WO2007030401A2

WO2007030401A2 - Antenna and rf terminal incorporating the antenna

Info

Publication number: WO2007030401A2
Application number: PCT/US2006/034351
Authority: WO
Inventors: Aviv Shachar; Maksim Berezin; Motti Elkobi
Original assignee: Motorola, Inc.
Priority date: 2005-09-05
Filing date: 2006-09-02
Publication date: 2007-03-15
Also published as: GB0518002D0; WO2007030401A3; GB2429845A; GB2429845B

Abstract

A dipole antenna (102) comprises a first section (103) which includes a first input port (107) and a folded configuration (115, 119, 123) and a second section (105) which includes a second input port (109) and a folded configuration (117, 121, 125) and is characterised by a reactive coupling (131, 133) between the first section and the second section. The reactive coupling may conveniently comprise a capacitive coupling formed between a conducting strip portion (131) of the first section and an adjacent conducting strip portion (133) of the second section. The antenna may be formed on a face of an insulating substrate 101.

Description

ANTENNA AND RF TERMINAL INCORPORATING THE ANTENNA

FIELD OF THE INVENTION

The present invention relates to an antenna and a RF terminal incorporating the antenna. In particular, the invention relates to an improved dipole antenna having folded configurations .

BACKGROUND OF THE INVENTION

Various antenna types are known for use in high frequency RF communication terminals, particularly portable communication devices. For example, monopole and dipole antennas, patch and so called planar inverted ¹F' (PIF) antennas are all known for this application.

In a dipole antenna the same signal for transmission by the antenna is applied to two input ^" ports of the antenna. The antenna has two separate radiative portions which extend from each of the input ports. In order to achieve good directional radiation pattern characteristics and at the same time provide good matching to a practical feed line, the length / of each of the radiative portions, e.g. where each is in the form of an elongate single wire element, is usually chosen to be λlA≤l ≤ λ where A is the wavelength of the electromagnetic radiation to be transmitted. The most widely used dipole antenna is one whose radiative portions each have an overall length of l≡Λ/2. In order provide good matching characteristics, variations of the simple dipole comprising two separate radiative portions are used. One simple known antenna form that can provide matching improvements is a conventional folded dipole antenna. In this form the conductor, e.g. wire, which forms the radiative portions of the simple dipole antenna is bent or folded at both ends to form a further portion which completes a closed loop. Usually, the loop is in the form of a very thin rectangle wherein the spacing s between the two long sides of the rectangle is s«λ, usually s < 0.05/1.

SOT-MARY OF THE INVENTION

According to the present invention in a first aspect there is provided a dipole antenna comprising a first section which includes a first input port and a folded configuration and a second section which includes a second input port and a folded configuration and characterised by a reactive coupling between the first section and the second section. The reactive coupling may comprise a capacitive coupling formed between a conducting strip portion of the first section and an adjacent conducting strip portion of the second section. Desirably, the reactive coupling is formed adjacent to the first and second input ports. For example, where the reactive coupling comprises a capacitive coupling formed between a conducting strip portion of the first section and an adjacent conducting strip portion of the second section the separation between the first input port and the conductive strip portion of the first section and the separation _? between the first input port and the conductive strip portion of the first section and between the second input port and the conductive strip portion of the second section is preferably not greater than 0.052 where l is a wavelength of electromagnetic radiation radiated or received by the antenna.

Each of the first section and the second section may include, extending from its input port, a first conducting strip portion and a second conducting strip portion connected to and at an angle, e.g. perpendicular, to the first conducting strip portion. Each of the first and second sections may include further strip portions which provide the folded configuration. For example, each of the first and second sections may include a third conducting strip portion conducting strip portion connected to the second conducting strip portion and, connected to the third conducting strip portion, a fourth conducting strip portion extending substantially parallel to the second conducting strip portion. The third conducting strip portion may conveniently be substantially perpendicular to the second and fourth conducting strip portions.

Each of the first and second sections may include a fifth conducting strip portion connected to the fourth conducting strip portion and extending substantially parallel to the first conducting strip portion.

Where the reactive coupling comprises a capacitive coupling formed between a conducting strip portion of the first section and an adjacent conducting strip portion of the second section, the conducting strip portion of the coupling in each of the first section and the second section may be a sixth conducting strip portion connected to and at an angle to, e.g. perpendicular to, the fifth conducting strip portion of the respective section.

