This article prepares, Mg_xCa_(0.90-x)Zn_0.10Fe₂O₄ nanoparticle-based new flexible microwave substrate materials to build flexible antennas. The Mg_xCa_(0.90-x)Zn_0.10Fe₂O₄ nanoparticles are synthesized using the sol-gel chemical method. There are three different weight percentages are chosen for “X” values i.e., X= 25%, 50%, and 75%. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) studies are used to evaluate the structural and morphological features of the produced nanoparticles. Dielectric Assessment Kit is also used to evaluate the nanoparticles' tunable dielectric characteristics (DAK). Loss tangents range from 0.00275 to 0.00675 while dielectric permittivity values range from 3.25 to 4.75. The magnetic characteristics of the produced samples are also investigated using the vibrating sample magnetometer (VSM). The Agilent 8501E module is used to calculate the adjustable magnetic permeability and magnetic loss tangent values. The magnetic permeability and magnetic loss tangent values obtained are in the range of 1.00 to 1.15 and 0.0035 to 0.0065, respectively. Later, a flexible antenna is designed on the prepared flexible substrate that operates within the frequency range of 1.60 GHz to 3.00 GHz with a maximum gain of 5.15 dBi. After that, this antenna is incorporated with a nine-antenna array-based portable microwave head imaging system. Finally, successful brain tumor detection is observed by post-processing the collected scattering parameters with an image reconstruction algorithm. The overall results ensure that the Mg_xCa_(0.90-x)Zn_0.10Fe₂O₄ nanoparticle-based new flexible microwave substrate materials can be a potential candidate for microwave head imaging and are suitable to fit with microwave devices.