The development of dielectric materials with low permittivity and low loss is a great challenge in wireless communication. In this study, LiLn(PO₃)₄ (Ln = La, Sm, Eu) ceramic systems were successfully prepared using the traditional solid-state method. X-ray diffraction analysis indicated that the LiLn(PO₃)₄ ceramics crystallized in a monoclinic structure when sintered at 850–940 ℃. The characteristic peak shifted to high angles with variations in the Ln element, which was ascribed to a reduction in cell volume. Further analysis by structure refinement revealed that the reduction of cell volume resulted from the decline of chemical bond lengths and compression of [LiO₄] and [PO₄] tetrahedra. Remarkably, the LiLn(PO₃)₄ ceramic system displayed exceptional performances at low sintering temperatures (910–925 ℃), including high Q·f of 41,607–75,968 GHz, low t_f ranging from −19.64 to −47.49 ppm/℃, low ε_r between 5.04 and 5.26, and low density (3.04–3.26 g/cm³). The application of the Phillips–Van Vechten–Levine theory revealed that the increased Q·f value of the LiLn(PO₃)₄ systems can be attributed to the enhanced packing fraction, bond covalency, and lattice energy, and the stability of t_f was associated with the increase of bond energy. Furthermore, a microstrip patch antenna prototype using the LiEu(PO₃)₄ ceramics was fabricated. Measurement results demonstrated excellent antenna performances with a bandwidth of 360 MHz and a peak gain of 5.11 dB at a central frequency of 5.08 GHz. Therefore, the low ε_r LiLn(PO₃)₄ ceramic systems are promising candidates for microwave/millimeter-wave communication.