Each of the first and second sections may beneficially include a second folded configuration formed inside a first folded configuration. The second folded configuration may be connected to the sixth conducting strip portion. The second folded configuration may comprise a seventh conducting strip portion extending substantially parallel with the fifth conducting strip portion, an eighth conducting strip portion connected to the seventh conducting strip portion and a ninth conducting strip portion connected to the eighth conducting strip portion and extending substantially parallel with the seventh conducting strip portion. The eighth conducting strip portion may be substantially perpendicular to the seventh and ninth conducting strip portions.

Each of the first and second sections may include at the end of its second folded configuration an end strip portion at an angle, e.g. perpendicular to the strip portion to which it is connected.

The antenna according to the first aspect is suitable for use in a variety of high frequency communication applications, examples of which are given later, but is particularly suitable to radiate and receive electromagnetic radiation signals in a frequency band which includes 2.4 GHz, e.g. in a WLAN (Wireless Local Area Network) system operating in accordance with the IEEE WLAN 802.11 standard.

In accordance with a second aspect of the present invention there is provided an antenna module including an insulating substrate and, deposited on a surface of the insulating substrate in the form of a shaped conducting layer, an antenna, wherein the antenna is an the antenna according to the first aspect . In accordance with a third aspect of the present invention there is provided a terminal for RF communications comprising a RF transceiver, an antenna and an RF feed line between the RF transceiver and the antenna, wherein the antenna comprises an antenna according to the first aspect.

Beneficially, as illustrated later, the novel antenna in accordance with the first aspect of the invention shows several improvements over the conventional folded dipole antenna. Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an antenna module embodying the invention.

FIG. 2 is a block schematic diagram illustrating an RF terminal embodying the invention. FIG. 3 is a graph of magnitude in dB versus frequency in GHz for the simulated reflection coefficient SIl of the antenna module of FIG. 1.

FIG. 4 is a Smith Chart for the simulated reflection coefficient SIl of the antenna module of FIG. 1.

FIG. 5 is a graph of phase angle in degrees versus frequency in GHz for the simulated reflection coefficient SIl of the antenna module of FIG. 1. FIG. 6 is a graph of simulated voltage standing wave ratio (VSWR) versus frequency in GHz for the antenna module of FIG. 1.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 is a plan view of an antenna module 100 including an antenna 102 embodying the invention. In the following description of the antenna module 100 and the antenna 102, the expressions 'vertical' and 'horizontal' (or 'vertically' and 'horizontally') refer to the orientation and relative directions as seen in FIG. 1. Of course, in practice, the use of antenna module 100 and the antenna 102 is not restricted to the particular orientation shown in FIG. 1. The antenna module 100 comprises an insulating substrate 101 on which the antenna 102 is deposited, on an upper face seen in FIG. 1. The substrate 101 may conveniently be prepared from a material employed to make printed circuit boards (PCBs), e.g. a glass fibre reinforced resin material. The substrate 101 could actually be part of a printed circuit board but, as indicated in FIG. 1, may be a separate component. The antenna 102 comprises a first antenna section 103 and a second antenna section 105. The first and second antenna sections 103 and 105 may be made of any suitable conducting material known for use in producing printed conductors, e.g. copper or a copper based alloy, formed in a known manner on the substrate 101, e.g. by depositing a coating or coatings of copper on a PCB substrate material and using a known computer controlled cutting operation applied to the coated substrate to remove copper and to leave the desired shapes of copper.

The first antenna section 103 and the second antenna section 105 are in the form of shapes which are mirror images of one another about an axis A-A which is parallel to the shorter sides of the rectangular shape of the top surface of the substrate 101. Thus, corresponding parts of the first antenna section 103 and the second antenna section 105 (i.e. parts which are 'images' of one another) have the same dimensions.

