Miniature antenna

Abstract: A miniature antenna comprises an elongated base portion (10) and an essentially circular top loop (12) that is perpendicular thereto. A U-shaped load (14) is connected between the partially helical (20) base portion (10) and the top loop (12), and its first leg (16) is connected essentially perpendicular to the elongated base portion (10) while its second leg (18) is connected essentially tangential to the top loop (12).

Description: TECHNICAL FIELD The present invention relates to a miniature antenna, preferably for pocket telephones, comprising a helical base portion with a longitudinal axis and an essentially circular top loop that is essentially perpendicular to said axis. BACKGROUND OF THE INVENTION Antennas for pocket telephones usually comprise an antenna rod with a length of a half or a quarter of a wave length. The half wave antenna has a current distribution in the form of half of a sine wave, that is the current is zero at the ends and has a maximum in the middle. This antenna type works well from a technical point of view, but it has the drawback that it is bulky. This is because at the usually used radio frequencies of 900 MHz a wave length of approximately 30 cm is obtained. This means an antenna length of approximately 15 cm, which by many users is considered unpractical and implies a risk that the antenna will be damaged when the pocket telephone is used. A quarter wave antenna is half as long, that is 7-8 cm, and is therefore more practical. However, this antenna type has an unfavourable current distribution in the form of a quarter of a sine wave, the current being zero at the top of the antenna and at a maximum at the antenna base. An antenna in accordance with the preamble of claim 1 is described in US-A-3 573 840. Furthermore, a circularly polarized antenna comprising an elongated base portion and an essentially circular top loop that is perpendicular thereto is known per se from "Antennas", Fig. P7-6, John D. Kraus, McGraw-Hill, second edition, 1988, page 339. An object of the present invention is to provide a linearly polarized miniature antenna that further to being short also provides an essentially rectangular current distribution between the point of connection at the antenna base and the antenna top. SUMMARY OF THE INVENTION In accordance with the present invention the above object is solved by a miniature antenna of the type mentioned in the introductory part having the characterizing features of claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawing, which shows a preferred embodiment of a miniature antenna in accordance with the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The miniature antenna in accordance with the present invention comprises a base portion 10 and an essentially circular top loop 12, that is essentially perpendicular to the base portion 10. Top loop 12 comprises at least one, preferably a bit more than one turn. A U-shaped load 14 is provided between base portion 10 and top loop 12. The first leg 16 of load 14 is connected essentially perpendicular to base portion 10, while its second leg 18 is connected essentially tangential to top loop 12. The U-shaped load 14 increases the current flow to top loop 12. This gives the desired rectangular current distribution. In the preferred embodiment of the invention shown in the drawing the plane in which the U-shaped load 14 lies is perpendicular to top loop 12. However, it is also possible to provide the U-shaped load 14 in a plane that is perpendicular to base portion 10. Base portion 10 has a helically wound mid portion 20 comprising for instance about 10 turns. In the drawing the helix has uniform diameter. In a preferred embodiment the whole antenna is formed by a single thread, which from base portion 10 extends into U-shaped load 14 and thereafter into top loop 12. The miniature antenna is suitably connected to a schematically shown transmitter/receiver 22, either directly or over a matching circuit comprising for instance a series capacitor 24 and a parallel inductor 30. Furthermore, a U-shaped metal band 26, forming an antenna aperture enlarging and chassis isolating metal grounding plane, can be provided between capacitor 24 and transmitter/receiver 22 at the current feeding point of the antenna. Such a band is especially suitable when the apparatus case comprises a metal frame. The purpose of the grounding plane is to decouple the antenna from the metal frame and to increase the antenna aperture. This is especially important for short antennas. By letting a resonant band shaped metal grounding plane with a total electrical length of a half wave length follow the frame structure at a distance of one or a few millimeters and by letting this band have the same or a few millimeters larger width than the frame and by connecting the band to the frame only at the antenna connection point, it is possible to avoid current induction in the frame. In order to shorten the band it can be folded to form one or several pockets as in shown in the left part of the drawing. Such a pocket has the function of a shortening series inductor. Since the band does not lie directly on the frame, except at the current feeding point, similar pockets are formed also between the frame and the ends of the band. These pockets should have an electrical length that corresponds to a quarter of a wave length, referred to the mid frequency of the working range of the antenna, i. e. they should have a high input impedance to prevent current from being transferred to the frame. When the band is shortened this becomes possible only by electrically extending the pocket by filling it with a dielectric that compensates for the shortening. If the antenna is arranged non-symmetrically as in the drawing the antenna radiation is reduced in the direction towards the closest corner of the apparatus frame. In this case the band can be extended more along the closest frame side to compensate for the radiation reduction. In a preferred embodiment of the miniature antenna in accordance with the present invention the antenna is embedded in a dielectric material, preferably with a dielectric constant of approximately 3, for instance silicone. The dielectric material can for instance be in the shape of a frustum of a cone 28, the thicker part of which surrounds elongated base portion 10 and the thinner part of which surrounds U-shaped load 14 and top loop 12. By this embedding in a dielectric material the antenna can be further shortened and further more be better protected against punches. At a carrier frequency of approximately 895 MHz the antenna, when combined with an ordinary sized pocket phone, preferably has the following data: Antenna length 31.5 mm Thread length (including inductor 30) 130 mm Thread diameter 0.75 mm Number of turns in base portion 11 Outer diameter 3.5 mm Length of wound portion 15.5 mm Number of turns in top loop 1.5 Outer diameter 8.5 mm Length of leg of U-shaped load 7 mm Height of load + top loop 3 mm Length between load and wound portion 3 mm Length between wound portion and pocket phone 10 mm Thread material silver plated copper Dielectric material in cone Sylgard 170 from DOW CORNING CORP, USA Cone diameter at top 12 mm Cone diameter at base 13 mm Capacitor 47 pf, ceramic Inductor 9 turns, outer diameter 2,5 mm, thread diameter 0,75 mm It will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departure from the spirit and scope thereof, which is defined by the appended claims. For instance the dimensions of the antenna can be changed to make it suitable for other frequencies, for instance frequencies around 450 MHz or 1700 MHz.

