Abstract: A method and an apparatus for determining the dielectric constant of a material wherein the material is loaded to a short-circuited microstrip and microwaves are fed to the microstrip for providing a reflected signal. The reflected signal is compared with a reference signal both under load and not load conditions of the microstrip for obtaining the modulus and the phase of the reflection coefficient, the dielectric constant being a function thereof. The measure of the phase angle between the reference and the reflected signal in both conditions is obtained by handling the reflected signal with a first phase shifter and a second calibrated phase shifter cooperating with a zero detector. The method and the apparatus of the invention is especially designed for a quick and easy measure of the amount of unburnt coal in the ash coming from a combustion heater.