Antenna apparatus and radio device using antenna apparatus

Abstract

Antenna apparatus is comprised of dipole antenna arranged proximate to a surface of metal housing including radio device circuit and having two quarter wavelength elements having opposite end portions being feed points, short-circuit element short-circuiting the quarter wavelength elements, and coaxial feed line having outer conductor and central conductor having one ends connected to the feed points, respectively, and connecting the feed points to radio device circuit. The coaxial feed line is arranged along part of the quarter wavelength element and part of the short-circuit element and branched from central portion of the short-circuit element. The outer conductor is electrically connected to part of the quarter wavelength element and part of the short-circuit element. Currents leaked onto the feed line and flowing through the two quarter wavelength elements are also divided to the short-circuit element. The currents are mixed at a position at which the coaxial feed line is branched and flow into the outer conductor of the coaxial feed line as a leak current from the dipole antenna. The currents branched from the two quarter wavelength elements to the short-circuit element are almost opposite in phase to each other. Thus, the currents cancel each other and the leak current becomes zero.