[0001] The present invention relates to a patch antenna and particularly to a patch antenna
for use inside the housing of a portable device, such as a mobile telephone.
[0002] Many applications require small, lightweight and efficient antennas. Traditionally
patch antennas, such as microstrip patch antennas, have been a preferred type for
many applications. However, microstrip patch antennas typically are efficient only
in a narrow frequency band. Otherwise, the advantage of patch antennas is that they
are mountable in a small space, have a high gain and can be constructed in a rugged
form.
[0003] There have been a number of efforts in the past to provide an antenna inside a portable
communication unit for signal reception and transmission. Such efforts have thought
at least to reduce the need to have an external broad or bandwidth antenna because
of the inconveniences of handling and carrying such a unit with the external antenna
extended. This is particularly true for portable telephone units operating in relatively
high frequency ranges, such as the GSM frequency range of 890 MHz to 960 MHz.
[0004] Patch antennas and microstrip patch antennas have been known in the art for applications
in which thin and small antennas were required. These kinds of antennas comprise a
conductive patch as a resonator element, which is generally parallel to and spaced
from a conducting ground element by an insulator consisting of a dielectric material.
US 5 777 581 discloses such a microstrip patch antenna, which is designed for use
in the UHF band. US 4 980 697 discloses a microstrip type antenna comprising a stack
of alternate conductive and dielectric layers, so that the antenna is designed for
use in several frequency bands. WO95/24745 discloses an antenna unit for a handheld
receiving/transmitting apparatus. The antenna unit comprises a resonator element,
a ground element and a connector element, which are formed as conducting layers on
a dielectric body. The connector element electrically connects the resonator element
and the ground element at one end of the antenna unit, so that a closed end is formed.
In the region of the other end of the antenna unit, the resonator element and the
ground element are not connected, so that an open end is formed. The distance between
the resonator element and the ground element increases from the open end to the closed
end. The transmission and reception of radiation mainly takes place at the open end
of the antenna unit. Since the entire space of the antenna unit inside the ground
element and the resonator element is occupied by the solid dielectric material, this
known antenna unit is quite heavy, so that the use of this known antenna unit in portable
devices is very inconvenient. Further, the antenna unit is costly to produce, since
the entire antenna volume is filled with dielectric material.
[0005] The object of the present invention is therefore to provide a patch antenna according
to the preamble of claim 1, which is lightweight and which can be produced at low
cost.
[0006] This object is achieved by a patch antenna comprising a ground element being electrically
conductive, a resonator element being electrically conductive and a solid dielectrid
element located between said resonator element and said ground element. The ground
element and the resonator element are electrically connected at a first end of the
resonator element so that a closed end is formed and not electrically connected at
a second end of said resonator element so that an open end is formed. The patch antenna
of the present invention is characterized in that the solid dielectric element is
only provided in the region of the open end and not in the region of the closed end
of the antenna. Thereby, the patch antenna according to the present invention can
be produced at low cost, since not the entire volume between the ground element and
the resonator element is filled with solid dielectric material. Further, the patch
antenna according to the present invention can be produced having a light weight,
since only the part of the antenna in the region of the open end is provided with
solid dielectric material, whereas the part at the region of the closed end of the
antenna can e. g. comprise air, so that the weight of the antenna is significantly
reduced compared to known antennas.
[0007] Although in the state of the art most patch antenna types or microstrip patch antenna
types are described as having a very large ground element compared to the resonator
element, i. e. the patch, the patch antenna according to the present invention is
advantageously of a design, in which the ground element has approximately only the
double length of the resonator element, whereby the width of the resonator and the
ground element is essentially the same. The ground element and the resonator element
of the patch antenna are at least partially parallel to each other.
[0008] The use of the solid dielectric material in the region of the open end of the patch
antenna has the further advantage, that the tolerances of the ground element and the
resonator element in this very tolerance critical part of the antenna can be set accurately.
