Field of the invention
[0001] The invention relates to a new technique used to tune planar antennas by means of
a shorting wall.
[0002] 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
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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
[0024] 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.
[0025] Figures 1A and 1B show a conventional rectangular patch antenna, and that same antenna
with a length-variable shorting wall, respectively.
[0026] Figure 2 shows three-dimensional view of a folded patch-antenna, wherein a shorting
wall has been applied.
[0027] Figure 3 shows three-dimensional view of a coplanar multi-layer transformer patch-antenna,
wherein a shorting wall has been applied.
[0028] Figure 4 shows a possible embodiment of the means for varying the length of the shorting
means.
Detailed description of the drawings
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
1. 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.
2. Method according to claim 1, wherein said shorting means comprises a planar metallic
wall (30).
3. 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.
4. Method according to claim 3, wherein said discrete connections consist in posts or
vias.
5. 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.
6. 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).
7. 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.
8. Planar antenna according to claim 7, wherein said shorting means comprises a planar
metallic wall.
9. 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.
10. Planar antenna according to claim 9, wherein said discrete connections consist in
posts or vias.
11. 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.
12. 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).
13. Planar antenna according to any of claims 7-11, wherein said antenna is a single-layer
or a multi-layer antenna.