Circularly Polarized Complementary Antenna With Patch and Dipole Elements

This invention relates to a complementary antenna, and more particularly to an isotropic, improved complementary antenna having a circular polarization characteristic.

GPS (global positioning system) antennas or other antennas for communication of a moving body such as an artificial satellite must have a property of clockwise circular polarization. Although there are various possible methods for obtaining such an antenna, it is very difficult to establish both a hemispherical or isotropic directivity and a circular polarization characteristic. In this situation, a conventional practical solution was array-arrangement of dipole, loop or other planar radiating elements, or alternatively the use of spiral tridimensional radiating elements.

These prior art antenna arrangements certainly enabled realization of an antenna substantially satisfying to a certain degree the aforementioned characteristics. However, it requires much experience and know-how. Besides this, the prior art antennas involve a certain compromise between the aforementioned characteristics, and various disadvantages, i.e. large scale and complicated structure of the entire antenna, difficult handling and cost increase.

It is therefore an object of the invention to provide a quasi-planar antenna which is small-scaled, readily handled, simple-structured and provided with the aforementioned characteristic.

In order to obtain the object, the invention is characterized in that a half-wavelength dipole antenna bent into the form of a semicircle is disposed relatively adjacent, as compared to the specified wavelength, to a patch antenna in the form of a semicircle open at its circumferential portion and short-circuited at its straight portion, so that they form a pair of complementary antennas which are fed with phase 90.degree. apart to realize a circular polarization characteristic.

Since the inventive half-wavelength dipole antenna and patch antenna have a complementary relationship, when the respective antennas are fed with phase difference of 90.degree., the former antenna acts as a substantially quasi-isotropic electric current source, whereas the latter antenna acts as a magnetic current source which is fed with phase difference of 90.degree. from the former antenna. Since the sum of radiated electric fields from both antennas composes a circular polarization characteristic, a complementary antenna having the aforementioned characteristic is obtained.

FIGS. 1 through 3 show an embodiment of the invention;

FIG. 4A is a diagram showing current and charge distribution on a full-wavelength loop antenna;

FIG. 4B is a diagram showing a corresponding electric field and magnetic field of the loop of FIG. 4A;

FIG. 5A is a diagram showing current and charge distribution on a full circular patch antenna;

FIG. 5B is a diagram showing a corresponding electric field and magnetic field of the loop of FIG. 5A;

FIG. 6 is a directivity characteristic diagram of a half-wavelength dipole antenna;

FIG. 7 is a directivity characteristic diagram of a semicircular half-wavelength dipole antenna;

FIGS. 8A-8D present views which show how a semicircular patch antenna is composed; and

FIG. 9 shows a further embodiment of the invention.

The invention is explained below, referring to preferred embodiments illustrated in the drawings.

A half-wavelength loop antenna 1 shown in FIG. 1 is made by bending a cord-shaped antenna element 1a into the form of a semicircular ring, and reference numeral 1b denotes a feeding point to be connected to a coaxial cable, etc.

In contrast, a patch antenna 2 shown in FIG. 2A consists of an upper conduction plane 2a on a Teflon (trademark) plate 2 or the like and a ground plane member 2b. The upper conduction plane 2a is a semicircular conduction plane and acts as a radiation element. The circumferential portion 2d of the upper conduction plane 2a is open and its straight portion 2e is short-circuited to the ground plane member 2b. To a feeding point 2f is connected a coaxial cable, etc. Both antennas are disposed in confrontation through a small distance as compared to the wavelength, as shown in FIG. 3. The respective antennas are fed with phase 90.degree. apart, using a phase shifter 6 as shown in FIG. 2C.

The above mentioned antenna assembly exhibits a desired circular polarization characteristic as explained below.

Since the sum of radiated electric fields from an electric current source and from a complementary magnetic current source with phase 90.degree. apart composes a circular polarization characteristic, the use of this nature makes it possible to establish an antenna having a circular polarization characteristic.

The first step for obtaining such an antenna is to determine the configuration of a loop antenna (or dipole antenna) so that the absolute value of the radiation pattern disregarding the polarization characteristic coincides with the absolute value of a radiation pattern having a desired radiation characteristic.

The next step is to form a patch antenna which becomes a complementary magnetic current source with the electric current source on the loop antenna. Then by confronting but not coupling these two complementary antennas and feeding them with phase 90.degree. apart, a desired circular polarized wave radiating characteristic is obtained.

FIG. 4A shows distribution of current i and charges .sigma. of a full-wavelength loop antenna whose center is on the z axis on an x-y plane. The maximum current distribution points are on the x axis and the minimum ones on the y axis, the maximum charge distribution points on the y axis and the minimum ones on the x axis, respectively. FIG. 4B shows the resulting electric field E and magnetic field H on the x-y plane.