[0009] Advantageously, the distance between the resonator element and the ground element
in the region of the open end is smaller than in the region of the closed end of the
antenna. By this measure, the antenna length, which is defined as the direction extending
from the closed end to the open end of the antenna, can be further decreased. This
is particularly advantageous if the antenna has to be integrated in mobile devices,
in which the valuable space is very refined. Further, increasing the distance between
the resonator element and the ground element in the region of the closed end of the
antenna as compared to the open end of the antenna increases the frequency bandwidth
and the efficiency of the antenna according to the present invention significantly.
[0010] Particularly in comparison to the antenna disclosed in WO95/24745, the increased
distance between the resonator element and the ground element in the region of the
closed end of the antenna as compared to the open end of the antenna together with
the fact, that no solid dielectric material is provided in the region of the closed
end of the antenna, where the distance is increased, tremendously lowers the weight
of the patch antenna according to the present invention.
[0011] Further advantageously, the ground element has a stepped shape with a first and a
second step section, whereby the second step section corresponds to the region of
the open end at which the solid dielectric material is provided. And said first step
section corresponds to the region of the closed end and has a larger distance to the
resonator element than said second step section. Also, said resonator element can
have essentially stepped shape with a first and a second step section, whereby the
second step section corresponds to the region of the open end at which the solid dielectric
material is provided. And said first step section corresponds to the region of the
closed end and has a larger distance to the ground element than said second step section.
The stepped shapes of the ground element and the resonator element, respectively,
provide a simple and at low cost producable design for the patch antenna according
to the present invention. The second step sections respectively are located adjacent
to the solid dielectric material and the first step sections provide the larger distance
between the resonator element and the ground element in the region of the closed end
of the antenna, where no solid dielectric material is provided, so that a very light
structure is enabled.
[0012] Advantageously, the resonator element and the ground element are parallel in a region
of the open end where the solid dielectric material is provided. In the region of
the open end of the antenna, the tolerances of the ground element, the resonator element
and the dielectric element are critical in view of the resonance frequency and the
efficiency of the antenna. Therefore, it is an important feature, that in this region
the resonator element and the ground element are essentially parallel to each other.
In the region of the closed end, the ground element and the resonator element need
not necessarily be parallel to each other, since the tolerances in the region of the
closed end are not that critical. This is also the reason why no solid dielectric
material needs to be provided in the region of the closed end of the antenna to support
the resonator element and the ground element to thereby fix their dimensional relation
and reduce their dimensional tolerances.
[0013] Advantageously, the solid dielectric element has a sheetlike shape with a first and
a second main surface, which respectively comprise a printed metal layer. The metal
layers respectively contact the resonator element and the ground element. In this
way, the solid dielectric element can be produced as a printed circuit board, so that
a mass production with low cost is possible. Further, by providing the metal layers
on the first and second main surface of the dielectric material, the tolerances, e.
g. the distance between the resonator element and the ground element can be set accurately,
which is very important in the region of the open end of the antenna.
[0014] Further advantageously, the length of the resonator element is essentially half of
the length of the ground element. The length direction is thereby the direction between
the open end and the closed end of the antenna. Thereby, the length of the resonator
element can e. g. correspond to a quarter wavelength of the resonance frequency of
the antenna.
[0015] The patch antenna of the present invention is described in more detail in the following
description relating to the drawings, in which
figure 1 shows schematically a first embodiment of the patch antenna according to
the present invention
figure 2 shows schematically a second embodiment of the patch antenna according to
the present invention, and
figure 3 shows schematically a third embodiment of the patch antenna according to
the present invention.
[0016] The patch antennas shown in figure 1, 2 and 3 are designed to be used as internal
antennas of a GSM mobile phone. However, the proposed antennas can also be used in
other applications, in which lightweight, small and low cost antennas are required.
[0017] Generally, applications in portable devices require lightweight and small antennas.
However, there are some fundamental limits on how small an antenna can be designed
if a special bandwidth and efficiency is required. The so-called Q-value is the ratio
between the resonance frequency and the frequency bandwidth of the antenna. The Q-value
is a property of the antenna, which depends mainly on the antenna dimensions. Increasing
the dimensions of an antenna lowers the Q-value significantly. Thus a good antenna
with a large bandwidth is usually a physically large antenna. On the other hand, small
antennas to be integrated in a portable device necessarily have a high Q-value and
thus a small frequency bandwidth.
