WO2012177899A2

WO2012177899A2 - Antenna system

Info

Publication number: WO2012177899A2
Application number: PCT/US2012/043573
Authority: WO
Inventors: Ole Jagielski; Simon Svendsen; Finn HAUSAGER
Original assignee: Molex Incorporated
Priority date: 2011-06-21
Filing date: 2012-06-21
Publication date: 2012-12-27
Also published as: WO2012177899A3

Abstract

An antenna system is disclosed that includes a PCB that supports a coupler. An isolated resonating element is inductively coupled to the coupler. The resonating element includes curl portions that lower the frequency response of the resonating element. The resonating element can be positioned along a single edge of a PCB if desired. The resonating element can be configured to vary its frequency response based on tuner, which can change the inductance or capacitance of the curl portions.

Description

Antenna System

RELATED APPLICATIONS

[0001] This application claims priority to United States Provisional Application No. 61 ,499,312, filed June 21 , 201 1 , which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to the field of antennas, more specifically to compact antennas suitable for use in wireless devices.

DESCRI TION OF RELATED ART e

[0003] Antenna feeding, based on inductive coupling, is a known feeding technique for slot antennas, microstripe patch antennas and even standard dipole antennas. However many of these applications are not limited in terms of available volume, influence of users, coexistence with close by antennas and/or electrical size of the device. Also, these known concepts are normally for applications were the electrically size of the device is close to or greater than the half wavelength of the desired resonance frequency. Such antenna system are less suitable for mobile devices, such as cellular phones, that are often unable to accept such a large antenna.

[0004] Compact isolated antennas have been used in mobile devices. The physical nature of a compact isolated antenna is normally limited to narrow impedance bandwidth antennas (High Q) and single resonance antennas, which makes this concept suitable for many non cellular systems, like ISM 868 MHz, GPS, Wifi/Bluetooth, etc. Applications, such as GSM cellular system have proven less compatible with such narrow-band antenna systems. Existing designs would also benefit from being able to decrease the size, particularly for lower frequencies where the length of a resonating element must be fairly large relative to a compact mobile device. Certain individuals would therefore appreciate an improved compact isolated antenna design.

BRIEF SUMMARY

[0005] An antenna system is disclosed that includes a PCB that supports a coupler. An isolated resonating element is inductively coupled to the coupler. The resonating element includes curl portions that lower the frequency response of the resonating element. The resonating element can be positioned along a single edge of a PCB if desired. It has been determined that such a configuration can provide good performance in a much smaller package than was previously available. If desired, the antenna can coupled to the coupler via a loop coupled extension line so as to allow the antenna to be positioned further away from the PCB. T e resonating element can be configured to vary its frequency response based on tuner positioned adjacent the curl portions as the tuner can change the inductance or capacitance of the curl portions. It has been determined that by combining the compact isolated antenna with a tuner it is possible to use the modified compact isolated antenna in broad band applications like cellular systems.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The present invention is illustrated by way of example and not limited in the accompanying figures in which like reference numerals indicate similar elements and in which:

[0007] Fig. 1 illustrates a perspective view of an embodiment of an isolated antenna system.

[0008] Fig. 2 illustrates a perspective view of a coupler inductively coupled to a resonating element.

[0009] Fig. 3 illustrates a perspective view of another embodiment of an isolated antenna system. [0010] Fig. 4A illustrates a perspective view of another embodiment of an isolated antenna system.

[0011] Fig. 4B illustrates an impedance plot of the embodiment depicted in Fig. 4A.

[0012] Fig. 5A illustrates a perspective view of another embodiment of an isolated antenna system.

[0013] Fig. 6A illustrates a perspective view of an embodiment of an isolated antenna system.

[0014] Fig. 6B illustrates an impedance plot of the embodiment depicted in Fig. 6A.

[0015] Fig. 7 A illustrates a perspective view of the embodiment depicted in Fig. 6A but with less inductive coupling.

[0016] Fig. 7B illustrates an impedance plot of the embodiment depicted in Fig. 7A.

[0017] Fig. 8A illustrates a perspective view of the embodiment depicted in Fig. 7A but with less inductive coupling.

[0018] Fig. 8B illustrates an impedance plot of the embodiment depicted in Fig. 8A.

