Patch antenna and method for making the same

Abstract: A patch antenna has a driven patch (4) which is fed with a high-frequency signal and radiates a high-frequency electromagnetic field from its one surface; a parasitic patch (2A) receives the high-frequency electromagnetic field from the driven patch at its one surface and reradiates the high-frequency electromagnetic field from its other surface; and a radome (1A) contains the driven patch and the parasitic patch inside of the radome to protect them and holds the parasitic patch. The parasitic patch (2A) is held by the inner surface of the radome (1A) which contacts the other surface of the parasitic patch and a holder (11A) which protrudes from the inner surface of the radome and covers a part of the one surface of the parasitic patch.

Description: This invention relates to a patch antenna, and more particularly to, a patch antenna which has a driven patch with a microstrip structure, a parasitic patch which reradiates an electromagnetic field received from the driven patch and a radome containing these patches to protect them, and a method for making the same. It is known that a patch antenna composed of a driven patch and a parasitic patch which are structured as two electromagnetic wave radiating layers has an excellent broad-band performance. By containing such patch antenna inside a radome, a patch antenna structure which presents advantages in terms of portability as mobile communication means, reliability and aesthetical appearance can be provided. For example, such structures are disclosed in Japanese patent application laid-open No.2-141007(1990) and Japanese patent application laid-open No.3-74908(1991), where the former discloses a microstrip antenna in which a parasitic patch is adhered to inner surface of a radome and the latter discloses a stacked microstrip antenna in which a parasitic patch is buried in the wall of a radome. However, in the conventional patch antennas disclosed therein, there is a problem that the number of fabrication steps must increase due to requiring the adhesion of the parasitic patch to the radome or the burying of the parasitic patch in the wall of the radome. Also, there is a problem that it is difficult to position the parasitic patch at a proper position on the radome or in the wall of the radome. Accordingly, it is an object of the invention as disclosed in claim 1 to provide a patch antenna in which a parasitic patch is precisely positioned and firmly fixed onto the inner surface of a radome. It is a further object of the invention as disclosed in claim 6 to provide a method for making a patch antenna which facilitates the positioning and fixing of a parasitic patch onto the inner surface of a radome. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in more detail in conjunction with the appended drawings, wherein: FIG.1A is a cross sectional view showing a patch antenna in a first preferred embodiment according to the invention, FIG.1B is an enlarged partial view showing the patch antenna in FIG.1A, FIG.2 is a perspective view showing the separated state of the patch antenna in the first embodiment, FIGS.3A to 3D are cross sectional views showing a method of making a patch antenna in a preferred embodiment according to the invention, FIG.4A is a partial plan view showing a patch antenna in a second preferred embodiment according to the invention, FIG.4B is a cross sectional view cut along the line A1-A2 in FIG.4A, FIG.5A is a partial plan view showing a patch antenna in a third preferred embodiment according to the invention, and FIG.5B is a cross sectional view cut along the line B1-B2 in FIG.5A. DESCRIPTION OF THE PREFERRED EMBODIMENTS A patch antenna in the first preferred embodiment will be explained in FIGS.1A to 2, where FIG.1A is a cross sectional view thereof and FIG.1B is an enlarged view showing a part in FIG.1A and FIG.2 is a perspective view showing a radome 1A and the other parts separated of the patch antenna in FIG.1A. As shown in FIG.1A and 2, in this patch antenna, a microstrip structure is formed by a feeding section 3, a radiating element 4 which are processed of a printed board and a dielectric substrate 5, a grounding conductor 6 which retain the original shape of a printed board. A metal patch 2A and the radiating element 4 form a broad- band microstrip antenna. The high-frequency wave fed through the feeding section 3 is resonated by the metal patch 2A and the radiating element 4. If the diameter and resonance frequency of the radiating element 4 are D1, f1, respectively, the diameter and resonance frequency of the metal patch 2A are D2, f2, respectively and D2>D1 is given, f1>f2 is obtained. Further, if a center frequency f0 is given by f0=(f1+f2)/2, a frequency band, which is given by (f1f2)/f0, is nearly equal to 0.1. Thus, the radiating element 4 fed though the feeding section 3 resonates near at the frequency f1 and radiates an electric wave. On the other hand, the metal patch 2A is electromagnetic-coupled