EP1933418A1

EP1933418A1 - Antenna tuning method based on an variable shorting means and planar antenna with variable shorting means

Info

Publication number: EP1933418A1
Application number: EP06077256A
Authority: EP
Inventors: J. I. Epson Europe Electronics GmbH Cairo Molins; Lluis Universitat Politecnica Jofre Roca; Raquel Universitat Politecnica Serrano Calvo; Sebastian Universitat Politecnica Blanch Borios
Original assignee: Seiko Epson Corp
Current assignee: Seiko Epson Corp
Priority date: 2006-12-15
Filing date: 2006-12-15
Publication date: 2008-06-18

Abstract

Method of tuning an IC compatible planar antenna (1), said antenna comprising a lower ground plane, an upper active layer and feeding means (20), the method comprising:
- providing shorting means (30) from the active plane to the ground plane at one of the end walls of said antenna

characterised in that the method further comprises:
- providing means for varying the length of the shorting means from a first length L1 to a second length L2; and
- when the shorting means are at its first length L1 the antenna matching frequency seen at the feeding means has a first value f1 and when the shorting means are at its second length L2 the antenna matching frequency seen at the feeding means has a second value f2.

The invention also relates to an IC compatible planar antenna with length-variable shorting means.

Description

Field of the invention

The invention relates to a new technique used to tune planar antennas by means of a shorting wall.
The general application fields of the invention are digital communications, particularly wireless/mobile digital communications, the mentioned multi-layer antennas being ultra-thin or IC compatible, that is, having a thickness of about 1 mm.

Background of the invention

Antennas for portable devices need to be small, power efficient and compatible with the portable structure. More recently a big interest exists in finding antenna designs compatible with IC integrated technologies. At the habitual wireless application operating frequencies (from some hundreds of MHz to some GHz) wavelength (λ) dimensions are in the order of tens of centimetres. When compact solutions are intended, designs tend to reduce as much as possible transversal (width, W) and thickness (height, H) dimensions. In order to obtain enough efficiency and minimum bandwidth, current designs use widths (W) above one fifth of the wavelength (λ/5) and thickness above one fiftieth of the wavelength (λ/50). At the mentioned wireless frequencies that implies transversal dimensions in the order of several centimetres and thickness in the order of several millimetres. On the other hand, IC integrated technologies are able to deal with transversal dimensions (W) below the centimetre, and thickness below the millimetre. In consequence current solutions tend to combine the antenna and the associated electronics into a back to back architecture, which normally prevents the antenna to be fully integrated into the IC package. For example, at the current 1.8/1.9 GHz bands, transversal dimensions of 2 cm and thickness of 4 mm for single layer antennas are needed. When multilayer solutions are used for frequency bandwidth increase or multifrequency operation, several times the current thickness are additionally needed, which is totally incompatible with the IC dimensions.
In this regard, the solution proposed in the present invention provides a new IC compatible technique to tune planar antennas by means of a shorting wall and to create reconfigurable antennas.
In the prior-art, shorting walls have been used as a technique to reduce the overall size of a patch antenna. For example, the longitudinal dimension can be halved using a shorting wall in one of the ends of the structure, at the cost of halving the bandwidth.
In a standard patch antenna, it is possible to tune the antenna impedance to a certain value of interest by taking the front side of the antenna and shorting it with a shorting wall which has a defined length. Indeed, by shorting one of the end walls of the patch antenna to the ground plane as indicated before it is possible to reduce the size of the antenna, but it is also possible to adapt the antenna impedance seen at the feeding point to a desired value. This shorting wall can be made, for example, by means of a planar metallic wall.
The planar invert-F antenna (PIFA) is one of the most well-known and documented small patch antennas. Actually, the PIFA can be viewed as a shorted-patch (S-P) antenna. The F-antenna can be thought of as a tilted whip, where impedance matching is done by tapping the antenna at the appropriate impedance point. Because this antenna is reasonably compact, has an omnidirectional radiation pattern, good efficiency, and is very simple, it is used extensively in applications, including the mobile communications business. For such antennas the currents in the ground leg are high, and then a good sized ground plane is necessary to provide good efficiency.
However, given a specific patch antenna which has been tuned to a specific impedance using a specific shorting wall, said tuning of the shorted-patch antenna cannot be varied once that shorting wall has been carried out in the antenna.

