WO2007141505A1

WO2007141505A1 - A patch antenna

Info

Publication number: WO2007141505A1
Application number: PCT/GB2007/002052
Authority: WO
Inventors: Chicot Van Niekerk; Stefan Eben Goosen; Reinart Johan Moraal; Thavendran Reddy; Allan Linton-Walls; Hendrik Johannes Du Preez
Original assignee: Wavetrend Technologies Limited
Priority date: 2006-06-09
Filing date: 2007-06-04
Publication date: 2007-12-13
Also published as: TW200818606A; BRPI0712363A2; CA2653542A1; EP2027627A1; GB0611481D0; AU2007255168A1

Abstract

An antenna for communicating data, the antenna comprising a dielectric layer that separates an electrically grounded layer from a conductor layer, wherein the conductor layer has a first portion connected to a capacitive element. Increasing the capacitive element makes the patch antenna looking electrically longer than it physically is.

Description

A PATCH ANTENNA

Field of the Invention

The present invention relates to antenna technology, and in particular, but not exclusively to a patch antenna.

Background

Digital communications have become essential in the modern age in which data is transmitted between various locations around the world. hi particular, the field of wireless communications has exploded especially in the area of mobile phone communications and/or other wireless computer-related devices. Indeed, such has been the growth of wireless communications, specifically RF-type wireless communications, that the frequency spectrum for transmitting radio waves is becoming increasingly crowded.

One of the most important aspects of any RP (Radio Frequency) system is the design of an appropriate antenna that is able to transmit and receive wireless data as required, but additionally is able to meet the specific design requirements of the application in question.

There are a plurality of different types of antenna designs to chose from, each having their own strengths and weaknesses. The RP designer must try and select the type of antenna whose properties are most suitable for the relevant application. For example, for a mobile phone application the RF designer will typically look for a compact antenna design having a low-power properties that occur when size, weight and portability are important as they are in the wireless field.

There are a plurality of different antenna geometries, for example the standard dipole or loop antenna configurations.

However in the area of RPID (radio frequency identification) tags it is desirable to have an antenna possessing certain properties, for example: small in size, a low profile and lightweight. Such antennas can be used as transmitters, receivers or transceivers that can be easily attached to a package or other moveable asset to be tracked. For this type of application a micro strip antenna is often most suitable. A micro strip antenna is often referred to as a patch antenna since it consists of a patch of metallisation overlying, yet separated from, a ground plate. Specifically, a patch antenna is often manufactured by etching an antenna element pattern in a metal trace that is bonded to an insulating substrate which separates the ground plate from the etched antenna element. Other advantages of such antennas is that they are easy to manufacture and mechanically rugged. Moreover, patch antennas have the ability for polarisation diversity.

A further concept in antenna technology is the idea of a so-called "electrically short antenna", in which the electrical conductor of the antenna is physically short in length and often significantly shorter than the wavelength of the resonating frequency of the antenna Again the advantage of such short antennas is a reduction in size, which is particularly useful in the field of RFK) tags or identifier antennas.

However, normal microstrip patch antennas are still required to be of a certain size to transmit at a particular resonating frequency. It is especially desirable to reduce the size of such antennas that are attached to assets for tracking purposes.

Therefore, it is an aim of an embodiment of the present invention to reduce the physical size of a patch antenna.

Summary of the Invention

According to one aspect of the present invention there is provided an antenna for communicating data, the antenna comprises a dielectric layer that separates an electrically grounded layer from a conductor layer, wherein the conductor layer has a first portion connected to a capacitive element

Advantageously, the capacitive element makes the patch antenna looking electrically longer than it physically is. This means that the size of the antenna can be reduced, while still operating at the same frequency.

Moreover, a further advantage is that it is possible to vary the operating frequency of the antenna in an inversely proportional manner to the value of the capacitive element. Thus, for an antenna of a given size it possible to lower the resonating frequency by increasing the value of the capacitive element. Preferably, wherein the capacitive element is a plurality of capacitors.

Advantageously, the plurality of capacitors are evenly spaced producing a more uniform electromagnetic field radiated from the edge of the antenna.

Preferably, wherein the first portion is a first edge of a rectangular patch antenna.

Preferably, wherein the conductor layer has a second portion that is connected to the ground plate.

Preferably, wherein the antenna forms part of an RFID tag attached to an object such that movements of the object can be tracked.