The first antenna section 103 includes an input port 107. A first conducting strip portion 111 of the first antenna section 103 extends horizontally outward from the input port 107, and a second conducting strip portion 115, connected to the first conducting strip portion 113, extends vertically downward from the first strip portion 111. A short third conducting strip portion 119, connected to the second strip portion 115, extends horizontally inward from the second strip portion 115 to form a folded connection to a fourth conducting strip portion 123. The fourth conducting strip portion 123 extends vertically upward from the third portion 119. A fifth conducting strip portion 127, connected to the fourth conducting strip portion 123, extends horizontally inward from the fourth conducting strip portion 127 to a sixth conducting strip portion 131. The sixth conducting strip portion 131, connected to the fifth strip portion 127, extends vertically upward from the fifth strip portion 127. The sixth conducting strip portion 131 has a short length and has a width which is enlarged compared with the width of the other conducting strip portions (excluding the input port 107) of the antenna section 103. A seventh conducting strip portion 135, connected to the sixth conducting strip portion 131, extends horizontally outward from the sixth conducting strip portion 131. A short eighth conducting strip portion 139, connected to the seventh conducting strip portion 135, extends horizontally downward from the seventh conducting strip portion 135 to form a folded connection to a ninth conducting strip portion 143. The ninth conducting strip portion 143 extends horizontally inward from the eighth conducting strip portion 139. A tenth conducting strip portion 147, connected to the ninth conducting strip portion 143, forms an end portion in the form of a vertical arm or ^VT' junction at the inner end of the ninth conducting strip portion 147 and thereby extends vertically above and below the ninth conducting strip portion 143. The seventh conducting strip portion 135, the eighth conducting strip portion 139 and the ninth conducting strip portion 143 together form a folded configuration or combination which is inside a region between the first conducting strip portion 111 and the fifth conducting strip portion 127, in other words within a folded configuration provided by the portions from the first conducting strip portion 111 to the fifth conducting strip portion 127.

The second antenna section 105 includes an input port 109 which is close to the input port 107 of the first antenna section 103. A first conducting strip portion 113 of the second antenna section 105 extends horizontally outward from the input port 109, and a second conducting strip portion 117, connected to the first conducting strip portion 113, extends vertically downward from the first conducting strip portion 117. A short third conducting strip portion 121, connected to the second conducting strip portion 117, extends horizontally inward from the second conducting strip portion 117 to form a folded connection to a fourth conducting strip portion 125. The fourth conducting strip portion 125 extends vertically upward from the third conducting strip portion 121. A fifth conducting strip portion 129, connected to the fourth conducting strip portion 125, extends horizontally inward from the fourth conducting strip portion 125 to a sixth conducting strip portion 133. The sixth conducting strip portion 133, connected to the fifth conducting strip portion 129, extends vertically upward from the fifth conducting strip portion 129. The sixth conducting strip portion 133 has a short length and has a width which is enlarged compared with the width of the other strip portions (except the input port 109) of the second antenna section 105. The sixth conducting strip portion 133 of the second antenna section 105 is close to the sixth conducting strip portion 131 of the first antenna section 103. A seventh conducting strip portion 137 of the second antenna section 105, connected to the sixth conducting strip portion 133, extends horizontally outward from the sixth conducting strip portion 133. A short eighth conducting strip portion 141, connected to the seventh conducting strip portion 137, extends vertically downward from the seventh conducting strip portion 137 to form a folded connection to a ninth conducting strip portion 145. The ninth conducting strip portion 145 extends horizontally inward from the eighth conducting strip portion 141. A tenth strip portion 149, connected to the ninth conducting strip portion 145, is an end portion in the form of a vertical arm or 'T junction' at the inner end of the ninth conducting strip portion 145 and thereby extends vertically above and below the ninth conducting strip portion 145. The seventh conducting strip portion 137, the eighth conducting strip portion 141 and the ninth conducting strip portion 145 together form a folded configuration or combination which is inside a region between the first strip portion 113 and the fifth strip portion 129, in other words within a folded configuration provided by the portions from the first conducting strip portion 111 to the fifth conducting strip portion 127. FIG. 2 shows schematically an RF communication terminal 150 including the antenna module 100 (which includes the antenna 102 as described above with reference to FIG. 1) . The terminal 150 includes an RF transceiver 151 which serves as both transmitter and receiver of RF signals. A feed line 152 is connected between the RF transceiver 151 and the antenna module 100. In particular, the feed line 152 supplies from the RF transceiver 151 an input RF electrical signal to both of the input ports 107 and 109 (FIG. 1) of the antenna 102 at the same time. The feed line 152 may be any of the feed lines (transmission lines) known in the art. For example, the feed line may comprise a coaxial cable having an inner conductor which is galvanically connected, e.g. by soldered contacts, to both of the input ports 107 and 109. Alternatively, the feed line may comprise a microstrip connection to both of the input ports 107 and 109.

In use, the antenna sections 103 and 105 of the antenna 102 act together as a combination to radiate RF electromagnetic signals supplied via the feed line 152 from the RF transceiver 151. These signals may be sent over the air to a distant RF receiver (not shown) .

