Patent ID: 8472907

Claim:
An antenna device comprising: a substrate; a radiating electrode formed on said substrate; a ground electrode formed on said substrate and disposed opposite said radiating electrode; a feed line as a distributed constant transmission line connected via a feed point positioned at one end of said feed line to said radiating electrode; an impedance matching element for impedance-matching said radiating electrode at a prescribed signal frequency by being connected in parallel with said radiating electrode to said feed line at a position a prescribed distance away from said feed point; and a switch, interposed between said impedance matching element and said feed line, for connecting or disconnecting said impedance matching element to or from said feed line in accordance with a prescribed control signal, wherein one end of said impedance matching element is connected to said switch and the other end of said impedance matching element is connected to said ground electrode, and wherein the other end of said feed line is connected to a communication circuit, and wherein said prescribed distance is determined so that when said impedance matching element is connected to said feed line, combined conductance of said radiating electrode and said feed line becomes equal to the conductance of the communication circuit, and wherein said prescribed distance is determined by equation 1 = 1 β ⁢ ⁢ tan - 1 [ ⁢ - X f ⁢ ⁢ 0 ⁢ Z 0 ± ( X f ⁢ ⁢ 0 ⁢ Z 0 ) 2 - ( Z 0 2 - R f ⁢ ⁢ 0 ⁢ Z 0 ) ⁢ ( X f ⁢ ⁢ 0 2 + R f ⁢ ⁢ 0 2 - Z 0 ⁢ R f ⁢ ⁢ 0 ) Z 0 2 - R f ⁢ ⁢ 0 ⁢ Z 0 ⁢ ] ⁢ ⁢ β = 2 ⁢ ⁢ π λ eff ( 1 ) where I represents said prescribed distance, f 0 represents said prescribed signal frequency, R f0 represents a real component of the impedance that said radiating electrode has at said prescribed signal frequency, X f0 represents an imaginary component of the impedance that said radiating electrode has at said prescribed signal frequency, Z 0 represents the characteristic impedance of said feed line, λ eff represents the wavelength of a signal having said prescribed signal frequency, as computed by considering wavelength shortening due to the material of said substrate, and β is a phase constant.