[0018] A bandwidth of an antenna with a given Q-value can be increased by proper matching,
but only within some limits. The bandwidth that can be matched with a certain matching
loss can e. g. be increased by increasing the number of reactive elements in the matching
network of the antenna. However, even for an infinite number of reactive elements
in the matching network, the bandwidth that can be matched with a certain matching
loss is limited for a given Q-value. In reality, a matching section with zero loss
does not exist. Therefore, it is usually not a good idea to put more than a single
section into the matching network in the case of GSM antennas with high Q-values.
An increase of this number of sections will normally give an insertion loss increase
larger than the matching loss decrease.
[0019] One possibility to design an antenna for GSM mobile phones is thus to design the
antenna to half of the necessary bandwidth and to use a range switching network to
switch the antenna resonance frequency between the transmission and the reception
band. The second possibility is to use a corresponding matching network to match the
antenna to the entire GSM band.
[0020] One problem with an integrated antenna for a GSM mobile phone, as stated above, is
that the antenna has got to be relatively small which results in a high Q-value and
therefore a small bandwidth. Another even worse problem is that an integrated antenna
inherently will be placed much closer to the ground plane than an antenna which is
located on top of the phone. Therefore, the distance between the resonator plane and
the ground plane for a handportable phone will typically be less than 1 cm for the
GSM frequency band. Bending e. g. a monopol so that is in parallel to the ground plane
in the distance of about 1 cm transforms it into a kind of microstriplike transmission
line which is open in one end. In this case, the capacity to the ground plane is tremendously
increased, so that the Q-value also is significantly increased.
[0021] The only way to cope with the ground plane vicinity problem is to use an antenna
type which is actually intended to work close to a ground plane. A patch antenna is
such an antenna type. Patch antenna theory usually assumes a ground plane with an
infinite length compared to the length of the resonator plane. In the patch antenna
of the present invention, however, the ground plane can have a length L in the range
of the length of the resonator plane, e. g. the double length or the triple length.
In the patch antennas shown in figure 1, 2 and 3, the ground plane has a length L,
which is approximately the double of the length of the resonator element. Further,
in the patch antenna according to the present invention, the width w of the ground
plane and the resonator plane are generally the same.
[0022] A patch antenna is an antenna with a narrow frequency bandwidth. The frequency bandwidth
can thereby be decreased to some extent by increasing the distance from the resonator
plane to the ground plane. Making this distance too large, however, creates surface
waves which limit the bandwidth since more energy will be stored.
[0023] One of the main items in view of patch antennas for portable phones is the frequency
bandwidth. As stated above, the frequency bandwidth is mainly determined by the dimensions
of the antenna. A variation in the dimensional tolerances of the patch antenna leads
to tolerances of the resonance frequency of the antenna. If, e. g. the tolerance of
the resonance frequency is ± 1 %, this means for GSM applications a resonance frequency
tolerance of ± 1 % × 925 MHz = ± 9,25 MHz. 925 MHz is thereby the center frequency
of the GSM frequency band of 890 MHz to 960 MHz. The ± 1 % tolerance of the resonance
frequency thereby results in an increased bandwidth of (2 × 9,25) MHz + 70 MHz = 88,5
MHz. If the tolerance of the antenna resonance frequency can be lowered, the insertion
loss will also be lowered for a given antenna Q-value. The frequency bandwidth of
an antenna with a given Q-value can be increased by proper matching, as stated above,
but only within some limits. The matching loss thereby increases with an increasing
frequency bandwidth to be matched.