[0019] Fig. 9illustrates a perspective view of another embodiment of an isolated antenna system.

[0020] Fig. 10A illustrates a perspective view of an embodiment of a compact isolated antenna system.

[0021] Fig. 10B illustrates an impedance plot of the embodiment depicted in Fig. 10A.

[0022] Fig. 1 1 A illustrates a perspective view of another embodiment of a compact isolated antenna system.

[0023] Fig. 1 1 B illustrates an impedance plot of the embodiment depicted in Fig. 1 1 A. [0024] Fig. 12 illustrates results of tests conducted on sample antennas.

[0025] Fig. 13A illustrates a perspective view of another embodiment of a compact isolated antenna system with a loop coupled extension line.

(0026] Fig. 13B illustrates an enlarged view of the loop coupled extension line depicted in Fig. 13 A.

10027] Fig. 14 illustrates impedance plots of an antenna system with and without a loop coupled extension line.

(0028] Fig. 15 illustrates results of tests conducted on sample antennas with and without a loop coupled extension line.

[0029] Fig. 16 illustrates a perspective view of an embodiment of a compact isolated antenna system that is tunable.

[0030] Fig. 1 7 illustrates a perspective view of a simplified embodiment of a tunable curl portion.

[0031 ] Fig. 18 illustrates a frequency response of the embodiment depicted in Fig. 1 7 at different tuning values.

[0032] Fig. 1 9 illustrates a perspective view of another embodiment of a compact isolated antenna system that is tunable. |

[0033] Fig. 20 illustrates a perspective view of another embodiment of a compact isolated antenna system that is tunable.

DETAI -,ED DESCRIPTION

[0034] The detailed description that follows describes exemplary embodiments and is not intended to be limited to the expressly disclosed combination(s). Therefore, unless otherwise noted, features disclosed herein may be combined together to form additional combinations that were not otherwise shown for purposes of brevity.

[00351 Exemplary details of a compact isolated antenna system are explained by the example shown in Fig. 1 , which consists of a resonating element 40, which can be a standard half wave dipole antenna, and a coupler 60, which as depicted is a small inductive coupler integrated on a printed circuit board (PCB) 20. As can be appreciated, the coupler 60 is placed at the center and very close to the resonating element 40, in order to maximize the inductive coupling. As can be appreciated from Fig. 2, some of the magnetic fields generated by the resonating element 40 pass through the coupler 60, whereby a current is generated on the conductive loop of the coupler 60, inducing a voltage across feed 80.

[0036] The result is an inductive feed technique on an isolated half wavelength antenna. However, a certain flux needs to pass through the coupler in order to achieve a useful result. It has been determined that the flux is mainly determined by four factors:

1 ) The size of the coupler (a bigger coupler will catch more flux);

2) The distance between coupler and dipole (a shorter distance will generate higher flux in the coupler);

3) The position of the coupler relative to the center of the dipole (the highest currents are at the center of the dipole, where also the highest flux is generated in the coupler); and

4) The Q of the dipole (a higher Q will generate higher currents on the dipole and thereby higher flux in the coupler).

[0037] Similar inductive feeding techniques are used for certain slot antenna designs and patch antenna designs, where the electrically size of the conductive parts is equal to or larger than half the wavelength of the desired resonance frequency. The concepts described are targeting compact devices with electrically sizes smaller than half wavelength of the desired resonance frequency or devices were the distance between the coupler and the antenna element is too large for achieving a sufficient flux in the coupler. (0038] The concept shown in Fig. 1 requires a relatively large coupler in order to obtain a sufficient flux, which is not desirable for compact devices. The coupling can be increased by having the resonating element follow the shape of the coupler, as shown in Fig. 3. A resonating element 40' includes a coupling loop 45' that is configured to inductively couple to a coupler 60'. As noted above, this allows the system to induce a voltage across feed 80, as previously noted. As the inducing of a voltage across the feed is a standard part of the antenna system, further discussion of the feed will be omitted for the sake of brevity.

