WO2010122220A1 - Internal monopole antenna - Google Patents

Internal monopole antenna Download PDF

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Publication number
WO2010122220A1
WO2010122220A1 PCT/FI2010/050296 FI2010050296W WO2010122220A1 WO 2010122220 A1 WO2010122220 A1 WO 2010122220A1 FI 2010050296 W FI2010050296 W FI 2010050296W WO 2010122220 A1 WO2010122220 A1 WO 2010122220A1
Authority
WO
WIPO (PCT)
Prior art keywords
antenna
operating band
main element
upper operating
resonance frequency
Prior art date
Application number
PCT/FI2010/050296
Other languages
French (fr)
Inventor
Heikki Korva
Original Assignee
Pulse Finland Oy
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Pulse Finland Oy filed Critical Pulse Finland Oy
Publication of WO2010122220A1 publication Critical patent/WO2010122220A1/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q1/00—Details of, or arrangements associated with, antennas
    • H01Q1/12—Supports; Mounting means
    • H01Q1/22—Supports; Mounting means by structural association with other equipment or articles
    • H01Q1/24—Supports; Mounting means by structural association with other equipment or articles with receiving set
    • H01Q1/241—Supports; Mounting means by structural association with other equipment or articles with receiving set used in mobile communications, e.g. GSM
    • H01Q1/242—Supports; Mounting means by structural association with other equipment or articles with receiving set used in mobile communications, e.g. GSM specially adapted for hand-held use
    • H01Q1/243—Supports; Mounting means by structural association with other equipment or articles with receiving set used in mobile communications, e.g. GSM specially adapted for hand-held use with built-in antennas
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q5/00—Arrangements for simultaneous operation of antennas on two or more different wavebands, e.g. dual-band or multi-band arrangements
    • H01Q5/30—Arrangements for providing operation on different wavebands
    • H01Q5/307—Individual or coupled radiating elements, each element being fed in an unspecified way
    • H01Q5/342—Individual or coupled radiating elements, each element being fed in an unspecified way for different propagation modes
    • H01Q5/357—Individual or coupled radiating elements, each element being fed in an unspecified way for different propagation modes using a single feed point
    • H01Q5/364—Creating multiple current paths
    • H01Q5/371—Branching current paths
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q5/00—Arrangements for simultaneous operation of antennas on two or more different wavebands, e.g. dual-band or multi-band arrangements
    • H01Q5/30—Arrangements for providing operation on different wavebands
    • H01Q5/378—Combination of fed elements with parasitic elements
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q9/00—Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements
    • H01Q9/04—Resonant antennas
    • H01Q9/30—Resonant antennas with feed to end of elongated active element, e.g. unipole
    • H01Q9/42—Resonant antennas with feed to end of elongated active element, e.g. unipole with folded element, the folded parts being spaced apart a small fraction of the operating wavelength
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q9/00—Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements
    • H01Q9/04—Resonant antennas
    • H01Q9/44—Resonant antennas with a plurality of divergent straight elements, e.g. V-dipole, X-antenna; with a plurality of elements having mutually inclined substantially straight portions

Definitions

  • the invention relates to a monopole antenna inside a device, especially intended for small-sized mobile terminals.
  • the antenna is preferably placed inside the covers of the device, the space spared for the antenna has become ever smaller, when the devices have become smaller. This means that the antenna design becomes more difficult. Also the fact that the antenna usually has to function in two or more separate frequency bands has an influence in the matter.
  • the internal antenna of the portable radio devices is often of PIFA type (Planar Inverted-F Antenna), in which case the antenna includes a radiating plane and a ground plane one on the other and connected to each other by a short-circuit conductor.
  • PIFA type Planar Inverted-F Antenna
  • the antenna includes a radiating plane and a ground plane one on the other and connected to each other by a short-circuit conductor.
  • good characteristics can be achieved by means of such a structure.
  • the characteristics of PIFA such as the bandwidth, are unsatisfactory, because in that case the distance between the radiating plane and ground plane stays clearly behind its optimum value.
  • the problem caused by the flatness of the radio device can be alleviated by making the antenna into one with a monopole structure, in which case a small height is enough for the antenna, because its ground plane is beside the radiator and not below it.
  • a monopole antenna inside a device, known from the publication WO 2008/081077.
  • the circuit board PCB of a radio device and two radiating elements 120, 130 of the antenna are seen in the drawing.