[0024] The patch antenna 1 shown in figure 1 is a first simple embodiment of an antenna
according to the present invention. The antenna comprises a ground plane 3 of electrically
conductive material, e. g. metal, and a resonator plane 2 of electrically conductive
material, e. g. metal. The length L of the ground plane 3 has generally the double
value of the length of the resonator plane 2. The resonator plane 2 is electrically
connected at a first end by means of a connector 5 to the ground plane 3, so that
a closed end is formed. The second end of the resonator plane 2 opposite to the first
end is not connected to the ground plane 3 so that an open end is formed. A solid
dielectric material is provided in the region of the open end between the resonator
plane 2 and the ground plane 3.
[0025] The resonator plane 2 thus works essentially as a quarter wave resonator with 0 V
at the closed end and a voltage maximum at the open end. This high voltage at the
open end creates an E-field to the ground which propagates into space. Therefore the
radiation mainly comes from the slot at the open end of the antenna 1.
[0026] The ground plane 3 and the resonator plane 2 have generally the same width w, whereby
the width of the resonator plane 2 might be a little smaller than the width of the
ground plane 3. The ground plane 3 and the resonator plane 2 are essentially parallel
to each other and spaced by a uniform distance h. As stated above, the length of the
resonator plane 2 corresponds to a quarter wavelength and the length L of the ground
plane 3 is a little bit shorter than one half wavelength. The normal microstrip antenna
theories states that the radiation resistance only depends on the width of the resonator
plane. The wider the resonator plane is, the lower the radiation resistance is. According
to the known theory, the radiation resistance does not really depend on the distance
h between the ground plane and the resonator plane or the dielectric constant of the
solid dielectric material 4 between the resonator plane 2 and the ground plane 3.
As shown in figure 1, the solid dielectric material 4 is only provided in the region
of the open end of the resonator plane 2 and not in the region of the closed end.
[0027] In normal microstrip antenna theory the resonator plane is considered to be a very
wide microstrip line. This is not a reasonable assumption in the case of a patch antenna
as shown in figure 1, where the resonator plane and the ground plane essentially have
the same width. For a patch antenna as shown in figure 1, the radiation resistance
does not really depend on a distance h between the resonator plane 2 and the ground
plane 3. Further, the Q-value of a patch antenna 1 as shown in figure 1 depends on
the distance h, whereby a large value for h gives a low value for Q. Thus, the distance
h between the resonator plane 2 and the ground plane 3 should be as large as possible
to achieve a low Q-value. Further, the Q-value depends on the relative dielectric
constant of the solid dielectric material 4 in that a large relative dielectric constant
gives a large Q-value. The Q-value of the patch antenna 1 shown in figure 1 further
depends on the width of the resonator plane in that a larger width gives a lower Q-value.
The efficiency of the patch antenna 1 shown in figure 1 can also be increased by increasing
the distance h between the resonator plane 2 and the ground plane 3.
[0028] The patch antenna 1 is fed with a direct probe feeding by means of a coaxial line
6. As shown in figure 1, the feeding line is connected to a side edge of the resonator
plane 2 in the region of the closed end of the antenna. The impedance level of the
patch antenna 1 can be decreased by moving the feeding point closer towards the closed
end. It is further important, that the ground of the coaxial feeding line 6, i. e.
the outer line of the coaxial line 6 is connected to the ground plane 3 as close as
possible to the feeding point at the side edge of the resonator plane 2. Another possibility
is to feed the resonator plane 2 by electromagnetic coupling from a feeding line,
whereby the impedance level of the antenna can be changed by changing the distance
from the coupling feeding line to the resonator plane 2 and/or to the ground plane
3.
[0029] From the above observations, the physical dimensions of the patch antenna can be
determined. The width of the resonator element 2 should be as large as possible and
the distance a between the resonator plane 2 and the ground plane 3 should be as large
as possible in order to gain as much frequency bandwidth and efficiency as possible.
However, if the distance h becomes too large, surface waves will occur. The space
available for the patch antenna according to the present invention in a mobile phone
is usually very small, so that the distance h is already restricted in this respect
and surface waves should not be an item. The length of the resonator plane corresponds
to a quarter wavelength, so that the physical length of the resonator plane is approximately
determined by the frequency and the dielectric material. The length of the resonator
plane 2 can thereby be decreased by increasing the distance between the resonator
plane 2 and the ground plane 3 in the region of the closed end compared to the open
end of the antenna. This feature is realized in the patch antenna 7 shown in figure
2 and the patch antenna 16 shown in figure 3.