(0039] It is desired to further increase the coupling, which can be done by increasing the Q of the antenna element while keeping the size of the coupler unchanged. The Q of the antenna element can be increased by having part of the resonating element close together, by doing this symmetrically around the center of the resonating element 140, one can ensure that the currents of these parts will have the same amplitude and be 180° out of phase, whereby the radiation is canceled, while maintaining the electric length of the resonating element 140. This technique is shown in Fig. 4. As depicted, the resonating element 140 is connected to a coupler loop 145 via transmission section 150, which has two symmetric sides of the resonating element 140 positioned closely together. A coupler 160 is supported by the PCB 120 and functions as previously discussed.

|0040] A plot 90a of the resonance obtained from this is depicted in Fig. 4B and is not self-matched but can be moved into the center of the smith chart by external components or by further increasing the Q of the antenna element, as shown in Fig. 5A. As depicted, a coupling loop 145' is provided on resonating element 140' and longer transmission section 150' is used. This allows for coupling to coupler 160' that is supported by PCB 120'. This provides a plot 90b, as depicted in Fig. 5B.

(0041] For this case the Q of the antenna element is now generating too high flux in the coupler, thereby not being self-matched. The flux can be reduced by decreasing the Q of the antenna. Another way to reduce the flux is to offset the coupling loop of the resonating element and the coupling loop supported by the PCB, as is shown in Fig. 6A, which as can be appreciate, depicts a coupling loop 245 (which is part of a resonating element) only partially overlapping with a coupler 260 supported by a PCB 220 and this provides an impedance plot 90c that is depicted in Fig. 6B. Figs. 7A and 8A illustrate further separation between the coupler 260 and the coupling loop 245, while Fig. 7B illustrates an impedance plot 90d associated with Fig. 7 A and Fig. 8B illustrates an impedance plot 90e associated with Fig. 8A.

[0042) The previously configurations are not suitable solutions for compact devices, since the resonating element is occupying a large volume compared to the PCB. The Q of the antenna element, as depicted in Fig. 9, is increased by folding a resonating element 340 along two edges C, D of PCB 320. The half wavelength at 850 MHz is around 175 mm, which will require a relative large PCB, even when using the above folded configuration. In practice, however, the expected size of a reference PCB that could be used in commercial applications could be about 15 mm x 40 mm and as can be appreciated, such dimensions would provide a resonating element that would be far from sufficient to achieve resonance at 850 MHz when using the configuration shown in Fig. 9.

[0043] In traditional designs where the antenna element is connected to the PCB, the resonance of the resonating element and PCB could be forced down to 850 MHz by adding an inductor between the resonating element and the PCB. However, in this case the use of an inductor is not an option because the resonating element is an isolated antenna.

[0044) To address this issue, an embodiment of antenna system is depicted in Fig. 10A, where the inductance of the resonating element 440 is increased by providing a curl portion 446. By curling up the high current center part of a resonating element 440 (like a wire wound inductor) it is possible to provide an isolated compact inductive loaded antenna (ICILA). The size of a PCB 420 can be 15 mm x40 mm and includes a coupler 460 that is configured to couple to coupling element 445. As can be appreciated from Fig. 10B, the resonance frequency is at 877 MHz, as shown by impedance plot 90f.

[0045] Such an embodiment is a beneficial solution for electrically very small PCB, but will also be efficient on electrically larger PCB. The disadvantage of embodiment of ICILA depicted in Fig. I OA is that it occupies three sides C, D, E of PCB 420, which is not desirable for many compact stack-ups. The 1C1LA version depicted in Fig. 10A can be rotated 90 degrees, as shown in Fig. 1 1 A, and is thereby only occupying one side of the PCB.

[0046] As can be appreciated, the embodiment depicted in Fig. 1 1 A includes a PCB 420' that supports a coupler 460' positioned along an edge C of the PCB 420'. Resonating element 440' includes a U-shaped element 445' that allows for symmetrically separation of two curl portions 446' that are positioned on sides A and B of PCB 420'. As the PCB 420' includes a cutout 422', the resonating element 420' can be compactly position so that it extends along a single edge but is positioned on both sides A, B of the PCB 420'. As can be appreciated, such a construction tends to minimize the space from a vertical standpoint as the PCB is positioned at a mid-point (vertically speaking) of the resonating element 420' but it should be noted that if desired the coupler and the resonating element could be shifted upward or downward so as to position the resonating element in a more off-set manner.

