section arranged along the first direction; the first monopole section is connected to the ground plane by a first microstrip section at one end of the monopole section; the first antenna comprises a second microstrip section connected to the first monopole section at an intermediate point on the monopole section; and the antenna is fed by an input provided on the second microstrip section.

6. The antenna of claim 1, wherein the second antenna comprises an inverted L-antenna.

7. The antenna of claim 1, wherein the second antenna comprises a monopole section arranged along the second direction, and the second monopole section is connected to the ground plane by a microstrip section at one end of the monopole section.

8. The antenna of claim 7, wherein the monopole section of the second antenna is connected to the ground plane by a first microstrip line section extending in the first direction connected in series with a second microstrip line section extending in the second direction.

9. The antenna of claim 7, wherein the one or more electrical components are connected in series between the monopole section of the second antenna and the ground plane.

10. The antenna of claim 1, wherein the one or more electrical components form a phase shift circuit.

11. The antenna of claim 1, wherein the one or more electrical components comprise: one or more resistors connected in series with one or more inductors; and/or one or more resistors connected in series with one or more capacitors.

12. The antenna of claim 1, wherein the one or more electrical components comprise one or more surface mounted electrical components.

13. The antenna of claim 1, wherein the first antenna comprises an inverted F-antenna and the second antenna comprises an inverted L-antenna.

14. The antenna of claim 1, wherein the second antenna comprises a monopole section arranged along the second direction, and the second monopole section is connected to the ground plane by a microstrip section at one end of the monopole section, and wherein the monopole section of the second antenna is connected to the ground plane by a first microstrip line section extending in the first direction connected in series with a second microstrip line section extending in the second direction.

15. A microstrip antenna configured to emit and/or receive elliptically or circularly polarized radiation, the microstrip antenna comprising: a substrate; a conductor arranged on a first side of the substrate; and a ground plane; wherein the conductor comprises a first antenna extending generally in a first direction, and a second antenna extending generally in a second direction, wherein the second direction is orthogonal to the first direction; wherein the second antenna is connected to the ground plane via one or more electrical components comprising one or more resistors, one or more inductors and/or one or more capacitors; and wherein the first antenna comprises an inverted F-antenna and the second antenna comprises an inverted L-antenna.

16. A microstrip antenna configured to emit and/or receive elliptically or circularly polarized radiation, the microstrip antenna comprising: a substrate; a conductor arranged on a first side of the substrate; and a ground plane; wherein the conductor comprises a first antenna extending generally in a first direction, and a second antenna extending generally in a second direction, wherein the second direction is orthogonal to the first direction; wherein the second antenna is connected to the ground plane via one or more electrical components comprising one or more resistors, one or more inductors and/or one or more capacitors; and wherein the second antenna comprises a monopole section arranged along the second direction, and the second monopole section is connected to the ground plane by a microstrip section at one end of the monopole section, and wherein the monopole section of the second antenna is connected to the ground plane by a first microstrip line section extending in the first direction connected in series with a second microstrip line section extending in the second direction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Certain preferred embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which:

[0045] FIG. 1 shows schematically an antenna configured in accordance with an embodiment;

[0046] FIG. 2 shows schematically an antenna configured in accordance with an embodiment;

[0047] FIG. 3 illustrates schematically circularly polarized radiation;

[0048] FIG. 4 illustrates schematically an equivalent circuit of a phase shift circuit configured in accordance with an embodiment;

[0049] FIG. 5 illustrates schematically of a phase shift circuit configured in accordance with an embodiment; and

[0050] FIG. 6A illustrates the far field pattern; and

[0051] FIG. 6B illustrates the axial ratio (AR) of an antenna configured in accordance with an embodiment.

DETAILED DESCRIPTION

[0052] Antennas that are configured to emit and/or receive circularly polarized radiation (“circularly polarized antennas”) are widely used in various applications such as wireless communications, Internet of Things (IoT) devices, Global Position System (GPS) device, and so on. Circularly polarized antennas can beneficially be used to provide positioning and polarization diversity. It can be desirable to provide a compact circularly polarized antenna, e.g. for miniature and handheld applications.

[0053] FIGS. 1 and 2 illustrate a microstrip antenna in accordance with various embodiments that is configured to emit and/or receive circularly polarized radiation.

[0054] The antenna is a microstrip antenna, and so is formed from a substrate 1, an electrically conducting layer 2 arranged on one side of the substrate, and a ground layer 3b arranged on the opposite side of the substrate.

[0055] The substrate 1 may comprise a layer of electrically insulting material such as, for example, Duroid, Teflon or FR4. The conducting layer 2 may comprise a thin layer of a metal such as copper, and may have a uniform thickness.