  • the surface of the circuit board is largely of conductive signal ground GND, which also functions as the ground plane of the antenna.
  • the radiating element 120 is the main radiator. It is located at the end of the circuit board, almost totally outside it seen from above the board, the ground plane thus being located beside the radiator.
  • the main radiator 120 Seen from the feed point FP, the main radiator 120 is divided into two arms 121 , 122 of different lengths for implementing two separate operating bands.
  • the main radiator with its arms is mostly on a plane, which is parallel to the circuit board PCB.
  • An inductive component can be connected to the ground near the feed point FP for improving the antenna matching.
  • the second radiating element 130 is a parasitic element, which is connected to the ground plane GND from the ground point GP at one end of the element.
  • the ground point GP and said feed point are located side by side near a corner of the circuit board, seen from above.
  • the lower operating band of the antenna is based on the resonance of the longer arm 121 of the main element, and the upper operating band is based both on the resonance of the shorter arm 122 of the main element and the resonance of the parasitic element 130.
  • the frequencies of the two latter resonances are different, but in any event so close to each other that a united and relatively wide upper operating band is achieved.
  • the monopole antennas like the one described above are superior in performance of a PIFA made to a space of equal size. However, insufficient bandwidth still is a drawback: If the device has to operate in addition to the GSM systems (Global
  • the upper operating band of the antenna remains too narrow.
  • a further drawback is that exciting a resonance in the parasitic element weakens the upper resonance of the main element, for which reason the coupling between the parasitic and main element can not be made very strong.
  • the resonances of the main element have a diminishing effect on each other, which causes degradation, besides in the bandwidths, also in the antenna efficiency.
  • An object of the invention is to reduce said disadvantages related to the prior art.
  • An antenna according to the invention is characterized by what is set forth in the independent claim 1. Some advantageous embodiments of the invention are disclosed in the sub-claims.
  • the antenna of a radio device is of monopole type and comprises a radiating main element and parasitic element.
  • the main element has a resonance frequency both in the lower and upper operating band of the antenna, and the parasitic element has a resonance frequency in the upper operating band.
  • the length of the part of the main element, which corresponds to its lower resonance is increased by shaping the element so that a harmonic of the basic resonance frequency lowers relatively close to the other resonance frequencies in the upper operating band.
  • the shift of the lower resonance frequency of the main element which takes place at the same time, is compensated by adding a serial capacitor to the feed circuit of the antenna.
  • the part of the main element which corresponds to its upper resonance, is divided into two arms so that one of these arms is the part of the main element nearest to the parasitic element.
  • An advantage of the invention is that the upper operating band of an antenna can be made considerably wider compared with the corresponding known antennas. This is due to the fact that the above-mentioned harmonic resonance of the main element is utilized in the antenna matching in the upper operating band.
  • the coupling between the main and parasitic element can be relatively strong without degradation in the antenna matching in the upper operating band, because only a fraction of the part of the main element, which corresponds to its upper resonance, is located alongside the parasitic element.
  • the strength of the coupling between the elements results in that the size of the parasitic element resonating at a certain frequency becomes smaller compared with the known technique, in which case the unfavourable coupling of the parasitic element to the other conductive parts of the radio device, such as a neighbouring battery, reduces.
  • Fig. 1 shows an example of the known internal monopole antenna
  • Fig. 2 shows an example of the internal monopole antenna according to the invention
  • Fig. 3 shows the antenna according to Fig. 2 from the side
  • Fig. 4 shows another example of the antenna according to the invention
  • Fig. 5 shows an example of the band characteristics of the antenna according to the invention
  • Fig. 6 shows an example of the efficiency of the antenna according to the invention.
  • Fig. 1 was already described in conjunction with the description of the prior art.
  • Fig. 2 shows an example of the internal monopole antenna according to the invention.
  • the epithet 'internal' means in this description and claims that the antenna does not form a protruding part from the outer cover of a radio device.
  • the antenna is a dual-band one so that it has a lower and upper operating band.
  • a part of a relatively flat radio device RD is seen in the drawing.
  • the antenna is located at the end of the radio device.
  • the radiators of the antenna are of conductive coating of a dielectric frame FRM, which forms the cover of the end part of the radio device.
  • the antenna comprises two radiating elements, the main element 220, in which the antenna feed point FP is, and the parasitic element 230.