According to a further aspect of the present invention there is provided a rectangular patch antenna comprising: a ground plate that is electrically grounded; a conduction layer having electrical conductors; a dielectric layer for separating the ground plate from the conduction layer; and wherein the conduction layer having a first edge connected to the ground layer and a second edge having a plurality of capacitors for enhancing the edge capacitance.

According to a further aspect of the present invention there is provided a rectangular patch antenna comprising: a ground plate that is electrically grounded; a conduction layer having electrical conductors; a dielectric layer for separating the ground plate from the conduction layer; and wherein the conduction layer having a first and a second radiating edge that each have a plurality of capacitors for enhancing their respective edge capacitance.

List of the Drawings

Embodiments of the present invention will now be described by way of example and with reference to the following drawings:

Figures 1 shows a plan view of a basic patch antenna;

Figure 2 shows a side view of the patch antenna;

Figure 3 shows a perspective view of a patch antenna according to a first embodiment;

Figure 4 shows a plot of the desired frequency response of the antenna; Figure 5 shows an impedance graph of the antenna;

Figure 6 shows a perspective view with defined dimensions of the antenna according to the first embodiment;

Figure 7 shows a side view with the electromagnetic field according to the first embodiment;

Figure 8 shows an equivalent transmission line circuit according to the first embodiment;

Figure 9 shows a perspective view of the patch including the capacitors according to the first embodiment;

Figure 10 shows a perspective view with defined dimensions of the antenna according to an alternative embodiment;

Figure 11 shows a side view with the electromagnetic field of the alternative embodiment;

Figure 12 shows an equivalent transmission line circuit of the alternative embodiment

Figure 13 shows a perspective view of the patch including the capacitors for the alternative embodiment;

Figures 14a shows the reduction in patch size when no capacitance is added to the edges; and

Figure 14b shows the reduction in patch size when capacitance is added.

Description

Figure 1 shows a plan view of a basic patch antenna, whereas Figure 2 shows a side view of the antenna. Specifically, the patch antenna shown in Figure 1 has a circular shape, but it should be appreciated that other shapes of patch antenna are possible, for example a square or a rectangular shape. Indeed, Figure 3 shows patch antenna according to a preferred embodiments of the present inventions comprising a rectangular shape.

Figures 1 and 2 show a patch antenna has an underlying ground plate element 100, a dielectric layer 140 located on the ground plate, and a conduction layer 110 located on the dielectric layer. In practice the patch antenna is usually printed on a circuit board and has a radiation pattern in any direction above the ground plane in a hemispherical area.

The thickness of the dielectric layer 140 determines the conduction layer 110 separation from ground 100, which effects the bandwidth of the patch antenna. Generally, the thicker the dielectric layer, the higher the bandwidth.

It is possible to reduce the size of the patch antenna, but shortening the length of the conductor layer 110 (i.e. physical size of the patch) has an impact on performance. Accordingly, the resonating frequency at which the antenna operates increases as the antenna size is reduced.

It should also be appreciated that there exists an inverse relationship between the physical size of the antenna and the resonating frequency of the antenna. That is, if the patch size is reduced then the resonating frequency will increase and vice versa.

The trade-off when designing such patch antennas, for a certain resonating frequency, is that performance often deteriorates as the size of the patch is reduced, but for RFID applications it is desirable to reduce the size of the patch antenna as far as possible while still achieving adequate performance.

Figure 3 shows an exploded view of the geometry of a patch antenna according to a preferred embodiment of the present invention. Specifically, the electrical short patch antenna is a rectangular patch antenna having a ground plate 300, with a separating dielectric substrate 340 and a top printed layer 310. The top layer 310 comprises the conductor arrangement 320 of the antenna.

Figure 3 shows that one edge 350 of the top layer 310 has vias which are able to be electrically connected to the ground plate 300. For example metallic vias can placed in respective holes formed in the dielectric layer to connect one edge 350 of the top conduction layer 310 to the ground plate 300, effectively shorting that edge of the antenna to ground. The top layer 310 additionally is also shown as comprising a plurality of capacitors, Cl, C2, C3, C4, C5 and C6 located along the opposite edge 360 of the top layer. These capacitors increases the edge capacitance of the patch antenna. The plurality of capacitors Cl to C6 shown in Figure 3 are connected in parallel. According to one embodiment each of the capacitors Cl to C6 is connected to the patch by having one plate of each capacitor grounded and the other plate connected to the conduction layer 310. Each plate of the capacitor may be grounded by connecting the plate to a relevant via, which is fitted through a relevant hole 395 (see for example Figures 9 and 13) in the dielectric layer, and connects to the ground plate 300.

