Microstrip antenna having unipole antenna

Abstract: An antenna arrangement includes a microstrip antenna which comprises a ground conductor plane and a radiating conductor plane arranged on opposite sides respectively of a dielectric substrate, and a connecting conductor plane connecting together the radiating conductor plane and the groun conductor plane, and a unipole antenna coupled to the radiating conductor plane at one end thereof.

Description: BACKGROUND OF THE INVENTION This invention relates to an improvement for a microstrip antenna. Conventionally, microstrip antennas of a small and thin structure have been used inside of an automobile. Such a microstrip antenna is generally placed on the rear side of the back seat in view of availability in space and simplicity in mounting. Accordingly, to receive radio waves through the rear window, it is desirable to use a unidirectional antenna having a strong directivity in the direction of the rear window rather than microstrip antennas having the directivity in the direction of ceiling or generally in the horizontal direction. In the specification of a French patent which was published under No. 2,507,825 on December 17 1982, there are described various embodiments of high frequency directional antennae which provide particular field configurations. In one embodiment there are two flat conductors providing large faces and a third conductor which connects the two flat conductors together; the space between the conductors being filled with a dielectric material. In an embodiment to be described there is an antenna including a microstrip antenna comprising a ground conductor plane and a radiating conductor plane arranged on both sides of a dielectric substrate to oppose each other and a connecting plane conductor which connects the ground conductor plane and the radiating conductor plane, and a unipole antenna coupled to the radiating conductor plane on one end thereof. The invention is claimed in claim 1. Embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, wherein: Fig. 1 is a vertical cross section of an automobile having an indoor antenna installed; Fig. 2 is a perspective view of a microstrip antenna according to this invention; Figs. 3A and 3B are a vertical cross section and an equivalent circuit diagram to explain the antenna shown in Fig. 2; Fig. 4 is a view to explain the radiation field of the antenna shown in Fig. 2; Figs. 5 through 7 are perspective views of another embodiment of a microstrip antenna according to the present invention; and Figs. 8A and 8B show computed radiation patterns of the antenna shown in Fig. 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, a microstrip antenna 1 of this invention may be placed on a rear board 51 inside an automobile 50. Radio waves arrive at places like this more from the direction 3 of the rear window than from the direction 2 of the front window. An antenna of a unidirectivity is more desirable for such a place, but there has not yet been put into practical use an indoor microstrip antenna having such advantageous characteristics. Fig. 2 is a schematic view of an embodiment of the antenna according to this invention. This antenna (which is hereinafter referred to as a U-MS antenna) includes a unipole antenna 6 and a microstrip antenna (hereinafter referred to as an MS antenna) comprising a ground conductor plane 4 which extends in the yz plane, a radiating conductor plane 5, a connecting conductor plane 7 connecting the conductor planes 4 and 5, and a dielectric element 9 placed between the conductors 4 and 5. The length Ls (in the direction z) of the MS antenna (4,5,7,9) is selected to be about λ/4 ( λ = λ o /√ ε r , where λ represents a wavelength used; λ o , a free space wavelength; and ε r, the relative dielectric constant of the substrate 9). The width W (in the direction y) and the thickness t (in the direction x) of the MS antenna are determined depending on the relative bandwidth. The unipole antenna 6 is placed on the radiating conductor plane 5 at a position which is spaced by W/2 from both ends of the radiating conductor plane 5 (in the direction y), i.e. at the symmetry axis, and spaced from the connecting plane conductor 7 by d (in the direction z). A coaxial cable 8 for feeding power is connected at a feeding location S (in the direction z) in a manner to connect the outer conductor thereof to the ground plane conductor 4 and the central conductor to the radiating plane conductor 5, respectively. The location S is selected so that the cable 8 cause no impedance mismatching. The operation of the U-MS antenna of this invention may be explained by separating it into a unipole antenna 6 and an MS antenna (4,5,7,9). More particularly, it is assumed in Fig. 3A that the letters Vf, If denote respectively the voltage and the current at the feeding point 8; Vu and Iu, the voltage and the current of the unipole antenna 6; and Vs and Is, the voltage and the current of the MS antenna (4,5,7,9), and that the electric field inside the MS antenna (4,5,7,9) distributes in sinewave in length (in the direction z) and uniformly in width (in the direction y). On that assumption, the equivalent circuit of this antenna can be expressed by Fig. 3B using an ideal transformer 10 of the turn ratio of sin(ks) : 1 and an ideal transformer 11 of the turn ratio of sin(ks) : sin(kd). In Fig. 3B, the letter Zs denotes the impedance of the MS antenna (4,5,7,9); Zu, the impedance of the unipole antenna 6; and k , the propagation constant inside the MS antenna (4,5,7,9). The constant k is expressed as k = 2 π √ ε r /λ o . Although there exists mutual coupling between the unipole antenna 6 and the MS antenna (4,5,7,9), the mutual coupling is disregarded in description herein for the sake of simplicity. As illustrated in Fig. 3B, the unipole antenna 6 and the MS antenna (4,5,7,9) are separately and respectively fed power and the unipole current Iu can be obtained from Vu / Zu. The radiation fields of the unipole antenna 6 and the MS antenna (4,5,7,9) can be obtained from Iu and Vs, and the radiation field of the present U-MS antenna can be obtained by summing these radiation fields. If we assume that power is fed at the phase of Fig. 3A and consider the directivity of the U-MS antenna qualitatively, we will find that the radiation fields of the unipole antenna 6 and the MS antenna (4,5,7,9) are generated at the phases 12 and 13 in Fig. 4. Therefore, the two radiation fields offset each other in the negative direction on the axis Z, while in the positive direction they intensify each other. The directivity of the U-MS antenna becomes unidirectional and the maximum radiation lies in the positive z direction. In order to effect excellent unidirectivity in the U-MS antenna, it is necessary to effectively make radiation fields of the two antennas offset in the negative z and yet to make them intensified in the positive z. To achieve such purposes, the unipole antenna 6 is positioned mainly at the tip end of the radiating conductor plane 5 (d ≈ Ls) and the length thereof is determined to be around λ o /4 so that the reactance of the unipole antenna 6 becomes substantially zero. Further, the size of the MS antenna (4,5,7,9) is determined so as to make the radiated powers from the MS antenna (4,5,7,9) and the unipole antenna 6 substantially equal. If the necessary bandwidth of the MS antenna (4,5,7,9) is narrow, the MS antenna can be reduced in size by reducing the width W and the thickness t. Since the impedance Zs of such compact MS antenna (4,5,7,9) becomes considerably larger than the impedance Zu of the unipole antenna 6, a desirable unidirectivity characteristic cannot be obtained in the U-MS antenna which uses a linear unipole antenna like the one shown in Fig. 2. In such a case, the unipole should be folded as shown in the embodiment shown in Figs. 5 and 6, so that the impedances Zu of the unipole antenna becomes large enough to provide an excellent unidirectivity. The unipole antenna of the U-MS antenna of this invention may be constructed to have a bent tip end and a low height. Fig. 7 shows an embodiment of the U-MS antenna using a bent type unipole antenna. Figs. 8A and 8B are examples of the gain in directivity of a U-MS antenna using a unipole antenna of about λ o / 4 when the ground plane conductor is infinity. Fig. 8 illustrates the result of calculation made taking into account the coupling between the unipole antenna and the MS antenna, where ε r = 1 , t = λ o /30 , W = λ o /2 , and D = Ls ≈ λ o /4 . As is shown in Fig. 8A, the directivity is oriented to the direction ϑ = 0° (z axis direction) on the E plane (X-Z plane), and an excellent unidirectivity is obtained. As described in the foregoing, the U-MS antenna can perform as an antenna having a unidirectivity simple by selecting an appropriate size. When the necessary bandwidth is narrow, the width and the thickness of the MS antenna can be reduced. The unipole antenna may have the height of less than λ o /4 by bending the tip end and making the structure in inverted L-shape. The U-MS antenna according to this invention can therefore be made compact enough to be conveniently used indoors.