[0047] Consequentially, the 1CILA embodiment depicted in Fig. 1 1 A only occupies one edge of the PCB and can be used on any size of PCB. The use of any PCB is possible because the design provides a self-resonating antenna element, thus the impedance and Q of the antenna will be more or less independent on the size of the PCB. Thus, there is less dependence on the size of the PCB as compared to a case for a direct fed antenna where the PCB is part of the radiation structure. A coupler 460' is placed parallel to the resonating element 440' in order to maximize the flux through the coupler. An impedance plot 909 of the embodiment depicted in Fig. 1 1 A is shown in Fig. 1 1 A.

[0048] Both embodiment depicted in Figs. 10A and 1 1 A were built and tested and test results of both versions are provided in Fig. 12. As can be appreciated, the efficiency results are significant better than what can be obtained with a direct fed antenna concept. It is believed that this is mainly due to contact resistances and soldering of the direct fed antenna. When working with extremely high antenna Q factors such as 180 to 290, any small resistance (e.g., a spring contact) or low conductivity regions (e.g., a solder joint) will generate high loss due to the very high currents created by the high Q antenna elements. The loss observed in Fig. 12 is believed to mainly be caused by the conductivity of copper, which is one of the best conductors available, and thus is kept relatively low.

[0049] The configurations described in the Figs. 10A- 12 are embodiment suitable to provide compact antenna element designs that are positioned close to electrically small PCBs, thus the designs are suitable for physically small devices. However, some application might require that the antenna element is placed offset from the PCB, having no option to extend the PCB to the resonating element. The embodiments depicted in Figs. 10A-12 will be less useful in such a configuration because the coupling will be very small. A configuration that can help overcome the issues of small coupling is depicted below in Fig. 13 A, where a loop coupled extension line (LCEL) 570 is placed between the coupler on the PCB and the coupler on the antenna element. The basic idea of the LCEL is that it includes two coupling element 571 , 572 (both of which can be loops as depicted) with coupling element 571 picking up the flux at the coupler on the PCB and then transfers flux to coupling loop 542 via coupling element 572, using a transmission section 573 that has minimal radiating. A close up view of the configuration depicted in Fig. 13A is provided in Fig. 13B.

|0050] When a signal is excited at the feed the coupler 560 of PCB 520, the induced magnetic field will generate a current running in the first coupling element 571 close to the PCB. Assuming that the conductivity of the LCEL is high, the same current level will tend to be present at the second coupling element 572. The area between the two conductors in the transmission section should be minimized in order to reduce the imposed flux, and maximize the flux in the second coupling element 572.

(0051] As can be appreciated, an embodiment using the LCEL consists of two coupling areas each them having a similar area as the single coupling area illustrated in other embodiments. This doubling of the coupling area means that it will be necessary that twice the flux is picked up at the coupling at the PCB, in order to get the same flux delivered to the antenna as before, assuming that little or no flux was lost in the transmission line. A twisted pair, as illustrated in Fig. 13B, or a coax cable both represent transmission line concepts with very little flux loss. Of course, as noted above, a full overlap may provide too much flux pick up and therefore the use of a LCEL concept can beneficially reduce the amount of flux pick up while providing other benefits such as the ability to position the resonating element further away from the PCB.

10052 J When considering the matched part of the antenna impedance the length of the LCEL is, in theory, insignificant if the LCEL is fully balanced. This is difficult to achieve in practical implementations and small dependencies are observed. It should be noted that the path of the transmission line between the two loops could be of any desirable form. An impedance plot 90j of the antenna system using the LCEL is shown in Fig. 14.

[0053 J As can be appreciated, a phase shift is caused by the phase delay in the LCEL line, and a resonance curl of the impedance plot is reduced is due to the reduced flux transferred to the antenna. An ICILA with and without a LCEL were test and the test results, which were obtained using a bazooka-balun for both impedance and efficiency measurements, are provided in Fig. 15.

[0054] As can be appreciated, the embodiments described in Figs. 1 OA- 15 are all narrow impedance bandwidth antenna concepts. Thus these embodiments are poorly suited for use in wide band cellular system like GSM, WCDMA & LTE. It has been determined, however, that it is possible to cover wide-band systems by combining such an antenna system with an inductive coupled tuner. An embodiment of such a system is shown in Fig. 16.