According to the embodiment of Figure 3, the capacitors Cl to C6 are located on the same side of the patch as the RF feed point 370. The capacitors are spaced equally along the feed point 370 edge of the patch surface. There is more than one capacitor on each side of the feedpoint such that current is distributed uniformly along the edge 360 of the patch antenna. This advantageously allows an even electromagnetic field distribution for the antenna.

The dielectric layer 340 being of a thickness of 1.6mm and having an RF feedpoint that is located using impedance matching.

Figures 6 to 9 shows an embodiment in which one edge of the conduction layer 310 is shorted to the ground plate (as shown in the embodiment of Figure 3), whereas Figures 10 to 13 show an alternative embodiment in which both of the opposing edges have capacitors connected (i.e. no edge is grounded).

Specifically, Figure 6 shows a perspective view of the patch antenna embodiment of Figure 3 with one edge shorted. Figure 6 defines various dimensions such as the length L and Width W of the conduction layer 310, as well as the thickness h of the dielectric layer 340.

Figure 7 shows a side view of the antenna embodiment shown in figure 3 and in particular shows that one side is shorted to ground, whereas on the other side fringing fields give rise to an edge capacitance that is responsible for the radiated field. Figure 7 also illustrates the distribution of the electromagnetic field under the patch antenna. By adding lumped capacitors to the edge that is not grounded, the edge capacitance may be artificially increased, thereby increasing the end-effect extension, enabling a physical reduction in patch antenna size.

Figure 8 shows a transmission line model of the rectangular shorted patch antenna with one edge shorted. L₀ is the length of the patch antenna, Z₀ is the characteristic impedance of the patch antenna, C is the edge capacitance and G is the radiation conductance. The length L₀ for a half wave rectangular patch antenna is calculated using:

L₀ = — _j= .X₀ - AL Equation ( 1 )

where

L₀ is the patch length in meters,

e_r is the substrate relative dielectric constant,

λ_o is the wavelength in free space in meters, and

ΔL is the end effect extension in meters.

The quarter wave shorted patch antenna is simply half the length of the half wave antenna.

The end effect extension makes the patch antenna look electrically longer than it actually is. Because of this effect, the physical length of a patch antenna is a little shorter than a quarter wavelength. The electrical length however, is exactly a quarter wavelength at the operating frequency (i.e. resonating frequency).

The end effect extension ΔL is directly related to the amount of capacitance at the capacitive edge 350 and can be represented by the following function:

arctan(CωZ_n ) _ . ,_.

AL = - °— Equation (2)

where

C is the patch edge capacitance in Farads,

ω is the operating frequency in radians/s, and

Z₀ is the patch characteristic impedance in ohms. By increasing the capacitance artificially with lumped element capacitors (Cl to C6), equation 2 suggests that the end effect extension will become larger, thus reducing the physical length of the patch. This effect is shown in Figures 14a and 14b, which show the patch as viewed from above (i.e. a plan view). The solid line represents the physical length of the antenna patch (i.e. as manufactured), whereas the dotted line represents the electrical length of the patch antenna.

Figure 14a shows the reduction in the size of the antenna patch without any capacitance added, whereas Figure 8b shows that the physical size (length) of the patch can be reduced further by adding capacitance on the edge 360. That is, by adding capacitance to the circuit the physical length (size) of the antenna can be reduced dramatically, while still maintaining the electrical length of a quarter wavelength. Or put another way, the size of a patch antenna operating at a particular resonating frequency can be significantly reduced by adding capacitance.

Figures 10 to 13 show an alternative embodiment in which both of the opposing edges have capacitors connected. That is Figure 10 shows a perspective view of the patch antenna in which the conduction layer 310' has no edges that are grounded. Instead both edges of the conduction layer 310' with length W have a plurality of capacitors connected. This is not shown in Figure 10, but is shown in Figure 13, in which the edge connected to the feedpoint

370' has a first set of capacitors 1300 and the opposite edge having a second set of capacitors 1310.

Thus both edges of the patch have capacitive edges and this is reflected in Figure 11 , which shows the electromagnetic field radiating from both edges. The electromagnetic distribution tapers towards the centre of the patch. Figure 12 shows the equivalent transmission line diagram for the embodiment where both edges are capacitive edges.

It should be appreciated that this can be extended to various patch geometries including, but not limited to, the quarter-wave shorted patch antennas.