with the radiating element 4, thereby resonating near at the frequency f2 and radiating an electric wave. As a result, a high-frequency signal with circularly polarized wave electromagnetic field is radiated in the direction of the outer surface 13 of the radome 1A which is made of plastics with a low loss. The microstrip structure including the radiating element 4 and the metal patch 2A are covered with the inner surface of the radome 1A and an antenna board 7 on which the microstrip structure is fixed, thereby protected from the external environment of the radome 1A. Further explaining this patch antenna in detail, the feeding section 3 and the radiating element 4 are formed on the same surface of a printed board by conductive board etching. The radiating element 4, which is formed like a disk, is fed with a phase difference of 90° from two positions with an angle α=90° to each other branched from the feeding circuit 3. By this feeding, the radiating element 4 radiates the circularly polarized wave electromagnetic field from its one surface. The metal patch 2A, which is made of a thin plate of aluminum and is formed like a disk, receives the circularly polarized wave electromagnetic field at its one surface and then reradiates it from the other surface. The other surface of the metal patch 2A is in contact with the inner surface 12 of the radome 1A, and the metal patch 2A is held between the inner surface 12 of the radome 1A and a holder 11A which protrudes from the inner surface 12 and covers a part of the lower surface of the metal patch 2A. The holder 11A is formed, as explained after, by processing the tip of a boss 11a. The radome 1A is pressed upon the antenna board 7 as it is guided along the circumference of the microstrip structure and is then fixed to the antenna board 7 by screws 8a, 8b. The means for fixing the radome 1A to the antenna board 7 is not limited to the screw, and another fixing means by which the position relationship between the radome 1A and the antenna board 7 are precisely determined may be employed. Next, the fixing of the metal patch 2A to the radome 1A will be explained in detail. The metal patch 2A has a boss hole 21 that is a through-hole penetrating from its one surface to other surface at the center of the surface, i.e., at the position of a circle center P1. The inner diameter r of the boss hole 21 is preferably around 5R/100, where R is the outer diameter of the metal patch 2A, taking into account the deterioration in the radiation characteristic of circularly polarized wave electromagnetic field and the holding strength of the metal patch 2A to the radome 1A. For example, if the service frequency of the patch antenna is 1.5 GHz to 1.6 GHz, it is suitable that the metal patch 2A has an outer diameter of R≒80 mm, a thickness of t≒0.5 mm and a boss hole inner diameter of r≒4 mm. On the inner surface 12 of the radome 1A, the holder 11A is formed to cover a part of the lower surface of the metal patch 2A which neighbors on the boss hole 21. The holder 11A is a structure which is obtained by processing the tip of the boss 11a that protrudes from the inner surface of the radome 1A. The height h of the boss 11a is greater than the thickness t of the metal patch 2A, and the outer diameter r1 of the boss 11a is a little shorter than the inner diameter r of the boss hole 21 of the metal patch 2A. The boss 11a is positioned such that the circle center P1 of the metal patch 2A is located, as shown in FIG.1A, just above the circle center P0 of the radiating element 4 when the radome 1A is fixed on the antenna board 7. After the upper surface of the metal patch 2A is pressed upon the inner surface 12 of the radome 1A as the metal patch 2A is guided by the boss 11a, the tip of the boss 11a is softened by heating and compressed in the direction of the inner surface 12, thereby transformed into a form to cover a part of the lower surface of the metal patch 2A, then cooled and hardened to provide the holder 11A as shown. The radome 1A can be made of polycarbonate and the like. The thermosoftening temperature(glass transition temperature) Tg where polycarbonate is softened by heating is around 150°C, crystal-melting temperature Tm where it is fluidized is around 240°C and heat-resisting temperature where it is used in the solid state is around 125°C. FIGS.3A to 3D show a process of fixing the metal patch 2A to the radome 1A in the patch antenna in FIG.1A, where FIGS.3A to 3D correspond to the first to fourth steps, respectively. In the first step, the radome 1A, on the inner surface of which protruded is the boss 11a which has a height h greater than the thickness t of the metal patch 2A and has an outer diameter r1 a little shorter than the inner diameter r of the boss hole 21 of the metal patch 2A, is made. In the second step, the metal patch 2A is pressed upon to contact the inner surface 12 as it is guided by the circumference of the boss 11a. In the third step, the tip of the boss 11a is pressed by a hot plate 30 