  • the feed conductor of the antenna comes to the feed point FP from a slot between the frame FRM and the rest of the device cover, the slot being relatively near the first end of the frame, or the end on the side of the first side surface of the device.
  • the main element comprises a first part 221 to implement the lower operating band and a second part 222, 223 to implement the upper operating band.
  • the first part comprises a first portion starting from the feed area towards the second end of the frame FRM, a U-shaped bend portion and a tail portion parallel to the first portion backwards.
  • the first portion is located on the rear surface of the radio device RD (the upper surface in the figure), and the tail portion is located on the head surface of the device.
  • the second part is divided to the first 222 and second 223 arm, which together resonate in the upper operating band of the antenna.
  • the arm formed by the first part 221 and the first and second arm of the second part have a shared area, or the feed area, joining the feed point FP.
  • the invention is characterized in that the first part 221 of the main element seen in Fig. 2 is longer than what the intended place of the lower operating band on the frequency scale requires. In other words, the length of the first part is greater than the length, which corresponds to the first part's basic resonance frequency falling into the lower operating band.
  • the basic resonance frequency of the main element would then be lower than the 'right' frequency, and correspondingly the harmonic frequency of the basic resonance frequency is lower than what it would be without lengthening the first part.
  • the latter circumstance is the intention of the solution:
  • the harmonic frequency is arranged relatively close to the resonance frequencies of the parasitic element and the second part of the main element so that the width of the upper operating band further increases and the matching improves especially at the higher end of the upper operating band.
  • the shift of the basic resonance frequency of the main element must be rectified, which is effected by means of a discrete capacitor seen in Fig. 3. This capacitor does not affect the harmonic frequency.
  • the harmonic frequency in question is the third, when the basic resonance frequency is counted as the first harmonic.
  • the first arm 222 of the second part of the main element 220 is directed from the feed area to the area between the first and tail portions of the first part 221.
  • the second arm 223 of the second part again starts from the feed area to a clearly different direction than the first arm and extends next to the tail end of the first part 221 on the end surface of the device.
  • the second arm is located between the first part 221 and the parasitic element. Because of this solution the coupling between the main and parasitic element can be strengthened by decreasing the distance of the second arm 223 and parasitic element 230 without much degrading the antenna matching in the upper operating band. If in the corresponging known antennas the coupling between the main and parasitic element is strengthened, the resonances in the upper operating band weaken, and the matching therefore degrades.
  • Fig. 2 the relatively strong coupling between the main and parasitic element results in that the electric size of the parasitic element is clearly bigger than its physical size. This further results in that the electromagnetic coupling of the parasitic element to the other conductive parts of the radio device, such as neighbouring battery, reduces. Such a coupling to the other conductive parts of the radio device naturally weakens the resonance of the parasitic element.
  • the parasitic element 230 is for widening the upper operating band of the antenna. Its resonance frequency is then close to the resonance frequency of the second part of the main element.
  • the parasitic element is located in the area between the main element and the first side surface of the device. It is short-circuited to the ground plane of the antenna from the grounding point GP, which is located next to the feed point FP in the slot between the frame FRM and the rest of the device cover. Starting from the grounding point, the parasitic element 230 travels first beside said feed area and then alongside the second arm 223 of the second part of the main element extending slightly to the end surface of the device.
  • the parasitic element comprises an extension from the grounding point GP towards the first side surface of the device.
  • Fig. 3 shows the antenna according to Fig. 2 from the side so that also the circuit board PCB of the radio device RD is visible.
  • the main element 220 of the antenna is connected to the antenna port AP of the radio device by the feed conductor FC.
  • FC feed conductor
  • TCA tuning capacitor mounted on the circuit board PCB.
  • the serial capacitance decreases the electric size of the main element, for which reason, with a suitable capacitance value, the basic resonance frequency of the main element can be shifted upwards to the planned point in the lower operating band.
  • the ground plane GND On the surface of the circuit board there is also the ground plane GND, necessary for the function of the antenna, the edge of the ground plane being at a certain distance from the radiating elements of the antenna.
  • Fig. 4 shows another example of the antenna according to the invention.
  • the radiating elements are in this example on the same circuit board as the ground plane GND.
  • the structure comprises the main element 420 with the first and second part, coupled to the antenna port through a serial capacitor TCA, and the parasitic element.