[0055] As can be appreciated, the configuration includes the resonating element 440', as shown in Fig. 1 1 A, along an edge C of PCB 420', along with coupler 460. Two curl portions 446' are provided in notch 422'. However, to provide for tuning, an inductive coupled tuner 485 is integrated with the PCB 420'.

[0056] As can be appreciated from a simplified model depicted in Fig. 17, a tuner 585 acts as a secondary coil inserted into the curl portion 546 of a resonating element. The insertion of a secondary coil will change the self-resonance frequency of the curl portion 546 into the resonating element, thereby changing the inductance of the curl portion into the resonating element, leading to a change of the resonance frequency of the resonating element. Adding a tuning component 586, such as a tunable capacitor, in series with the secondary coil will make it possible to change the inductance of the secondary coil and thereby also change the resonance frequency of the resonating element.

[0057] The simulated tuning range for this simplified example in shown in Figure 18. As can be appreciated, the self-resonating frequency of the curl portion 546 is lowered when the capacitance of the tuning capacitor is increased as this causes the inductance for a given frequency to be is increased. Increasing the inductance of the curl portion on an ICILA is equivalent to decreasing the resonance frequency. Consequentially, it is possible to provide frequency tuning by adjusting the inductance of the curl portion 546.

[0058] It has been determined that the inductance of the secondary tuning coil can be increased by adding more turns to the tuner 585, as depicted in Fig. 19. As depicted, a tuner 685, which is supported by PCB 620 and tuning element 686, has two turns between curl portions 646 of resonating element 640.

[0059] As can be appreciated, the tuning examples described in Figs. 16- 19 are based on inductive coupling between the resonating element and the tuner. An embodiment depicted in Fig. 20 includes a resonating element 740 that is coupled to a coupler 760 supported by a PCB 720. As above, curl portions are positioned on sides A and B of the PCB 720. Somewhat differently, however, a tuner 785 is capacitively coupled to the curl portion 746. Thus, the depicted embodiment uses a tuner 785 that is capacitively coupled to the resonating element 740 to change the self-resonance of the curl portion 746 integrated into the resonating element 740.

[0060] It should be noted that inductive tuning and capacitive tuning, in theory, provide similar results. The difference is the inductive tuning changes the inductance of the resonating element, while the capacitive tuning changes the capacitance of the resonating element. Increasing either the inductance or capacitance of the resonating element will in both cases reduce the resonance frequency of the resonating element. [0061 ) The disclosure provided herein describes features in terms of preferred and exemplary embodiments thereof. Numerous other embodiments, modifications and variations within the scope and spirit of the appended claims wii! occur to persons of ordinary skill in the art from a review of this disclosure.

Claims

We claim:

1 . An antenna system, comprising:

a printed circuit board (PCB);

a coupler supported by the PCB;

a resonating element electrically isolated from the coupler, the resonating element configured to inductively couple to the coupler; and

a first and second curl portion integrated into the resonating element, the curl portions configured to inductively load the resonating element.

2. The antenna system of claim 1 , wherein the PCB includes an edge and resonating element extends along the edge.

3. The antenna system of claim 2, wherein the PCB includes a first side and a second side and the first and second curl portion are respectively positioned on the first and second side.

4. The antenna system of claim 3, wherein the PCB include a notch and the first and second curl portion are aligned with the notch.

5. The antenna system of claim 1 , wherein the resonating element is offset from the PCB, the system further comprising a loop coupled extension line configured to inductively couple to the coupler and to inductively couple to the resonating element.

6. The antenna system of claim 5, wherein the loop coupled extension line includes a first loop configured to couple to the coupler, a second loop configured to couple to the resonating element, and a transmission line extending between the first and second loop.

7. The antenna system of claim 6, wherein the transmission line is a twisted pair.

8. The antenna system of claim 1 , further comprising a tuner positioned between the first and second curl portions, the tuner configured to adjust the inductance of the curl portions.

9. The antenna system of claim 1 , further comprising a tuner positioned adjacent the first and second curl portions, the tuner configured to adjust the capacitance of the curl portions.