To concentratedly install substantially a plurality of antennas to a narrow space by mixing a metallic chip consisting of two kinds of metals or over with resin powder and applying pressure and hot forming to the mixture under a high pressure, while supplying a high current thereto so as to form a porous board, thereby miniaturizing the antenna.

The resin powder of a board 2 strongly fusion each metallic chip is preferably an insulting resin with a low loss at high frequencies. After placing a mold release sheet on a bottom face of a die frame, metal chips and the resin powder sufficiently mixed are uniformly put in the die and a mold release sheet is placed furthermore on the mixture. For example, the thickness of the metal chips and the resin powder before pressure application is 30 to 70 mm, then the press die is descended until the current reaches 4,000 to 6,500 amperes to pressurize the mixture under a pressure of 210-340 kg/cm2 in general. This pressure application is continued for a prescribed time, and when the current passing through the die frame reaches a nearly constant value, the compact is taken out.