  • the shapes of the elements differ to some extent from the ones in Fig. 2:
  • the first part 421 of the main element comprises, starting from the feed area FA, a first portion in the longitudinal direction of the radiating structure, a transverse second portion, a longitudinal third portion backwards, a short transverse fourth portion towards the first portion and lastly a longitudinal fifth portion towards the second portion.
  • the first arm 422 of the second part of the main element is directed, alongside the first portion, from the feed area to the inner area confined by the first, second and fifth portions.
  • the second arm 423 of the second part of the main element comprises a transverse starting portion starting from the feed area and a longitudinal tail portion away from the first part 421.
  • the parasitic element 430 comprises, starting from its grounding point GP, a transverse portion beside the feed area FA and the starting portion of the second arm 423 of the second part, a longitudinal portion beside the tail portion of the second arm 423 and a transverse final portion so that the element has the shape of the U letter.
  • Fig. 5 shows an example of the band characteristics of the antenna according to the invention and of the corresponding known antenna as curves of the reflection coefficient S1 1.
  • Curve 52 relates to the antenna according to Fig. 2 and, for comparison, curve 51 relates to a corresponding known antenna.
  • the subject of review is primarily the upper operating band.
  • Curve 51 shows that the upper operating band is based on the resonance ra of the shorter part of the antenna's main element and on the resonance rb of the parasitic element. If the value -5 dB of the reflection coefficient is used as criterion for the boundary frequency of a band, the upper operating band is about 1.66-2.12 GHz, which well covers the frequency ranges used by the systems GSM1800 and GSM1900.
  • Curve 51 also shows that the known antenna additionally has a clear resonance re at the frequency of about 2.8 GHz. This resonance is a harmonic of the basic resonance of the longer part of the main element, and it is useless, because the range of 2.8 GHz is not utilized.
  • Curve 52 shows that the harmonic resonance re of the basic resonance of the longer part of the main element has shifted downwards below the frequency 2.5 GHz.
  • the shift of the harmonic improves the antenna matching in the range of 2110-2170 MHz (w1 in the figure), which is the receiving band of the mobile terminals in the WCDMA system.
  • the matching in the range w1 is improved by the shift of the frequency of the resonance rb of the parasitic element slightly upwards.
  • the reflection coefficient improves an average of about 6 dB.
  • Resonance re can be utilized also in the WCDMA7 system, which will start up and the frequency range w2 of which is 2500-2690 MHz.
  • the frequency range w2 of which is 2500-2690 MHz.
  • Curve 52 also shows that in the example the antenna according to the invention has a resonance also a little below the frequency of 2.7 GHz. This resonance is not planned. However, it can be exploited in said WCDMA7 system.
  • the solution according to the invention has hardly any effect on the lower operating band.
  • the effect tn of this tuning has been marked in Fig. 5: Without the tuning capacitor the basic resonance frequency corresponding to the lower operating band is about 720 MHz and after the tuning about 860 MHz (transition +19%).
  • the lower operating band is outside the frequency range 824-960 MHz used by the systems GSM850 and GSM900 together, and in the latter case it covers this range.
  • the third harmonic of the basic resonance frequency in question shifts upwards only 15 MHz, or 0.6%, because of the tuning (not visible in the figure).
  • Fig. 6 shows an example of the efficiency of the antenna according to the invention.
  • the curves of the example show the fluctuation of the efficiency in free space as a function of frequency. The efficiency is signified in decibels, 0 dB then meaning the ideal, or lossless case.
  • Curve 62 relates to the same structure as the matching curve 52 in Fig. 5, and curve 61 relates to a corresponding known antenna.
  • the curves show that in the lower operating band the efficiency of the antenna according to the invention is about one decibel better than of the comparison antenna.
  • the efficiencies are nearly the same below the frequency of 2 GHz, but above the frequency 2 GHz the antenna according to the invention is clearly better.
  • the difference is about 2.5 dB.
  • the monopole antenna according to the invention has been described above. Its structure may in detail differ from those described in Figs. 2 and 4 within the limits defined by the independent claim 1.