Claims: A unidirectional antenna arrangement including a ground conductor plane (4), and a radiating conductor plane (5), each plane (4) (5) being arranged on a side of the dielectric substrate (g) which is opposite to the side on which the other is arranged, and a connecting conductor plane (7) which is connected at one edge to an edge of the radiating conductor plane (5) and at the opposite edge to the ground conductor plane (4) in order to connect together the radiating conductor plane (5) and the ground conductor plane (4), characterised in that there is further provided a unipole antenna (6) which is placed on the radiating conductor plane (5) at or adjacent to a point on the edge of the radiating conductor plane (5) that is opposite to the edge which is connected to the connecting conductor plane (7). An antenna arrangement as claimed in claim 1 characterised in that the unipole antenna (6) is placed substantially on an axis of symmetry of the radiating conductor plane (5). An antenna arrangement as claimed in claim 1, characterised in that there is further provided a power feeding means (8) whose outer and centre conductors are connected to the ground and radiating conductor planes (4,5) respectively. An antenna arrangement as claimed in claim 3, characterised in that the length of the unipole antenna (6) is one quarter of the wavelength of the frequency used by the antenna. An antenna arrangement as claimed in claim 3 characterised in that the unipole antenna (6) includes a bent unipole (17). An antenna arrangement as claimed in claim 5 characterised in that the bent unipole is substantially shaped in the form of a letter L at the tip end thereof.

Inventors: YOKOYAMA YUKIO C O NEC CORPORA, YOKOYAMA, YUKIO C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1985
Main classification: H01Q1
IPC codes: H01Q13, H01Q21, H01Q9, H01Q1