In this study, a compact antenna design, which operates in the 2.4, 5.2, and 5.8 GHz (WLAN) and 3.5 and 5.5 GHz (WiMAX) frequency bands, has been implemented to be compatible with the 802.11.ac/n standards. The proposed metal antenna is made of a copper plate of thickness 0.5 mm with a compact overall physical size of 20 mm $\times$ 30 mm. Although it is low-profile, it can work with high efficiency because it has a cheap planar metal structure and it does not contain any expensive dielectric material. The antenna is investigated in terms of S parameters, input impedance, efficiency, surface current distributions, and radiation pattern. The implemented antenna has been used in a USB WiFi adapter design for a desktop computer as an indoor WLAN application. The protector outer jacket of the WiFi adapter has been designed using a 3D printer, and the adapter card and driver are acquired commercially. As a result, the produced WiFi adapter has been realized in a size of approximately 60$\%$ smaller than the other modules using commercially available monopole antennas on the market. The WiFi adapter provides IEEE 802.11.n/g/b standards and supports USB 2.0. It has been observed that the speed measurement tests have been performed successfully and that the download-link and upload-link can reach 600 Mbps data rates. In addition, BPSK, QPSK, and 16QAM with OFDM modulation techniques are used.