Abstract

An internal monopole antenna intended for small-sized radio devices (RD). It comprises a radiating main element (220) and parasitic element (230). The main element has a resonance frequency both in the lower and upper operating band of the antenna, and the parasitic element has a resonance frequency in the upper operating band. The length of the part (221) of the main element, which corresponds to its lower resonance, is increased by shaping the element so that the third harmonic of the basic resonance frequency lowers relatively close to the other resonance frequencies in the upper operating band. The shift of the lower resonance frequency of the main element, which takes place at the same time, is compensated by adding a serial capacitor to the feed circuit of the antenna. The part of the main element, which corresponds to its upper resonance, is divided into two arms (222, 223) so that one (223) of these arms is the part of the main element nearest to the parasitic element. The upper operating band of an antenna can be made wider compared with the corresponding known antennas, because said harmonic resonance is utilized in the antenna matching in the upper operating band. In addition, a relatively strong coupling can exist between the main (220) and parasitic (230) element without degradation in the antenna matching in the upper operating band, in which case the parasitic element becomes smaller compared with the known technique and its unfavourable coupling to the other conductive parts of the radio device reduces.

Description

Internal monopole antenna
The invention relates to a monopole antenna inside a device, especially intended for small-sized mobile terminals.
In antenna design of the portable radio devices the available space is an important factor. Without limit on size, it is relatively easy to make a high-quality antenna.
However, because the antenna is preferably placed inside the covers of the device, the space spared for the antenna has become ever smaller, when the devices have become smaller. This means that the antenna design becomes more difficult. Also the fact that the antenna usually has to function in two or more separate frequency bands has an influence in the matter.
The internal antenna of the portable radio devices, such as mobile phones, is often of PIFA type (Planar Inverted-F Antenna), in which case the antenna includes a radiating plane and a ground plane one on the other and connected to each other by a short-circuit conductor. In respect of its size, good characteristics can be achieved by means of such a structure. However, in very flat radio devices the characteristics of PIFA, such as the bandwidth, are unsatisfactory, because in that case the distance between the radiating plane and ground plane stays clearly behind its optimum value.
The problem caused by the flatness of the radio device can be alleviated by making the antenna into one with a monopole structure, in which case a small height is enough for the antenna, because its ground plane is beside the radiator and not below it. In Fig. 1 there is an example of such a monopole antenna inside a device, known from the publication WO 2008/081077. The circuit board PCB of a radio device and two radiating elements 120, 130 of the antenna are seen in the drawing. The surface of the circuit board is largely of conductive signal ground GND, which also functions as the ground plane of the antenna. The radiating element 120 is the main radiator. It is located at the end of the circuit board, almost totally outside it seen from above the board, the ground plane thus being located beside the radiator. Seen from the feed point FP, the main radiator 120 is divided into two arms 121 , 122 of different lengths for implementing two separate operating bands. The main radiator with its arms is mostly on a plane, which is parallel to the circuit board PCB. For lengthening the longer arm 121 and for shaping the radiating pattern, at its outer end there is a portion which extends towards the geometric plane of the circuit board, for which reason the antenna has a certain height. However, this height remains smaller than the height required by all the other parts of the radio apparatus together. An inductive component can be connected to the ground near the feed point FP for improving the antenna matching. The second radiating element 130 is a parasitic element, which is connected to the ground plane GND from the ground point GP at one end of the element. The ground point GP and said feed point are located side by side near a corner of the circuit board, seen from above.
The lower operating band of the antenna is based on the resonance of the longer arm 121 of the main element, and the upper operating band is based both on the resonance of the shorter arm 122 of the main element and the resonance of the parasitic element 130. The frequencies of the two latter resonances are different, but in any event so close to each other that a united and relatively wide upper operating band is achieved.
The monopole antennas like the one described above are superior in performance of a PIFA made to a space of equal size. However, insufficient bandwidth still is a drawback: If the device has to operate in addition to the GSM systems (Global
System for Mobile telecommunications) also in the WCDMA system (Wideband
Code Division Multiple Access), the frequency range of which extends to 2170
MHz, the upper operating band of the antenna remains too narrow. A further drawback is that exciting a resonance in the parasitic element weakens the upper resonance of the main element, for which reason the coupling between the parasitic and main element can not be made very strong. On the other hand also the resonances of the main element have a diminishing effect on each other, which causes degradation, besides in the bandwidths, also in the antenna efficiency.
An object of the invention is to reduce said disadvantages related to the prior art. An antenna according to the invention is characterized by what is set forth in the independent claim 1. Some advantageous embodiments of the invention are disclosed in the sub-claims.
