Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
2000-12-27
2002-12-24
Le, Hoanganh (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
C343S702000
Reexamination Certificate
active
06498586
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to small-sized microstrip antennas that operate on many different frequency bands. In particular, the invention relates to internal antennas used in mobile phones, which are fed from one feeding point.
BACKGROUND OF THE INVENTION
In the present patent application, a frequency range comprises one or more frequency bands, i.e. a frequency band is part of the frequency range. Furthermore, by the reception band is meant a frequency band reserved for downlink data transmission and by the transmission band is meant a frequency band reserved for uplink data transmission.
In mobile stations, there is going on a changeover to terminals that operate in several frequency ranges. Solutions of several frequency ranges like this include so-called dual band terminals currently in use, which operate in two frequency ranges.
Dual band terminals have been implemented by both an external and internal antenna. The external antenna, which can be, for example, monopole, helix or their combination, is demanding as for its manufacturing technique, and it breaks easily. Therefore, in mobile stations, there is going on an increasing changeover to internal antenna structures implemented by microstrip antennas. The advantage of internal antennas compared to external antennas is the ease of the manufacturing technique and the speeding up of the serial production as the degree of integration increases, as well as the more durable structure than that of the external antennas.
A conventional microstrip antenna comprises a ground plane and a radiating antenna element that is insulated from the ground plane by an insulating layer. The resonance frequency of the microstrip antenna is determined on the basis of the physical dimensions of the antenna element and the distance between the antenna element and the ground plane. The operating principle and dimensioning of microstrip antennas are well known and they are described in the literature relating to the field.
FIGS. 1
a
and
1
b
show a microstrip antenna and an L-plane antenna according to prior art, which hereinafter in the present patent application will be called an L-antenna.
The microstrip antenna consists of a ground plane, a radiating antenna element, as well as a feeding line. In between and above the ground plane and the antenna element, there is either air or some other dielectric agent as an insulating material.
Traditionally, the L-antenna is a whip antenna that is bent near the ground plane parallel to the ground plane, whereupon the antenna has a low feed impedance. It is also possible to build of the L-antenna a microstrip antenna that consists of a ground plane, a radiating antenna element as well as a feeding line.
Normally, the length of the resonant proportion of the antenna in wavelengths is defined as the difference between the microstrip antenna and the L-antenna. The electric length of the microstrip antenna is half a wavelength whereas, traditionally, the electric length of the L-antenna is a quarter of a wavelength. From the electric length of the L-antenna it follows that the maximum current of the L-antenna is at the input.
Normally, the microstrip antenna is made on a double-sided substrate, one metallisation of which acts as the ground plane and on the other, the pattern of the antenna element is made by etching. The antenna element is fed by the feeding line, which is coupled to the antenna element either from one side (
FIG. 1
a
) or by taking the feeding line through the ground plane and the insulating material (
FIG. 1
b
). The resonance frequency of the microstrip and L-antennas is affected by the physical dimensions of the antenna element, the place of the feeding point, as well as, to some extent, the location of the antenna element with respect to the ground plane.
The size of the microstrip antenna has been reduced by developing a so-called PIFA antenna (PIFA, Planar Inverted F-Antenna), shown in
FIG. 2
b.
In the PIFA antenna, the antenna element is coupled to the ground plane by a grounding line. This being the case, the actual antenna element can be dimensioned so that it is considerably smaller than in the case of the microstrip antenna. Furthermore, by optimising the place of the feeding point, the feed impedance of the antenna can be changed to the desired impedance level, which is not possible in the L-antenna. The resonance frequency of the PIFA antenna is affected by the physical dimensions of the antenna element and the ground plane, as well as by the distance of the antenna element from the ground plane. The antenna element is fed either from one side (
FIG. 2
a
) or by taking the feeding line through the ground plane and the insulating material (
FIG. 2
b
). When narrowing the width of the grounding line, the resonance frequency of the antenna decreases. The grounding line can be as wide as the whole antenna element or, at its narrowest, merely a conductor.
Furthermore, it is well known to feed a microstrip antenna capacitively. In a capacitively fed microstrip antenna, there is a feeding element in between the antenna element and the ground plane, whereupon a capacitive coupling is formed between the antenna element and the feeding element. The feeding line is coupled to the feeding element, which radiates power further to the antenna element. The capacitive coupling can be implemented both in the microstrip antenna (
FIG. 3
) and the PIFA antenna (FIG.
4
).
The problem of microstrip antennas is the narrow bandwidth. The frequency ranges of 2
nd
generation mobile communication systems are reasonably narrow and, therefore, they can be implemented by microstrip antennas. For example, the frequency range of the GSM system is 890-960 MHz, wherein a transmission band is 890-915 MHz and a reception band is 935-960 MHz. Thus, the bandwidth required of one antenna element is no less than 70 MHz. Due to the production tolerances and the objects in the vicinity of the antenna, for example, the hand of a user, the bandwidth of the antenna element must be even wider. The frequency ranges required by 3
rd
generation mobile communication systems, for example, broadband CDMA systems are still considerably wider than, for example, the GSM system's and, therefore, their implementation with microstrip antennas is difficult. For example, a transmission band of the WCDMA system is 1920-1980 MHz and a reception band is 2110-2170 MHz. This being the case, the whole width of the frequency range is 250 MHz. This is why the bandwidth of microstrip antennas according to prior art described above has been increased as far as possible with solutions, where several resonance frequencies close to each other are implemented in one antenna element.
Solutions are known from prior art, where several resonance frequencies close to each other are implemented in one antenna element. In one solution, the number of resonance frequencies is increased by adding slots to the antenna element. However, the slots easily act in the case of small antennas as slot radiators, whereupon antenna elements that are resonating close to each other are strongly coupled to each other and form a resonator around the slot. This further follows that at the frequency in question the radiation resistance is low and the current densities in the vicinity of the slot are high, whereupon the loss of the antenna increases. Consequently, the adding of the bandwidth of a microstrip antenna in the manner in question only succeeds at the cost of gain and radiation efficiency. Hence, with the solution in question, for example, the gain values required by 3
rd
generation broadband CDMA systems cannot be achieved.
Of the microstrip antennas described above, an attempt has also been made to develop antenna structures that operate in several frequency ranges. For example, an antenna structure of two frequency ranges can be implemented by one common feeding point and an antenna element the resonance frequency of which can be adjusted by a switch and an electric load to the frequency range of another mobile communication
Le Hoang-anh
Nokia Mobile Phones Ltd.
Perman & Green LLP
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