at a temperature higher than the thermosoftening temperature Tg of the boss 11a and lower than the melting temperature Tm of the toss 11a, thereby a part of the boss 11a protruded from the upper surface of the metal patch 2A being softened and formed to provide the holder 11A that has the form to cover a part of the upper surface of the metal patch 2A. Here, between the hot plate 30 and the metal patch 2A, a disk-like spacer 31 with a thermal resistance is inserted. The spacer 31 has a thickness t1 and a hole with a diameter r2 in the center. The spacer 31 serves to control the thickness of the holder 11A when pressing the hot plate 30 upon the boss 11a. The height h of the boss 11a and the thickness t1 of the spacer 31 is set to give a strength that the thickness and diameter of the resultant holder 11A can sufficiently hold the metal patch 2A, and the diameter r2 of the spacer 31 is set to be greater than a range that the holder 11A extends in the direction of the circumference of the metal patch 2A. The hot plate 30 may be substituted by a soldering iron. In the fourth step, the hot plate 30 and the spacer 31 are removed and the holder 11A is then cooled and hardened, thereby the metal patch 2A being held between the inner surface of the radome 12 and the holder 11A. As explained above, in the first embodiment, the metal patch 2A can be fixed to the radome 1A by using the fabrication method as shown in FIGS.3A to 3D. The radome 1A with the shape shown in FIG.3A can be made by injection molding and the fixing step of the metal patch 2A to the radome 1A by the hot plate 30 is very simple. Thus, the patch antenna in the first embodiment can be made at a low cost. Furthermore, the metal patch 2A can be precisely positioned as it is guided by the boss 11a of the radome 1A which can be precisely formed. Therefore, it is not necessary for the patch antenna in the first embodiment to consider the deterioration in electrical performance caused by the position difference between the metal patch 2A and the radiating element 4. A patch antenna in the second preferred embodiment according to the invention will be explained in FIGS.4A and 4B, where FIG.4A is a plan view thereof and FIG.4B is a cross sectional view cut along the line A1-A2 in FIG.4A. In the second embodiment, only the radome 1A in the first embodiment as shown in FIG.1A is replaced by a radome 1B and the other parts in the first embodiment are used as well. The fabrication method in the first embodiment can be also used in the second embodiment. The radome 1B further comprises holders 11B, 11C and 11D in addition to the holder 11A of the radome 1A in FIG.1A. Each of the holders 11B, 11C and 11D has a form that partially covers the lower surface nearby the circumference of the metal patch 2A, when the metal patch 2A is fixed to the radome 1B. The holders 11B, 11C and 11D are, similarly to the holder 11A, formed by thermosoftening and shaping bosses 11b, 11c and 11d(not shown) which are preformed on the radome 1B, respectively, thereafter cooling and hardening them. The bosses 11b, 11c and 11d are nearly in contact with the circumference of the metal patch 2A and are protruding from the inner surface 12 of the radome 1B at symmetrical points. By processing the bosses 11b, 11c and 11d as described above, the holders 11B, 11C and 11D, which are made by processing parts of the bosses 11b, 11c and 11d, respectively, are formed to cover the lower surface nearby the circumference of the metal patch 2A. In the second embodiment, since the holders 11B, 11C and 11D, in addition to the holder 11A, hold the metal patch 2A onto the inner surface 12 of the radome 1B, the holding strength of the metal patch 2A onto the radome 1B can be increased, therefore enhancing the electrical and mechanical stabilities. A patch antenna in the third preferred embodiment according to the invention will be explained in FIGS.5A and 5B, where FIG.5A is a plan view thereof and FIG.5B is a cross sectional view cut along the line B1-B2 in FIG.5A. In the third embodiment, the radome 1A and the metal patch 2A in the first embodiment as shown in FIG.1A are replaced by a radome 1C and a metal patch 2B, respectively and the other parts in the first embodiment are used as well. The fabrication method in the first embodiment can be also used in the third embodiment. This patch antenna has a structure that the disk-like metal patch 2B is held onto the inner surface of the radome 1C at its circumference. Namely, there is formed no through-hole at the circle center P2 of the metal patch 2B. Also, the radome 1C does not include the holder 11A of the radome 1A as shown in FIG.1A, and it is provided with holders 11E, 11F, 11G and 11H in place of the holder 11A. Each of the holders 11E, 11F, 11G and 11H has a form that partially covers the lower surface nearby the circumference of the metal patch 2B, when the metal patch 2B is fixed to the radome 1C. The holders 11E, 11F, 11G and 11H are formed by thermosoftening and shaping arc-shaped