FIG. 5A shows corresponding distribution of current i and charge .sigma. on an E.sub.110 -mode full circular complementary patch antenna. The currents i are maximum on the y axis, and charges c are maximum on the x axis. FIG. 5B shows distribution of the corresponding electric field E and magnetic field H on the x-y plane. As is apparent, electric potential .phi..sub.e is zero (.phi..sub.e =0) on the y-z plane and magnetic potential is zero on the x-z plane.

As is apparent from a comparison of FIGS. 4B and 5B, the full-wavelength loop antenna and the full circular E.sub.110 -mode patch antenna exhibit complementary characteristics in which electric fields E and magnetic fields H are exchanged with each other, and this means that they are in a complementary relationship.

FIG. 6 shows that the radiation characteristic of a classical half-wavelength dipole antenna aligned to the y-axis has null points in the y-axis direction. However, when the half-wavelength dipole antenna has a semicircular configuration as described above and shown in FIG. 1, the radiation in the y-axis direction increases as shown in FIG. 7 and the null points are removed. In this case, the level difference between the maximum radiation direction, i.e. z-axis direction, and the minimum radiation direction, i.e. y-axis direction, is about 8 dB, which means that the characteristic of FIG. 7 does not yet achieve the isotropic characteristic. However, it may practically be regarded as a substantially isotropic characteristic because in a GPS satellite communication, etc. reception is effected in the upper half region expressed by z>0. Reception from the direction of the horizontal plane (x-y plane, z=0) is actually rare, and reception is normally effected in elevated angles above a certain value.

That is, the two semicircular antennas discussed above, the half-wavelength electric current source antenna and the half circular patch antenna (equivalent to a magnetic current source antenna), have substantially isotropic radiation characteristics, and can be realized by a transformal version of the full-wavelength loop antenna and full circular E.sub.110 -mode patch antenna.

Referring to the full-wavelength loop antenna of FIG. 4A, considering that the current is always zero at the (0,.+-.a,0) positions on the y axis, no change occurs in electric current and charge distribution on a half loop existing in x.gtoreq.0 when the antenna loop is cut at said positions and the other half loop expressed by x<0 is removed. Therefore, by removing a half of the loop, we arrive at the semicircular half-wavelength electric current source dipole antenna of FIG. 1.

In this case, the loop radius a is expressed by: ##EQU1## where .lambda.o is the wavelength in a vacuum.

An E.sub.110 -mode patch antenna is discussed below. As is apparent from FIG. 8A, since the electric potential .phi..sub.e is zero (.phi..sub.e =0) on the y-z plane, the electric and magnetic characteristics of a full space of this configuration do not change though a perfect conducting plane is put on the y-z plane as shown in FIG. 8B. Accordingly, when putting an infinitely extending full conductor C on the y-z plane and omitting the antenna assembly in the x<0 region as shown in FIG. 8C, the electric characteristic of the region of x>0 does not change.

By decreasing the full conductor C on the y-z plane into a dimension still large enough to short-circuit the end surface D of the patch antenna where the electric potential .phi..sub.e is zero (.phi..sub.e =0), the antenna comes to behave as shown in FIG. 8D as a semicircular magnetic source antenna, radiation also occurs in the x<0 region, and the radiation characteristic of the region approaches the characteristic of FIG. 7. Thus, the patch antenna of FIG. 2A is obtained.

Since the patch antenna is of the E.sub.110 -mode, the radius b thereof is indicated by: ##EQU2## where .lambda.o is the wavelength in a vacuum, and .epsilon..sub.r is the dielectric constant of dielectric member 2c. Formula 2 is somewhat different from Formula 1 of the above-indicated half-wavelength dipole antenna. However, as long as their radii are not extremely different, an acceptable complementary relationship between both antennas can be maintained.

FIG. 9 shows an active reception antenna taken as a further embodiment of the invention.

Antennas 1 and 2 are individually directly connected to amplifiers 3 and 4. An output of the amplifier 3 is connected to one input of an adder 5, and an output of the amplifier 4 is connected to the other input of adder 5 through a phase shifter 6, so that a circularly polarized wave can be received and obtained from an output terminal 7 of the adder 5.

The antennas 1 and 2 form a complementary pair having a quasi-isotropic characteristic as described above. Therefore, by connecting them directly to the amplifiers 3 and 4 in a significant power mismatching condition near a noise matching condition which allows to minimize the coupling of both antennas, and adding respective outputs in 90.degree. phase difference, an active antenna array capable of receiving circular polarized waves is obtained.

As explained above, the invention provides a complementary antenna having a quasi-isotropic, circular polarization characteristic which can be small-scaled, readily handled and inexpensive.