Claims: A miniature antenna comprising a helical base portion (20) with a longitudinal axis and an essentially circular top loop (12) connected essentially perpendicular to said axis, characterized by a U-shaped load (14) provided between said base portion (10) and said top loop (12), the first leg (16) of said U-shaped load being connected to an end of said helical base portion (20) and extending essentially perpendicular to said axis of said base portion and the second leg (18) of said U-shaped load being connected to an end of said top loop and extending essentially tangential to said top loop. The antenna of claim 1, characerized in that said top loop (12) comprises at least a full turn, preferrably 1.5 turns. The antenna of claim 1 or 2, characterized in that said helix (20) comprises approximately 10 turns. The antenna of claim 1, characterized in that the antenna is embedded in a dielectric material. The antenna of claim 4, characterized in that the dielectric material has a dielectric constant of approximately 3. The antenna of claim 5, characterized in that the dielectric material has the form of a frustum of cone (28), the thicker part of which surrounds said base portion (10) and the thinner part of which surrounds said top loop (12). The antenna of claim 6, characterized in that the antenna has a length of the order of 1/10 wave length or less. The antenna of claim 7, characterized in that the antenna has a length of approximately 30 mm, preferrably 31.5 mm for the frequency range 825-895 MHz. The antenna of claim 1, characterized in that said base portion (10) is connected to, either directly or over a matching circuit (24, 30), an antenna aperture enlarging and chassis isolating metal grounding plane (26) at the current feeding point of the antenna.

Inventors: CASSEL ERLAND, CASSEL JAN, CASSEL,ERLAND, CASSEL,JAN
Applicants: ERICSSON GE MOBILE INC, ERICSSON INC.
Application year: 1992
Main classification: H01Q1
IPC codes: H01Q1, H01Q7, H01Q9