There are plurality of different applications and fields of use for the patch antenna geometry described therein. Some of these include:

in any RF point-to- point link system. any RF point-to-multi-point link system,

any RFID tag whether it is passive or an active tag,

any RF transmitter, receiver and / or transceiver

any sensor application with an RF link relaying data.

The patch antenna according to an embodiment of the present invention was designed in two phases.

In the first phase, the desired frequency response of the antenna was simulated using electro magnetic simulation software, for example, Sonnet, IE3D, Microware Studio, etc. Figure 4 shows a desired frequency response of the antenna using a relevant simulation package. Specifically, Figure 4 shows that the resonating frequency occurs at about 435MHz.

By increasing the values of the capacitors Cl to C6, it is possible to decrease the resonating frequency of the antenna.

Figure 5 is a further representation of the same simulation of the small patch antenna according to the preferred embodiment, but whereas Figure 4 showed the frequency response, Figure 5 shows an impedance graph.

The second phase of antenna design involves prototyping the antenna, which can be constructed for example using ordinary FR-4 PCB material. The antenna is then calibrated and a good design rule is that the smaller the patch element, the more capacitance is needed for the antenna to function at the desired resonating frequency.

In terms of performance improvement, one can consider as an example, a shorted quarter wave patch antenna of a specific size has a resonant frequency of around 2GHz. In contrast, the same size patch antenna, but having the lumped capacitors introduced on its edge is able to operate at a significantly lower resonating frequency, and in this case shown in the plot of Figure 4, reduced by a factor of around 5. Specifically Figure 4 shows the frequency response of the patch antenna with the lumped capacitors which has a resonating frequency 40 of about 435MHz. It should be appreciated that although the preferred embodiment of Figure 3 provides a substantially rectangular- shaped patch antenna, other shapes are also possible.

Claims

CLAIMS:

1. An antenna for communicating data, the antenna comprises a dielectric layer that separates an electrically grounded layer from a conductor layer, wherein the conductor layer has a first portion connected to a capacitive element.

2. The antenna of claim 1, wherein increasing the capacitive element makes the patch antenna looking electrically longer than it physically is.

3. The antenna of claim 1 or 2, wherein increasing the capacitive element decreases the resonating frequency.

4. The antenna of any preceding claim wherein the capacitive element is a plurality of capacitors.

5. The antenna of claim 4, wherein the plurality of capacitors are evenly spaced producing a more uniform electromagnetic field radiated from the edge of the antenna.

6. The antenna of any preceding claim, wherein the first portion is a first edge of a rectangular patch antenna.

7. The antenna of any preceding claim, wherein the conductor layer has a second portion that is connected to the ground plate.

8. The antenna of any preceding claim, wherein the antenna forms part of an RPID tag attached to an object such that movements of the object can be tracked.

9. The antenna of any preceding claim, wherein the antenna is a patch antenna.

10. The antenna of claim 9, wherein the patch antenna is an electrically short patch antenna.

11. The antenna of claim 10, wherein the electrical short patch antenna is rectangular.

12.. The antenna of any preceding claim, wherein the plurality of capacitors are evenly spaced on either side of a feedpoint of the antenna for enabling a even electromagnetic radiation from that edge of the antenna.

13. The antenna of any preceding claim, wherein the antenna is constructed using a PCB.

14. The antenna of any preceding claim, wherein the antenna communicates RF data.

15. The antenna of any preceding claim, wherein the antenna is used of at least of a receiver, transmitter and a transceiver.

16. The antenna of any preceding claim, wherein the values of the plurality of capacitors are able to be selected so as to vary a resonating frequency of the antenna.

17. The antenna of any preceding claim, wherein the plurality of capacitors allows the antenna to maintain the electrical length of an antenna despite a reduction in physical length.

18. A rectangular patch antenna comprising:

a ground plate that is electrically grounded;

a conduction layer having electrical conductors;

a dielectric layer for separating the ground plate from the conduction layer; and

wherein the conduction layer having a first edge connected to the ground layer and a second edge having a plurality of capacitors for enhancing the edge capacitance.

19. A rectangular patch antenna comprising:

a ground plate that is electrically grounded;

a conduction layer having electrical conductors;

wherein the conduction layer having a first and a second radiating edge that each have a plurality of capacitors for enhancing their respective edge capacitance.

20. The antenna of claim 18 or 19, wherein the first and second edges are opposite edges of the conduction layer of the rectangular patch.

21. The antenna of claim 20, wherein a feedpoint of the rectangular patch antenna is connected to at least one of the first and second edges.