The basic idea of the invention is as follows: The antenna of a radio device is of monopole type and comprises a radiating main element and parasitic element. The main element has a resonance frequency both in the lower and upper operating band of the antenna, and the parasitic element has a resonance frequency in the upper operating band. The length of the part of the main element, which corresponds to its lower resonance, is increased by shaping the element so that a harmonic of the basic resonance frequency lowers relatively close to the other resonance frequencies in the upper operating band. The shift of the lower resonance frequency of the main element, which takes place at the same time, is compensated by adding a serial capacitor to the feed circuit of the antenna. In addition, the part of the main element, which corresponds to its upper resonance, is divided into two arms so that one of these arms is the part of the main element nearest to the parasitic element.
An advantage of the invention is that the upper operating band of an antenna can be made considerably wider compared with the corresponding known antennas. This is due to the fact that the above-mentioned harmonic resonance of the main element is utilized in the antenna matching in the upper operating band. In addition, the coupling between the main and parasitic element can be relatively strong without degradation in the antenna matching in the upper operating band, because only a fraction of the part of the main element, which corresponds to its upper resonance, is located alongside the parasitic element. The strength of the coupling between the elements results in that the size of the parasitic element resonating at a certain frequency becomes smaller compared with the known technique, in which case the unfavourable coupling of the parasitic element to the other conductive parts of the radio device, such as a neighbouring battery, reduces.
The invention is described in detail below. In the description it will be referred to the accompanying drawings where
Fig. 1 shows an example of the known internal monopole antenna,
Fig. 2 shows an example of the internal monopole antenna according to the invention,
Fig. 3 shows the antenna according to Fig. 2 from the side,
Fig. 4 shows another example of the antenna according to the invention,
Fig. 5 shows an example of the band characteristics of the antenna according to the invention, and Fig. 6 shows an example of the efficiency of the antenna according to the invention.
Fig. 1 was already described in conjunction with the description of the prior art. Fig. 2 shows an example of the internal monopole antenna according to the invention. The epithet 'internal' means in this description and claims that the antenna does not form a protruding part from the outer cover of a radio device. The antenna is a dual-band one so that it has a lower and upper operating band. A part of a relatively flat radio device RD is seen in the drawing. The antenna is located at the end of the radio device. In this example, the radiators of the antenna are of conductive coating of a dielectric frame FRM, which forms the cover of the end part of the radio device. The antenna comprises two radiating elements, the main element 220, in which the antenna feed point FP is, and the parasitic element 230. The feed conductor of the antenna comes to the feed point FP from a slot between the frame FRM and the rest of the device cover, the slot being relatively near the first end of the frame, or the end on the side of the first side surface of the device. The main element comprises a first part 221 to implement the lower operating band and a second part 222, 223 to implement the upper operating band. The first part comprises a first portion starting from the feed area towards the second end of the frame FRM, a U-shaped bend portion and a tail portion parallel to the first portion backwards. In this example, the first portion is located on the rear surface of the radio device RD (the upper surface in the figure), and the tail portion is located on the head surface of the device. The second part is divided to the first 222 and second 223 arm, which together resonate in the upper operating band of the antenna. The arm formed by the first part 221 and the first and second arm of the second part have a shared area, or the feed area, joining the feed point FP.
The invention is characterized in that the first part 221 of the main element seen in Fig. 2 is longer than what the intended place of the lower operating band on the frequency scale requires. In other words, the length of the first part is greater than the length, which corresponds to the first part's basic resonance frequency falling into the lower operating band. The basic resonance frequency of the main element would then be lower than the 'right' frequency, and correspondingly the harmonic frequency of the basic resonance frequency is lower than what it would be without lengthening the first part. The latter circumstance is the intention of the solution:
The harmonic frequency is arranged relatively close to the resonance frequencies of the parasitic element and the second part of the main element so that the width of the upper operating band further increases and the matching improves especially at the higher end of the upper operating band. The shift of the basic resonance frequency of the main element, on the other hand, must be rectified, which is effected by means of a discrete capacitor seen in Fig. 3. This capacitor does not affect the harmonic frequency. The harmonic frequency in question is the third, when the basic resonance frequency is counted as the first harmonic.
