“…In recent years, various approaches have been developed to design and produce multi-frequency antennas. The main methods include the multilayer method [ 22 , 23 , 24 ], slot method [ 25 , 26 ], branch method [ 27 , 28 ], the reconfigurable method [ 29 , 30 , 31 ], the metamaterial loading method [ 32 , 33 , 34 , 35 , 36 ], and Dipole multifrequency antenna [ 37 , 38 , 39 , 40 ]. For example, in 2010, Chen et al [ 22 ] proposed a dual-frequency microstrip antenna using low-temperature co-fired ceramic (LTCC) technology by adopting a multi-layer method, which can be used for GPS positioning.…”