[0030] The tolerances of the physical dimensions of the patch antenna according to the present
invention is a critical item mainly due to the low frequency bandwidth of the antenna.
However, tolerances of the distance h between the resonator plane 2 and the ground
plane 3 and of the width of the resonator plane 2 will not give any direct change
in the resonance frequency of the patch antenna, but will change the impedance level
of the antenna so that the matching network will be affected.
[0031] The solid dielectric material 4 provided between the resonator plane 2 and the ground
plane 3 in the region of the open end of the antenna has mainly two functions, namely
to reduce the size of the antenna due to the relative dielectric constant which is
larger than 1 and to work as a spacer between the resonator plane 2 and the ground
plane 3 to keep the tolerance of the distance h low. The relative dielectric constant
of the solid dielectric material 4 should be in the range of 2 to 4. A too low constant
makes the antenna too large and a too high constant makes the frequency bandwidth
too narrow. Further, the dielectric constant should be very constant in view of temperature
changes.
[0032] The position of the resonator plane 2 in relation to the ground plane 3 is another
feature to be considered. As can be seen in figures 1, 2 and 3, the ground plane 3
or the ground element extends with a much longer distance from the closed end of the
resonator plane 2 as from the open end. This is due to the fact that the Q-value depends
strongly on the length of the ground plane 3 between the open end of the resonator
plane 2 and the corresponding end of the ground plane 3. Further, the ground plane
3 has an optimum entire length with respect to the Q-value taking the dielectric material
between the resonator plane 2 and the ground plane 3 into account, this optimum length
is approximately one half wavelength.
[0033] All general statements above and below made in relation to the patch antenna 1 of
figure 1 apply identically to the patch antennas 7 and 16 shown in figures 2 and 3,
respectively, and vice versa.
[0034] Figure 2 shows a second embodiment of a patch antenna 7 according to the present
invention, in which the distance between the resonator element and the ground plane
3 in the region of the closed end is larger than the distance between the resonator
element and the ground plane 3 in the region of the open end of the antenna. A resonator
element consists essentially of a first step section 8 in the region of the closed
end and a second step section 9 in the region of the open end of the antenna. Both
step sections 8 and 9 are essentially parallel to the ground plane 3, but the distance
of the first step section 8 to the ground plane 3 is Larger than the distance of the
second step section 9 to the ground plane 3. Between the second step section 9 and
the ground plane 3, the solid dielectric material is provided, whereas no dielectric
material is provided between the first step section 8 and the ground plane 3. The
first step section 8 and the second step section 9 are connected by a connecting section
10 protecting upwardly from the end of the solid dielectric material 4. Due to the
stepped design of the resonator element, the length of the resonator element can be
made shorter compared to the resonator element formed as a resonator plane 2 of the
embodiment shown in figure 1. The first step section 8 of the resonator element of
the patch antenna 7 is connected to the ground plane by a connector 5 to form a closed
end like in the first embodiment. The first step section 8, the second step section
9 and the connecting section 10 consist of electrically conductive material, e. g.
metal.
[0035] Figure 3 shows a preferred embodiment of a patch antenna according to the present
invention. The patch antenna 16 shown in figure 3 comprises a ground element consisting
of a first step section 12 and a second step section 11. The resonator element consists
of a first section 14 and a second section 13. The first section 14 is located over
a part of the first step section 12 of the ground element and the second section 13
of the resonator element is located over and essentially parallel to a part of the
second step section 11 of the ground element. Between the second section 13 of the
resonator element and the corresponding part of the second step section 11 of the
ground element, the solid dielectric material 4 is provided.