Summary of the invention

With respect to the stated background, the present invention permits a shorted-patch antenna to be tuned at the volition of its user, by varying the length of the shorting wall.
The invention refers to method of tuning an IC compatible planar antenna according to claim 1 and to an IC compatible planar antenna according to claim 7. Preferred embodiments of the method and the antenna are defined in the dependent claims.
A first aspect of the invention relates to a method of tuning an IC compatible planar antenna, said antenna comprising a lower ground plane, an upper active layer and feeding means, the method comprising:

providing shorting means from the active plane to the ground plane at one of the end walls of said antenna.

According to the invention, the method further comprises:

providing means for varying the length of the shorting means from a first length L1 to a second length L2, and,
when the shorting means are at its first length L1 the antenna matching frequency seen at the feeding means has a first value f1; and when the shorting means are at its second length L2 the antenna matching frequency seen at the feeding means has a second value f2.

That is, by providing shorting means in one of the ends of the antenna the longitudinal dimension can be halved at the cost of halving the bandwidth; and by further varying or adapting the length of the shorting means, the characteristic impedance of the feeding point can be matched or adapted.
Said shorting means may comprise a planar metallic wall. Or the shorting means may comprise discrete connections between the active plane to the ground plane, and the distance between said discrete connections is in the order of a tenth of the wavelength or below. Said discrete connections may consist in small wires, posts or vias.
According to a preferred embodiment of the invention, the means for varying the length of the shorting means comprises an array of transistors each of which is connected at one end with each the post or via and at the other end with the ground plane. Such transistors act as a switch and are controlled or governed by a control signal which defines when and which of the posts or vias should be shorted. Such transistors may be implemented in CMOS technologies, for example, as well as the control circuitry for such switching transistors.
The means for varying the length of the shorting means may comprise an array of transistors, each of which is connected between two successive layers in a substantially aligned vertical point, creating a vertical short.
A second aspect of the invention relates to an IC compatible planar antenna, which comprises a lower ground plane, an upper active layer and feeding means, the antenna comprising:

shorting means from the active plane to the ground plane at one of the end walls of said antenna.

According to the invention, the antenna further comprises:

means for varying the length of the shorting means from a first length L1 to a second length L2; and
when the shorting means are at its first length L1 the antenna matching frequency seen at the feeding means has a first value f1 and when the shorting means are at its second length L2 the antenna matching frequency seen at the feeding means has a second value f2.

Said shorting means may comprise a planar metallic wall. Or the shorting means may comprises discrete connections between the active plane to the ground plane, and the distance between said discrete connections being in the order of a tenth of the wavelength or below. Said discrete connections may consist in small wires, posts or vias.
According to a preferred embodiment of the invention, the means for varying the length of the shorting means comprises an array of transistors each of which is connected at one end with each post or via, and at the other end with the ground plane. Or the means for varying the length of the shorting means may comprise an array of transistors, each of which is connected between two successive layers in a substantially aligned vertical point, creating a vertical short.
Such transistors act as a switch and are controlled by a control signal which defines when and which of the posts should be shorted. Such transistors may be implemented in CMOS technologies, for example, as well as the control circuitry for such switching transistors.
The resulting architecture is suitable for monolithic integration. In fact, the overall planar dimensions of the multi-layer antenna are in the order of a tenth of the wavelength λ/10; and the thickness of the multi-layer planar antenna is approximately a hundredth of the wavelength λ/100 or below.
It is important to underline that the solution proposed here provides an antenna geometry which is fully compatible with the multilayer IC technology, meaning that the antenna is designed using the multi-layer IC frame itself in such a way that small, efficient integrated antennas may be designed. For doing so transversal and thickness dimensions are simultaneously reduced due to the particular matching effect between the highly unequal input port impedance and the antenna radiation impedance produced by the compacted multi-layer structure where the thickness of the layers is in the order of tenths of millimetres (1/10's mm) very differently from the tens of millimetres (10's mm) used on the conventional multilayer antenna geometries. This is due to the fact that the tuning method of the present invention is preferably applicable to antennas as defined in EP application No. 05078048.5 , and so are the antennas themselves.

Short description of the drawings

A series of drawings aiding to better understand the invention and which are expressly related to a preferred embodiment of said invention, representing a nonlimiting example thereof, is very briefly described below.
Figures 1A and 1B show a conventional rectangular patch antenna, and that same antenna with a length-variable shorting wall, respectively.
Figure 2 shows three-dimensional view of a folded patch-antenna, wherein a shorting wall has been applied.
Figure 3 shows three-dimensional view of a coplanar multi-layer transformer patch-antenna, wherein a shorting wall has been applied.
Figure 4 shows a possible embodiment of the means for varying the length of the shorting means.

