As an example of wideband antenna according to the related art, a microstrip antenna (patch antenna) is known in which a radiating conductor element and a ground conductor plate are provided facing each other across a dielectric that is thin relative to the wavelength, and a parasitic conductor element is provided on the radiating surface side of the radiating conductor element. See, for example, Japanese Unexamined Patent Application Publication No. 55-93305 (Patent Document 1). The wideband antenna according to Patent Document 1 achieves bandwidth enhancement by exploiting electromagnetic coupling between the radiating conductor element and the parasitic conductor element.
Also, as another example of the related art, a configuration is known in which, in addition to the configuration according to Patent Document 1 mentioned above, two conductor plates that face each other with a gap are placed between the radiating conductor element and the parasitic conductor element, and these conductor plates are electrically connected to the ground conductor plate. See, for example, Japanese Unexamined Utility Model Registration Application Publication No. 4-27609 (Patent Document 2). In the wideband antenna according to Patent Document 2, the conductor plates are placed between the radiating conductor element and the parasitic conductor element. This makes the electromagnetic coupling between the radiating conductor element and the parasitic conductor element stronger, which can lead to increased bandwidth.