Conversely, the antenna sections 103 and 105 of the antenna 102 acting together as a combination can also act as a receptor for RF electromagnetic signals sent over the air from a distant transmitter (not shown) . Such received signals are supplied via the feed line 152 to the RF transceiver 151 for demodulation and processing. The antenna module 100 including the antenna 102 produced in the manner described earlier with reference to FIG. 1 is suitable to give improved performance in a selected high frequency band, e.g. in a band which includes 2.44 GHz and is suitable for WLAN communications in accordance with the IEEE WLAN 802.11 standard. The antenna 102 of the antenna module 100 has a form which is based on the folded dipole antenna. However, the antenna 102 has novel features and thereby shows improvements over the conventional folded dipole antenna. In particular, the sixth strip portion 131 of the first antenna section 103 and the sixth strip portion 133 of the second antenna portion 105 together form a reactive coupling, especially a capacitive coupling, which at high frequencies is analogous to a short circuit between the first and second antenna sections 103 and 105. This coupling allows several desirable antenna characteristics to be controlled and enhanced, for instance: (i) antenna bandwidth and resonance depth may be improved; (ii) antenna/transceiver matching may be improved; (iii) antenna efficiency may be improved. In addition, the antenna 102 is versatile and may be used in each of several applications, e.g. as a regular antenna or as a folded dipole antenna, with the feed line providing a balanced or unbalanced connection, and in a fixed or mobile terminal. Where the antenna 102 is used in a fixed terminal it may be employed in a housing fixed above the ground. Where the antenna 102 is used in a mobile terminal, the mobile terminal may for example be a wireless communication handset.

The frequency of operation of the antenna 102 is determined largely by the distance L which is the sum of the following lengths (to the first major fold of the section 103) :

(i) the horizontal length of the input port 107; (ii) the horizontal length of the first conducting strip portion 111; and

(iii) the vertical length of the second conducting strip portion 115.

This length L is also equal to the sum of the following lengths : (i) the horizontal length of the input port 109; (ii) the horizontal length of the first conducting strip portion 113; and (iii) the vertical length of the second conducting strip portion 117. The length L is the equivalent electrical length of each of the sections 103, 105 of the antenna 102 and is selected to be approximately equal to 0.5λ, where λ is the wavelength of radiation at the centre of the desired frequency band of operation. For example, a particular desired band for WLAN operation is from approximately 2.40 GHz to 2.48 GHz giving a centre frequency of 2.44 GHz. This is equivalent to a wavelength of 122 mm so, for this example, L is selected to be approximately 61 mm. The following parameters related to the shape and size of the antenna 102 may be selected by an antenna designer to provide tuning of the properties of the antenna 102 : (i) the horizontal separation X between the sixth conducting strip portion 131 and the tenth conducting strip portion 147 and between the sixth conducting strip portion 133 and the tenth conducting strip portion 149; (ii) the vertical length Y of each of the sixth conducting strip portion 131 and the sixth conducting strip portion 133; (iii) the horizontal width Z of each of the sixth conducting strip portion 131 and the sixth conducting strip portion 133;

(iv) the horizontal separation T between the sixth conducting strip portions 131 and 133 (giving the capacitive coupling referred to earlier) ; ^' (v) the vertical separation H between the tenth conducting strip portion 147 and each of the fifth conducting strip portion 127 and the seventh conducting strip portion 135; and between the tenth conducting strip portion 149 and each of the fifth conducting strip portion 129 and the seventh conducting strip portion 137;

(vi) the surface area B of each of the sixth conducting strip portion 131 and the sixth conducting strip portion 133 (this affects the capacitance of the coupling between the portions 131 and 133) . In particular, the parameters T and B allow major tuning of the characteristics of the antenna 102 and the parameters X and Z allow gentle fine tuning of the characteristics of the antenna 102 at the appropriate resonance band.