The first arm 222 of the second part of the main element 220 is directed from the feed area to the area between the first and tail portions of the first part 221. The second arm 223 of the second part again starts from the feed area to a clearly different direction than the first arm and extends next to the tail end of the first part 221 on the end surface of the device. The second arm is located between the first part 221 and the parasitic element. Because of this solution the coupling between the main and parasitic element can be strengthened by decreasing the distance of the second arm 223 and parasitic element 230 without much degrading the antenna matching in the upper operating band. If in the corresponging known antennas the coupling between the main and parasitic element is strengthened, the resonances in the upper operating band weaken, and the matching therefore degrades. In Fig. 2 the relatively strong coupling between the main and parasitic element results in that the electric size of the parasitic element is clearly bigger than its physical size. This further results in that the electromagnetic coupling of the parasitic element to the other conductive parts of the radio device, such as neighbouring battery, reduces. Such a coupling to the other conductive parts of the radio device naturally weakens the resonance of the parasitic element.
The parasitic element 230 is for widening the upper operating band of the antenna. Its resonance frequency is then close to the resonance frequency of the second part of the main element. The parasitic element is located in the area between the main element and the first side surface of the device. It is short-circuited to the ground plane of the antenna from the grounding point GP, which is located next to the feed point FP in the slot between the frame FRM and the rest of the device cover. Starting from the grounding point, the parasitic element 230 travels first beside said feed area and then alongside the second arm 223 of the second part of the main element extending slightly to the end surface of the device. In addition, the parasitic element comprises an extension from the grounding point GP towards the first side surface of the device.
Fig. 3 shows the antenna according to Fig. 2 from the side so that also the circuit board PCB of the radio device RD is visible. The main element 220 of the antenna is connected to the antenna port AP of the radio device by the feed conductor FC. In series with the feed conductor, thus between the feed point FP and one terminal of the antenna port, there is a discrete tuning capacitor TCA mounted on the circuit board PCB. The serial capacitance decreases the electric size of the main element, for which reason, with a suitable capacitance value, the basic resonance frequency of the main element can be shifted upwards to the planned point in the lower operating band.
On the surface of the circuit board there is also the ground plane GND, necessary for the function of the antenna, the edge of the ground plane being at a certain distance from the radiating elements of the antenna. There can be a coil MCO improving the antenna matching between the feed conductor FC and the ground.
Fig. 4 shows another example of the antenna according to the invention. The radiating elements are in this example on the same circuit board as the ground plane GND. As in Fig. 2, the structure comprises the main element 420 with the first and second part, coupled to the antenna port through a serial capacitor TCA, and the parasitic element. The shapes of the elements differ to some extent from the ones in Fig. 2: The first part 421 of the main element comprises, starting from the feed area FA, a first portion in the longitudinal direction of the radiating structure, a transverse second portion, a longitudinal third portion backwards, a short transverse fourth portion towards the first portion and lastly a longitudinal fifth portion towards the second portion. The first arm 422 of the second part of the main element is directed, alongside the first portion, from the feed area to the inner area confined by the first, second and fifth portions. The second arm 423 of the second part of the main element comprises a transverse starting portion starting from the feed area and a longitudinal tail portion away from the first part 421. The parasitic element 430 comprises, starting from its grounding point GP, a transverse portion beside the feed area FA and the starting portion of the second arm 423 of the second part, a longitudinal portion beside the tail portion of the second arm 423 and a transverse final portion so that the element has the shape of the U letter.
Fig. 5 shows an example of the band characteristics of the antenna according to the invention and of the corresponding known antenna as curves of the reflection coefficient S1 1. Curve 52 relates to the antenna according to Fig. 2 and, for comparison, curve 51 relates to a corresponding known antenna. The subject of review is primarily the upper operating band. Curve 51 shows that the upper operating band is based on the resonance ra of the shorter part of the antenna's main element and on the resonance rb of the parasitic element. If the value -5 dB of the reflection coefficient is used as criterion for the boundary frequency of a band, the upper operating band is about 1.66-2.12 GHz, which well covers the frequency ranges used by the systems GSM1800 and GSM1900. Curve 51 also shows that the known antenna additionally has a clear resonance re at the frequency of about 2.8 GHz. This resonance is a harmonic of the basic resonance of the longer part of the main element, and it is useless, because the range of 2.8 GHz is not utilized.
Curve 52 shows that the harmonic resonance re of the basic resonance of the longer part of the main element has shifted downwards below the frequency 2.5 GHz. The shift of the harmonic improves the antenna matching in the range of 2110-2170 MHz (w1 in the figure), which is the receiving band of the mobile terminals in the WCDMA system. On the other hand, the matching in the range w1 is improved by the shift of the frequency of the resonance rb of the parasitic element slightly upwards. Compared with the known antenna, in this range the reflection coefficient improves an average of about 6 dB.
