1. Field of the Invention
The present invention relates to a matching device suitable for use with a microstrip antenna.
2. Description of the Prior Art
A conventional microstrip antenna 10 is represented in FIG. 1 and this microstrip antenna 10 has a radiation element 13 provided on a dielectric layer 12 formed on a ground conductor 11. The microstrip antenna 10 is used for radio communications in airplanes, automobiles and so on where particularly UHF/SHF bands are used because the microstrip antenna 10 can provide a desired unidirectivity under with its simple structure and low-height installation.
However, since the microstrip antenna 10 has a high Q and a narrow frequency band width, it cannot be used in radio communications using two frequencies for transmission and reception, respectively.
To obviate the above shortcoming, it is proposed to mount a parasitic antenna element in front of the radiation element for widening the frequency band by the resulting double-resonant state. This proposal, however, has a problem that the height of the entire antenna is unavoidably increased because the parasitic antenna element is mounted in front of the radiation element.
Whereas, Japanese Laid-Open Patent Publication No. 62-279704 describes a technique such that a matching device including a stub is interposed between the antenna and the feed line as shown in FIG. 1.
As shown in FIG. 1, a matching device 20 has conductor lines 23 to 25 connected in series on a grounded conductor 21 with a dielectric layer 22 therebetween, and has a stub 26 of an L-letter configuration branched from a mid point P.sub.M, and connectors 27, 28 provided on the load and input sides so as to be connected to the conductor lines 23 to 25, respectively. A feed point 14 of the antenna 10 is connected to one connector 27 of the matching device 20 by way of a coaxial feed line 15 and a connector 16. The other connector 28 of the matching device 20 is connected with a feed line (not shown).
A length l1 between the feed point 14 and the mid point P.sub.M is selected so that, at two different frequencies f1, f2 (f1&lt;f2), the conductance components as viewed from the mid point P.sub.M of the matching device 20 toward the antenna 10-side are equal, but the susceptance components B1, B2 (.vertline.B1.vertline.&gt;.vertline.B2.vertline.) are opposite in sign.
Further, a length l2 and the characteristic impedance of the stub 26 are selected such that the susceptance components of the stub 26 as viewing from the mid point P.sub.M takes values -B1, -B2 at the frequencies f1, f2, respectively.
Accordingly, at the two desired frequencies f1, f2, the resultant admittances as viewed from the mid point P.sub.M toward both the stub 26 and the antenna 10 are equal to each other.
The intermediate conductor line 24 is a known quarter-wave-line transformer, which converts the resultant admittance as viewing from the mid point P.sub.M into a standard value [1] as viewed from the input side connector 28.
Thus, by the use of the matching device 20, it is possible to match the impedance values of the antenna 10 at the two desired frequencies f1, f2, the frequency band being thereby widened.
The above matching device 20 can be unitarily formed with the antenna 10 by making two grounded conductors thereof common as shown in FIG. 2. In FIG. 2, reference numeral 17 designates a connecting conductor, and 29 a non-grounded conductor. The non-grounded conductor 29 represents the feed line 15, the conductor lines 23 to 25 and the stub 26 shown in FIG. 1.
However, since the above matching device 20 has the stub 26 branched from the conductor line 23, the size of the matching device 20 is relatively large even though the stub 26 is of an L-letter shape.
In addition, if the matching device 20 is formed of coaxial conductors, then the matching device 20 becomes complicated in structure.