Recently, compact and multi-functional wireless mobile devices have been highly demanded. Therefore, miniaturization of radio-frequency (RF) components, including antennas, is essential. Regarding wireless charging networks for mobile devices, performance degradation of near-field communication (NFC) and wireless power charging (WPC) systems is often observed. This is because various components are integrated into a compact space. Moreover, active components, such as amplifiers and oscillators, cause wireless communication devices to experience unwanted or noise signals. The unwanted signals are transmitted and received by an antenna, thereby degrading the quality of wireless communication. Thus, the frequency filtering device is required to suppress the radiation of unwanted signals. However, this increases the volume of a wireless device, which is not desired. The aforementioned demands and issues can be addressed by employing magnetic materials and exploiting the unique characteristics of frequency dependent complex permeability. It is noted that magnetic loss is a concern because the loss degrades device performance. The objective of the dissertation is to develop low-loss ferrites and design magnetics based antennas to meet the aforementioned demands without sacrificing antenna performance. Low loss hexaferrites, including Ba3Co2Fe24O41, BaCo1.4Zn0.6Fe16O27, and Ba2Co2Fe28O46, have been developed with a conventional ceramic process and compared with other reported low-loss ferrites for the figure of merit. The figure of merit is defined as a ratio of the real part of complex permeability to the magnetic loss tangent. Design and fabrication of miniature antennas, such as the dual-polarized hexaferrite antenna for an unmanned aerial vehicle and low-profile multiband antenna for telematics applications, are based on the low-loss ferrites, which was developed in this dissertation. Furthermore, effects of magnetic materials loading on antenna miniaturization and performance were investigated. Excellent antenna performance was demonstrated with ferrite loading. Spinel ferrite, Ni0.38Zn0.47Cu0.15Fe2O4, offers enough magnetic isolation for WPC design, resulting in a high power transfer efficiency for the WPC system. Accordingly, a simple wireless power charging system was designed with the spinel ferrite loading and simulated for its power charging performance. Then, the loaded ferrite was evaluated for its applicability to a WPC system. Lastly, loading an antenna with multiple ferrite cores significantly suppressed harmonic radiation from the antenna by dissipating unwanted signals. On the other hand, the conventional harmonic suppressed antennas suppress the harmonic radiation by reflecting or redirecting unwanted signals, which is not desired. The simulated and experimental data from ferrite loaded antennas suggest that the magnetic materials can play various roles in antenna design and performance.