Resonance re can be utilized also in the WCDMA7 system, which will start up and the frequency range w2 of which is 2500-2690 MHz. In addition, it would in principle be exploitable in the WLAN system (Wireless Local Area Network), the frequency range of which is 2400-2483.5 MHz.
Curve 52 also shows that in the example the antenna according to the invention has a resonance also a little below the frequency of 2.7 GHz. This resonance is not planned. However, it can be exploited in said WCDMA7 system.
The solution according to the invention has hardly any effect on the lower operating band. True the lengthening of the first part of the main element has to be compensated by said tuning capacitor TCA. The effect tn of this tuning has been marked in Fig. 5: Without the tuning capacitor the basic resonance frequency corresponding to the lower operating band is about 720 MHz and after the tuning about 860 MHz (transition +19%). In the former case the lower operating band is outside the frequency range 824-960 MHz used by the systems GSM850 and GSM900 together, and in the latter case it covers this range. The third harmonic of the basic resonance frequency in question shifts upwards only 15 MHz, or 0.6%, because of the tuning (not visible in the figure).
Fig. 6 shows an example of the efficiency of the antenna according to the invention. The curves of the example show the fluctuation of the efficiency in free space as a function of frequency. The efficiency is signified in decibels, 0 dB then meaning the ideal, or lossless case. Curve 62 relates to the same structure as the matching curve 52 in Fig. 5, and curve 61 relates to a corresponding known antenna. The curves show that in the lower operating band the efficiency of the antenna according to the invention is about one decibel better than of the comparison antenna. In the upper operating band the efficiencies are nearly the same below the frequency of 2 GHz, but above the frequency 2 GHz the antenna according to the invention is clearly better. For example, in the WCDMA band 21 10-2170 MHz the difference is about 2.5 dB.
The monopole antenna according to the invention has been described above. Its structure may in detail differ from those described in Figs. 2 and 4 within the limits defined by the independent claim 1.

Claims

Claims
1. An internal antenna of a radio device, which has a lower and an upper operating band and comprises a main element (220; 420) to be coupled from its feed point (FP) to an antenna port of the radio device by a feed conductor (FC) and a parasitic element (230; 430) short-circuited from its grounding point (GP) to a ground plane (GND) of the antenna, which main element comprises a first part (221 ; 421 ), a basic resonance frequency of which is in the lower operating band, and a second part (222, 223; 422, 423), a resonance frequency of which in the upper operating band, and a resonance frequency of which parasitic element is in the upper operating band, characterized in that the length of the first part (221 ; 421 ) of the main element is greater than the length, which corresponds to the first part's basic resonance frequency falling into the lower operating band, to lower the third harmonic frequency of the basic resonance frequency for improving the matching in the upper operating band.
2. An antenna according to claim 1 , characterized in that it further comprises a tuning capacitor (TCA) in series with said feed conductor (FC) to compensate a shift of the basic resonance frequency of the first part (221 ; 421 ) of the main element.
3. An antenna according to claim 1 , characterized in that its main element (220) and parasitic element (230) are of conductive coating of a dielectric frame
(FRM), which forms a cover of an end part of the radio device.
4. An antenna according to claim 1 , characterized in that the first part (221 ; 421 ) of the main element comprises a first portion starting from an area encompassing the feed point (FP), or a feed area (FA), and at least one portion parallel to the first portion, and the second part comprises a first (222; 422) and second (223; 423) arm, which first arm is directed from the feed area (FA) to inner area confined by the first portion and other portions of the first part (221 ; 421 ), and the second arm (223; 423) is located between the first part and the parasitic element (230; 430), the parasitic element then being located beside the second arm (223; 423) of the second part of the main element and the feed area.
5. An antenna according to claim 4, characterized in that the second arm (223) of the second part of the main element (220) extends next to tail end of the first part (221 ) of the main element.
6. An antenna according to claim 1 , characterized in that an edge of its ground plane (GND) is, seen in the direction of the normal of the ground plane, beside the main and parasitic elements, at a certain distance from them.
7. An antenna according to claim 1 , characterized in that it further comprises a matching coil (MCO) connected between said feed conductor (FC) and the ground plane (GND).
PCT/FI2010/050296 2009-04-22 2010-04-13 Internal monopole antenna WO2010122220A1 (en)

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