[0056] A microstrip structure may be fabricated on printed circuit board (PCB) or as part of monolithic microwave integrated circuits (MMICs) using conventional methods known to the skilled person such as milling, screen printing, and chemical etching. A desired two-dimensional shape may be formed in the conducting layer 2, e.g. by etching or milling the conducting layer to remove unwanted conducting material.

[0057] The ground layer 3b comprises another layer of electrically conductive material (e.g. copper) formed on the opposite side of the substrate 1 to the conducting layer 2. The ground layer 3b may cover most of the substrate 1 on the side on which it is placed. The ground layer 3b should not cover the area directly beneath the first 5 and second 6 antennas.

[0058] As illustrated in FIGS. 1 and 2, a ground plane 3a in the conducting layer 2 is connected to the ground layer 3b by vias 4, i.e. electrical connections between the conducting layer 2 and the ground layer 3b in the form of through holes where the edges of the holes are coated in an electrically conducting material.

[0059] As illustrated in FIGS. 1 and 2, the conducting layer 2 is formed (e.g. etched or milled) into a shape that comprises a first microstrip antenna 5 extending generally in a first (x) direction, and a second microstrip antenna 6 extending generally in a second orthogonal (y) direction. A third (z) direction may be defined as the direction orthogonal to the plane of the substrate 1 and/or the plane of the conducting layer 2 (and so orthogonal to the first (x) and second (y) directions).

[0060] More specifically, in the present embodiment, the first microstrip antenna 5 is an inverted F-antenna, i.e. comprising a λ.sub.g/4 length monopole section extending in the first (x) direction, and being connected to the ground plane 3a a single microstrip section which is connected to one end of the monopole section. The inverted F antenna 5 also comprises an intermediate feeding microstrip section which is connected to an intermediate region of the monopole section. The whole antenna is fed via an input 7 provided on the intermediate feeding microstrip section.

[0061] The second microstrip antenna 6 is an inverted L-antenna, i.e. comprising a λ.sub.g/4 length monopole section extending in the second (y) direction, and being connected to the ground plane via a single microstrip section at one end of the monopole section.

[0062] As shown in FIG. 1, the monopole section of the second antenna 6 is connected to the ground plane 3a via an “L”-shaped microstrip section. Thus, the monopole section of the second antenna 6 is connected to the ground plane 3a via a first microstrip line section extending in the first (x) direction and an additional microstrip line section 9 that extends in the second (y) direction.

[0063] As is also illustrated in FIG. 1, the second antenna 6 is connected to the ground plane 3a via one or more electrical components 8 connected in series between a portion of the microstrip line section and the monopole section of the second antenna 6. The electrical components 8 comprise one or more resistors, one or more inductors and/or one or more capacitors.

[0064] Connecting the second antenna to the ground plane via one or more electrical components provides additional control and flexibility in the design of the antenna. In other words, the antenna of various embodiments has one or more additional degrees of freedom in addition to geometric degrees of freedom in its design.

[0065] The inverted F-antenna 5 is configured to emit and/or receive electromagnetic radiation that is linearly polarized in the second (y) direction, and the inverted L-antenna 6 is configured to emit and/or receive electromagnetic radiation that is linearly polarized in the first orthogonal (x) direction. Where the amplitudes of radiation emitted by/received by each of the antennas are equal, and where the phase difference between the radiation emitted by/received by the first 5 and second 6 antennas is 90° (i.e. where the amplitude and phase are balanced between the first 5 and second 6 antennas), the emitted/received radiation will be “perfectly” circularly polarized. Where, on the other hand, one or both of the amplitude and phase are not perfectly balanced, the emitted/received radiation will be elliptically polarized.

[0066] The electric field of a radiating polarized wave traveling in the third (positive z) direction, can be described by:

{right arrow over (E)}(t, z)={right arrow over (x)}.Math.E.sub.x0 cos(ωt−kz+φ.sub.x)+{right arrow over (y)}.Math.E.sub.y0 cos(ωt−kz+φ.sub.y)

[0067] where Ex0 is the maximum magnitude of the x component of the electric field, and Ey0 is the maximum magnitude of the y component of the electric field; ω is the radial frequency, ω=2πf; k is the propagation constant (also known as the phase constant, or wave number),

[00001]$k=\frac{2\pi }{\lambda };$

z is the axis of electromagnetic wave propagation; and ΔΦ is the phase difference between the two components, Δφ=φ.sub.y−φ.sub.x.