Detailed description of the drawings

Figure 1A shows a conventional λ/2 rectangular patch antenna 10, having the following dimensions: length L, width W, and thickness h, and with a feeding point 20. In this case it is fed my means of a coaxial cable. Nevertheless, other feeding means are also possible such as a microstrip feed or the different coplanar line geometries.
Figure 1B shows how the method of tuning in the present invention is achieved: with the same patch antenna 10 of figure 1A, one of its ends is shorted by a shorting wall 30, which has a variable length ΔL. This way, by variably shorting the wall in one of the ends of the antenna, the characteristic impedance seen at the feeding point can be matched or adapted.
This same concept can be applied in combination with other techniques mentioned earlier to reduce the antenna size as folding antenna, as shown in figure 2 and 3, respectively. In these cases a partial wall 30 is shorted in order to accommodate the impedance of the feeding port of the antenna to a specific desired value. In figure 2 the invention has been applied to a folded patch antenna, while in figure 3 it has been applied to a multi-layer transformer patch antenna.
A possible way of implementing the variable shorting wall is with the use of transistors (i.e., MOSFET transistors). As shown in Figure 4, each transistor 40 connects vertically two successive layers of the antenna 10 in a substantially aligned vertical point (the representation of the connections has been enlarged for the sake of a better comprehension of the left part of figure 4). If the number of layers is N, using N-1 transistors 40 all the layers can be connected or shorted at essentially the same vertical point, that is, creating a vertical short 41 (schematically shown on the right hand of figure 4). This technique can be reproduces many times to create the desired number of vertical shorts 41, 51, It is important that the distance between shorts is at least ten times smaller than the resonant wavelength of the antenna. Said transistors can be controlled in order the switching functionality, for example, by means of extra logic circuitry easily integrated on the silicon substrate.

Claims

Method of tuning an IC compatible planar antenna (1), said antenna comprising a lower ground plane, an upper active layer and feeding means (20), the method comprising:
- providing shorting means (30) from the active plane to the ground plane at one of the end walls of said antenna;
characterised in that the method further comprises:
- providing means for varying the length of the shorting means from a first length L1 to a second length L2; and

- when the shorting means are at its first length L1 the antenna matching frequency seen at the feeding means has a first value f1 and when the shorting means are at its second length L2 the antenna matching frequency seen at the feeding means has a second value f2.
Method according to claim 1, wherein said shorting means comprises a planar metallic wall (30).
Method according to claim 1, wherein said shorting means comprises discrete connections between the active plane to the ground plane, and the distance between said discrete connections is in the order of a tenth of the wavelength or below.
Method according to claim 3, wherein said discrete connections consist in posts or vias.
Method according to claim 4, wherein the means for varying the length of the shorting means comprises an array of transistors each of which is connected at one end with each post or via and at the other end with the ground or active plane.
Method according to claim 3, wherein the means for varying the length of the shorting means comprises an array of transistors (40) each of which is connected between two successive layers in a substantially aligned vertical point, creating a vertical short (41).
IC compatible planar antenna, which comprises a lower ground plane, an upper active plane and feeding means, the antenna comprising:
- shorting means from the active plane to the ground plane at one of the end walls of said antenna;
characterised in that the antenna further comprises:
- means for varying the length of the shorting means from a first length L1 to a second length L2; and

- when the shorting means are at its first length L1 the antenna matching frequency seen at the feeding means has a first value f1 and when the shorting means are at its second length L2 the antenna matching frequency seen at the feeding means has a second value f2.
Planar antenna according to claim 7, wherein said shorting means comprises a planar metallic wall.
Planar antenna according to claim 8, wherein said shorting means comprises discrete connections between the active plane to the ground plane, and the distance between said discrete connections is in the order of a tenth of the wavelength or below.
Planar antenna according to claim 9, wherein said discrete connections consist in posts or vias.
Planar antenna according to claim 10, wherein the means for varying the length of the shorting means comprises an array of transistors each of which is connected at one end with each post or via and at the other end with the ground plane.
Planar antenna according to claim 9, wherein the means for varying the length of the shorting means comprises an array of transistors (40) each of which is connected between two successive layers in a substantially aligned vertical point, creating a vertical short (41, 51).
Planar antenna according to any of claims 7-11, wherein said antenna is a single-layer or a multi-layer antenna.