Encapsulant for buried u.h.f. antennas

Certain applications for above-ground communications require antennas placed below the ground surface. Under these conditions, reasonable antenna performance can be achieved if the antenna is encapsulated in a uniform low-loss dielectric having a relative permittivity near that of the surrounding earth. It is shown that, with special conditioning, refractory concrete is a suitable encapsulant. It is castable, extremely durable and has a relative permittivity of 6.5¿7.0 and a loss tangent of 0.005¿0.006 at 400 MHz. Resonator measurements have shown that the desired electrical properties can only be achieved, however, if the material is dried at a temperature of 125¿150°C. Once dry, the concrete surface must be sealed to prevent water absorption.