Loop antenna

Abstract: An improved loop antenna (10) comprises first (12) and second (16) conductive loops. Each of the first and second conductive loops has a discontinuity (14, 18). The first and second conductive loops (12, 16) are arranged substantially concentrically to form a capacitive coupling such that the tuned frequency of the loop antenna (10) has improved protection from external capacitance effects.

Description: Field of the Invention This invention relates to loop antennae, and particularly but not exclusively to H-field loop antennae having a tuned frequency. Background of the Invention Short range radio data transmission systems often use E-H field type tuned loop antennae. Such loop antennae are usually very small in size and have to fit into very small spaces (e.g. remote car alarm keys). A normal H-field loop antenna consists of a metallic loop conductor and a tuning capacitor at its ends. The form of the loop itself is of secondary importance and known loops have different physical shapes such as rectangular or diamond etc. The tuning capacitance is typically placed in the centre of the loop but can equally be displaced from it. The use of a tuning capacitor with an H-field loop antenna has a series of significant drawbacks. For example, in an environment where hand held equipment is used, such as a remote car alarm key, human body capacitance can significantly de-tune the resonant frequency of the loop antenna at the capacitance intersection. US Patent US-A-2 479 337 (Fyler) describes a high frequency antenna system having two loops, one of which surrounds a substantially closed loop structure. The aim of the system is to provide a high degree of vertical directivity so that the horizontal field strength pattern is as circular as possible. UK Patent GB-A-1105 354 (Northrop) describes an antenna capable of receiving and transmitting signals whilst maintaining a substantially constant input impedance. US Patent US-A-5225 847 (Roberts et al) describes a tuning system for an antenna which modifies characteristics of the antenna in a stepwise using fashion until optimum conditions are reached. Furthermore the requirement for a discrete capacitor component adds to the cost of the loop antenna and the complexity of the design. This invention seeks to provide a loop antenna which mitigates the above mentioned disadvantages. Summary of the Invention According to the present invention there is provided a loop antenna and a loop antenna assembly as described in the accompanying claims. In this way a loop antenna is provided, which is less susceptible to capacitive effects of external structures, such as the human body. Brief Description of the Drawings An exemplary embodiment of the invention will now be described with reference to the drawing in which: FIG.1 shows a prior art remote car alarm key, incorporating a loop antenna. FIG.2 shows a prior art loop antenna. FIG.3 shows preferred embodiment of an improved loop antenna in accordance with the invention. Detailed Description of a Preferred Embodiment Referring to FIG.1, there is shown a remote car alarm key 5, comprising a loop antenna 7 and an integrated circuit 6. Referring now also to FIG.2, there is shown a prior art loop antenna 7, as used in the remote car alarm key 5. The loop antenna 7 includes a metallic loop conductor 8 having a capacitor 9 coupled between two ends of the loop conductor 8, and a coupling antenna 20, which is formed by a further loop. In operation, the prior art loop antenna 7 operates at a tuned frequency given by the characteristics of the capacitor 9 and the loop conductor 8. The coupling antenna 20 provides coupling to external circuitry, such as the integrated circuit 6 of the remote car alarm key 5. Referring now to FIG.3, there is shown an improved loop antenna 10 comprising first 12 and second 16 conductive loops, arranged concentrically. The first loop 12 has a discontinuity 14, and the second loop has a discontinuity 18, such that the ends of the first and second loops 12, 16 are open. In the preferred embodiment, the first and second loops are substantially circular, although it will be appreciated that other shapes and configurations are possible. For example, rectangular or diamond shaped loops could be used. The first and second loops 12 and 16 are arranged such that the discontinuities 14 and 18 are on opposite sides of the loops, i.e. diametrically opposed. The size and relative position of the discontinuities 14 and 18, and the size and dielectric spacing of the first and second loops 12 and 16 determine a particular tuned frequency, which is the tuned frequency of the improved loop antenna 10. In this way the tuning capacitance is distributed over the whole surface of the first and second loops 12 and 16 and as a result the improved loop antenna 10 is less affected by external capacitive effects, such as the close proximity of a human body. The improved loop antenna is coupled to a coupling antenna 20 in the same manner as the prior art, and in this way coupling is provided to external circuitry, such as the integrated circuit 6 of the remote car alarm key 5. It will be appreciated that alternative embodiments to the one described above are possible. For example, the discontinuities 14 and 18 need not be diametrically opposed, but could be in an alternative configuration.

Claims: A loop antenna having a tuned frequency, the wavelength corresponding to said tuned frequency being large compared to the dimensions of the antenna and the antenna comprising first and second conductive loops (12,16), for coupling with an external coupling loop (20), characterised in that each of the first (12) and second (16) conductive loops presents an electrical discontinuity (14,18), and the first and second conductive loops (12,16) are arranged substantially concentrically to form a capacitive coupling distributed over the surfaces of the first and second loops (12,16), whereby the size and relative position of the discontinuities (14,18), and the size and dielectric spacing of the first and second loops (12,16) define said tuned frequency of the loop antenna. A loop antenna as claimed in claim 1 wherein the first (12) and second (16) conductive loops are substantially circular in shape. A loop antenna as claimed in claim 1 or claim 2 wherein the discontinuities (14,18) of the first and second conductive loops (12,16) are substantially diametrically opposed. An H-field loop antenna assembly comprising a loop antenna as claimed in any preceding claim including said first and second conductive loops (12,16) and said external coupling loop (20).

Inventors: BURRI MICHEL, STAUFER HANS, BURRI, MICHEL, STAUFER, HANS
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1997
Main classification: H01Q7
IPC codes: H01Q7, H04B5, H01Q19