FIGS. 3 to 6 illustrate results obtained for simulated performance (radiative performance in free space) of a specific form of the antenna 102 made as described with reference to FIG. 1 as measured using a commercially available simulation software tool. The specific form of the antenna 102 had dimensions selected to give a resonance band near to 2.4 GHz as referred to earlier for use in WLAN applications. The results illustrated refer to the parameters SIl and VSWR. As is well known in the art, SIl is an input return loss, or reflection coefficient, indicating relative results between an input electromagnetic wave that arrived from an antenna feed port ( A^" ) and an output wave transmitted by the antenna ( A⁺ ) . SIl is given by:

SU=^-

A⁺

Antenna performance is also indicated by VSWR (Voltage Standing Wave Ratio) in the desired band. VSWR is related to SIl by the following equation:

VSWR is a measure of the resonance of the antenna in the desired bands. If VSWR in the desired band is small (less than 2.5) the antenna performance for receiving and transmitting is considered to be good. FIG. 3 shows a plot 200 of magnitude versus frequency for the SIl of the antenna made in accordance with an embodiment of the invention described with reference to FIG. 1. A distinct dip 201 is shown corresponding to a required resonance at about 2.5 GHz provided by the antenna. A resonance which peaks at or close to the required frequency of 2.44 GHz may be obtained by optimisation of the properties of the associated feed line (e.g. coaxial cable) employed to deliver RF signals between the RF transceiver and the antenna .

FIG. 4 shows a Smith chart plot of SIl for the same antenna. A plot 301 is obtained for frequencies in the range 1 GHz to 3 GHz. The regular shape of the plot 301 indicates good impedance matching across the desired band. The plot 301 crosses the horizontal diameter of the chart indicated by reference numeral 302 at a frequency of 2.5 GHz .

FIG. 5 shows a plot 400 of phase angle versus frequency for the SIl of the same antenna. A distinct step change 401 is shown corresponding to the centre of the resonance provided by the antenna at about 2.5 GHz . This shifts to a frequency at or close to the required frequency of 2.44 GHz with use of a suitably optimised feed cable. FIG. 6 is a plot 500 of VSWR versus frequency for the same antenna. A distinct trough 501 in the VSWR is seen reaching a minimum at a point ml corresponding to a peak of the resonance obtained. The minimum shifts to a frequency at or close to the required frequency of 2.44 GHz with use of a suitably optimised feed cable.

In addition, radiation field measurements indicated a substantially omnidirectional radiation pattern from the antenna 102. The directivity was found to be 2.216 dB and the gain was found to be 1.743 dB.

In summary, the antenna 102 of the antenna module 100 produced in the manner described earlier with reference to FIG. 1 has a form which is based on the folded dipole antenna. However, the antenna 102 has a novel form and thereby shows several improvements over the conventional folded dipole antenna as described earlier. The antenna 102 is suitable to give improved performance in selected high frequency bands, e.g. in ^' a band which includes 2.44 GHz and is suitable for WLAN communications. The antenna 102 may be used in several configurations and applications.

Claims

1. A dipole antenna comprising a first section which includes a first input port and a folded configuration and a second section which includes a second input port and a folded configuration and characterised by a reactive coupling between the first section and the second section.

2. An antenna according to claim 1 wherein the reactive coupling comprises a capacitive coupling formed between a conducting strip portion of the first section and an adjacent conducting strip portion of the second section.

3. An antenna according to claim 2 wherein the reactive coupling is formed adjacent to the first and second input ports .

4. An antenna according to claim 3 wherein the reactive coupling comprises a capacitive coupling formed between a conducting strip portion of the first section and an adjacent conducting strip portion of the second section, and wherein the separation between the first input port and the conductive strip portion of the first section and the separation between the first input port and the conductive strip portion of the first section and between the second input port and the conductive strip portion of the second section is not greater than

5. An antenna according to claim 4 wherein each of the first section and the second section includes, extending from its input port, a first conducting strip portion and a second conducting strip portion at an angle to the first conducting strip portion.

6. An antenna according to claim 5 wherein each of the first and second sections includes a further strip portion providing the folded configuration.

7. An antenna according to claim 6 wherein each of the first and second sections includes, connected to the second conducting strip portion, a third conducting strip portion and a fourth conducting strip portion connected to the third conducting strip portion and extending substantially parallel to the second conducting strip portion.

8. An antenna according to claim 7 wherein each of the first and second sections includes a fifth conducting strip portion connected to the fourth conducting strip portion and extending substantially parallel to the first conducting strip portion.

9. An antenna according to claim 8 wherein the reactive coupling comprises a capacitive coupling formed between a conducting strip portion of the first section and an adjacent conducting strip portion of the second section, and wherein the conducting strip portion of the coupling in each of the first section and the second section is a sixth conducting strip portion connected to, and at an angle to, the fifth conducting strip portion of the respective section.

10. An antenna according to claim 9 wherein each of the first and second sections includes a second folded configuration formed inside a first folded configuration.