[0068] As illustrated by FIG. 3, the ratio of the major axis to the minor axis is referred to as the axial ratio (AR), and it is equal to:

[00002]$AR=\frac{OA}{OB},1\le AR\le \infty .$

[0069] In the case of circular polarization,

[0070] E.sub.x0=E.sub.y0; Δφ=φ.sub.y−φ.sub.x=±(½+2n)π, n=0, 1, 2, . . . ,

[0071] and AR is close to or equal to 1.

[0072] In order to design an antenna with an AR close to 1 (and to achieve other desired properties), the geometric dimensions of the antenna layout can be adjusted. For example, amplitude balance E.sub.x0=E.sub.y0 may be achieved by appropriate choices for the lengths of the antennas 5, 6, the sizes of the gaps between the antennas 5, 6 and the ground plane 3a, the width of the microstrip line section 9, and so on.

[0073] The phase balance Δφ=±(½+2n) π can then be adjusted by providing an additional phase-shift circuit that introduces a phase shift Δφ.sub.S, as will now be described in more detail.

[0074] As illustrated by FIG. 4, a complex impedance may be provided by a resistor R connected in series with an inductor or a capacitor. The resistor provides the real part R of the complex impedance, and an inductor or a capacitor provides the imaginary part (the reactance X) of the complex impedance.

[0075] As illustrated by FIG. 5, the resistor may be an SMD (surface mounted device) resistor, the inductor may be an SMD inductor, and/or the capacitor may be an SMD capacitor.

[0076] Connecting the complex impedance Z to the input of the L-antenna (having input impedance Z.sub.LAin=R.sub.LA+jX.sub.LA) forms a phase-shift circuit that introduces a phase shift Δφ.sub.S.

[0077] In the case that an imaginary part X of the complex impedance Z is an inductance X.sub.L, the phase shift is:

[00003]${\mathrm{\Delta \phi }}_S=-\mathrm{arc}h\tan \frac{X_L}{R_{LA}},$

[0078] and in the case that the imaginary part X of the complex impedance Z is a capacitance X.sub.C, the phase shift is:

[00004]${\mathrm{\Delta \phi }}_S=\mathrm{arc}h\tan \frac{x_C}{R_{LA}},$

[0079] where R.sub.LA is the real part of the input impedance of the inverted-L antenna.

[0080] Such a phase shifting circuit can introduce a fixed phase shift over a wide frequency range.

[0081] This approach offers a very compact layout because lumped elements typically are much smaller than delay lines. This is an important consideration for low frequency designs (i.e. below X-band), since delay transmission lines can be quite large. The real part R of the complex impedance X decreases the quality factor

[00005]$Q=\frac{f}{\Delta f}$

of the phase shift circuit and increases the frequency bandwidth, since

[00006]$\Delta f=\frac{f}{Q}.$

[0082] It will be understood that the electrical components 8 can be selected so as to control the amplitude and/or phase balance between the first antenna 5 and the second antenna 6. This in turn allows simultaneous control of the axial ratio, the antenna gain and the input impedance of the antenna.

[0083] FIG. 6A illustrates the far field pattern G(θ,φ) of an antenna configured in accordance with an embodiment. A maximum gain of 1.5 dBi, and an efficiency of 85% are shown. It can also be seen that there are no deep zeroes (i.e. significant reduction of the antenna's gain) in the antenna's radiation characteristics.

[0084] FIG. 6B illustrates the axial ratio (AR) of an antenna configured in accordance with an embodiment. This figure demonstrates that the proposed antenna provides an AR lower than 3 dB in the θ angle range of 90° (θ from −60° to +30°) for φ=0° (XZ plane or the azimuth plane depending on antenna orientation relatively to the Earth's surface), which is satisfactory for various applications.

[0085] It will be appreciated that various embodiments provide an antenna configured to provide circular polarized electromagnetic radiation formed from an inverted-F antenna 5, and an additional inverted-L antenna 6 connected to the ground plane 3a through a microstrip line section 9 and serial complex impedance 8.

[0086] The inverted-F antenna 5 is the source of electromagnetic waves with a first linear polarization, and the inverted-L antenna 6 is the source of electromagnetic waves with a second linear polarization orthogonal to the first one. The input 7 of inverted-F antenna 5 is the input of the whole circularly polarized antenna.

[0087] The inverted-L antenna is connected to the ground plane 3a through the microstrip line section 9 and serial complex impedance 8. The microstrip line section 9 and the serial complex impedance 8, which connect the additional inverted-L antenna 6 to the ground plane 3a, are used to regulate the amplitude and phase balance between the two orthogonal radiating components, and to control the axial ratio, antenna gain and input impedance of the antenna.

[0088] The antenna is compact, and therefore especially useful for miniature sensors and handheld devices; low-cost; and has improved characteristics (axial ratio, gain, impedance matching) in comparison to conventional antennas.