protrusions 11e, 11f(not shown), 11g and 11h which are preformed on the radome 1C, respectively, thereafter cooling and hardening them. The arc-shaped protrusions 11e, 11f, 11g and 11h which are symmetrically formed are protruding from the inner surface 12 of the radome 1C, and the inner walls of the arc-shaped protrusions are nearly in contact with the circumference of the metal patch 2B. The holders 11E, 11F, 11G and 11H, which are made by processing parts of the arc-shaped protrusions 11e, 11f, 11g and 11h, respectively, are formed to cover the lower surface nearby the circumference of the metal patch 2B. In the third embodiment, since the holders 11E, 11F, 11G and 11F hold the metal patch 2B onto the inner surface 12 of the radome 1C, the holding strength of the metal patch 2B onto the radome 1C can be increased, therefore enhancing the electrical and mechanical stabilities. Though, in the third embodiment, the holders 11E, 11F, 11G and 11H are formed as parts of a ring, they may be replaced by an integrated holder which is formed by substituting a ringshaped protrusion for the arc-shaped protrusions 11e, 11f, 11g and 11h. Furthermore, the radome 1C may include a holder nearby the circle center P2 of the metal patch 2B. Although the invention has been described with respect to specific embodiment for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modification and alternative constructions that may be occurred to one skilled in the art which fairly fall within the basic teaching here is set forth.

Claims: A patch antenna, comprising: a driven patch (4) which is fed with a high-frequency signal and radiates a high-frequency electromagnetic field from its one surface; a parasitic patch (2A) which receives said high-frequency electromagnetic field from said driven patch (4) at its one surface and reradiates said high-frequency electromagnetic field from its other surface; and a radome (1A) which contains said driven patch (4) and said parasitic patch (2A) inside of said radome to protect them and holds said parasitic patch; wherein said parasitic patch (2A) is held by the inner surface (12) of said radome which contacts said other surface of said parasitic patch and a holder (11) which protrudes from said inner surface (12) of said radome (1A), wherein: said parasitic patch (2A) has a through-hole (21) at the center of its surface, and said holder (11) of said radome comprises a first holding member (11A) which penetrates said through-hole (21) of said parasitic patch (2A) from said inner surface (12) of said radome (1A) and covers said one surface of said parasitic patch nearby said through-hole. A patch antenna, according to claim 1, wherein: said holder (11) of said radome (1A) further comprises one or a plurality of second holding members (11B,11C,11D) which cover a part of the circumference of said parasitic patch (2A). A patch antenna, according to claim 1 or 2, wherein: said holder (11) of said radome (1A) covers a part or all of the circumference of said parasitic patch (2A). A patch antenna, according to any of claims 1 to 3, wherein: said holder (11) is formed by thermosoftening, shaping, cooling and then hardening its tip after positioning said other surface of said parasitic patch (2A) onto said inner surface (12) of said radome (11). A patch antenna, according to claim 4, wherein: said parasitic patch (2A) is positioned as it is guided by said holder (11) before said thermosoftening. A method for making a patch antenna, which comprises a driven patch (4) which is fed with a high-frequency signal and radiates a high-frequency electromagnetic field from its one surface, a parasitic patch (2A) which receives said high-frequency electromagnetic field from said driven patch (4) at its one surface and reradiates said high-frequency electromagnetic field from its other surface said parasitic patch (2A) having a through-hole (21) at the center of its surface, and a radome (1A) which contains said driven patch (4) and said parasitic patch (2A) inside of said radome to protect them and holds said parasitic patch, comprising the steps of: making said radome (1A) from which a boss (11) which has a height greater than a thickness of said parasitic patch (2A) is protruded at its inner surface (12); pressing said other surface of said parasitic patch (2A) to contact said inner surface (12) of radome (1A) along the circumferential shape of said boss (11); wherein the boss (11) penetrates the through-hole (21); thermosoftening a part of said boss (11) protruding from said one surface of said parasitic patch (2A) and forming it into a shape to cover a part of said one surface of said parasitic patch (2A) in order to provide a holder; and cooling and hardening said holder (11) to hold said parasitic patch between said radome and said holder.

Inventors: KURAMOTO AKIO, TANABE KOSUKE, KURAMOTO, AKIO, TANABE, KOSUKE
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1997
Main classification: H01Q9
IPC codes: H01Q9, H01Q1, B29C65, H01Q13