[0036] The end of the first section 14 of the resonator element on the opposite side of
the second section 13 of the resonator element is electrically connected to the ground
element by means of a connecting section 15 to form a closed end similar to the first
two embodiments. The connecting section 15 extends upright from the first step section
12 of the ground element. The distance between the first section 14 of the resonator
element and the first step section 12 of the ground element is larger than the distance
between the second section 13 of the resonator element and the second step section
11 of the ground element. Thereby, the first section 14 of the resonator element is
slant so that the distance between the first section 14 of the resonator element and
the first step section 12 of the ground element increases towards the closed end.
This proposed design allows to maximize the distance between the resonator element
and the ground element and thereby to optimize the bandwidth of the patch antenna.
The second step section 11 of the ground element can also serve as shield for circuitry
located underneath. The first section 14 and the second section 13 of the resonator
element as well as the connecting section 15 are electrically conductive and can e.
g. be metal sheets. The solid dielectric material 4 is low cost, low loss material
with a low relative dielectric constant, e. g. polystyrene or polyethlylen.
[0037] The three embodiments of the patch antenna according to the present invention shown
in figures 1, 2 and 3 provide a lightweight and low cost patch antenna particularly
useful for applications as internal antenna in portable devices. The solid dielectric
material 4 is only provided in the region of the open end of the resonator element,
since this part is the main transmitting/receiving part of the antenna and therefore
the tolerances in this part of the antenna are more critical than in the other parts.
Further, by making the distance between the resonator element and the ground element
in the region of the open part of the antenna considerably smaller than in the region
of the closed end of the antenna, Less dielectric material is needed and the antenna
becomes even cheaper to produce. The solid dielectric material 4 can in all three
embodiments comprise printed metal layers on its first and second main surface. These
metal layers are in contact with the corresponding parts of the resonator element
and the ground element, so that the dimensional tolerances, namely the distance between
the resonator element and the ground element in the region of the open part of the
antenna can be accurately set. The patch antenna according to the present invention
has a high efficiency and a very good radiation pattern and gain.
1. Patch antenna, comprising
a ground element (3; 11, 12) being electrically conductive,
a resonator element (2; 8, 9; 13, 14) being electrically conductive, and
a solid dielectric element (4) located between said resonator element and said ground
element, whereby said ground element (3; 11, 12) and said resonator element (2; 8,9;
13, 14) are electrically connected at a first end of the resonator element so that
a closed end is formed and not electrically connected at a second end of said resonator
element so that an open end is formed,
characterized in,
that said solid dielectric element (4) is only provided in the region of the open
end and not in the region of the closed end of the antenna.
2. Patch antenna according to claim 1,
characterized in,
that the distance between said resonator element (2; 8, 9; 13, 14) and said ground
element (3; 11, 12) in the region of said open end is smaller than in the region of
said closed end.
3. Patch antenna according to claim 2,
characterized in,
that said ground element (3; 11, 12) has a stepped shape with a first and a second
step section, whereby said second step section corresponds to said region of the open
end at which the solid dielectric material (4) is provided and said first step section
corresponds to the region of the closed end and has a larger distance to the resonator
element than said second step section.
4. Patch antenna according to claim 2 or 3,
characterized in,
that said resonator element (2; 8, 9; 13, 14) has essentially a stepped shape with
a first and a second step section, whereby said second step section corresponds to
said region of the open end at which the solid dielectric material (4) is provided
and said first step section corresponds to the region of the closed end and has a
larger distance to the ground element than said second step section.
5. Patch antenna according to one of the claims 1 to 4,
characterized in,
that said resonator element (2; 8, 9; 13, 14) and said ground element (3; 11, 12)
are parallel in the region of the open end where the solid dielectric material (4)
is provided.
6. Patch antenna according to one of the claims 1 to 5,
characterized in,
that said solid dielectric element (4) has a sheet-like shape with a first and a second
main surface, which respectively comprise a printed metal layer, the metal layers
contacting said resonator element (2; 8, 9; 13, 14) and said ground element (3; 11,
12), respectively.
7. Patch antenna according to one of the claims 1 to 6,
characterized in,
that the length of said resonator element (2; 8, 9; 13, 14) is generally half of the
length of the ground element (3; 11, 12).