  We report on the microwave (mw) properties of coaxial cavities built by using
bulk MgB2 superconductor prepared by reactive liquid Mg infiltration
technology. We have assembled a homogeneous cavity, by using an outer MgB2
cylinder and an inner MgB2 rod, and a hybrid cavity by using an outer copper
cylinder and the same MgB2 rod as inner conductor. By the analysis of the
resonance curves, in the different resonant modes, we have determined the
microwave surface resistance, Rs, of the MgB2 materials as a function of the
temperature and the frequency, in the absence of DC magnetic fields. At T = 4.2
K and f = 2.5 GHz, by a mw pulsed technique, we have determined the quality
factor of the homogeneous cavity as a function of the input power up to a
maximum level of about 40 dBm (corresponding to a maximum peak magnetic field
of about 100 Oe). Contrary to what occurs in many films, Rs of the MgB2
material used does not exhibit visible variations up to an input power level of
about 10 dBm and varies less than a factor of 2 on further increasing the input
power of 30 dB.
