We have developed a 3.5-mm coaxial microcalorimeter that will become a national power standard up to 26.5 GHz. Several core parts such as a thermostat, an adiabatic line, a thermopile, and reference standards are described in detail. Some measurement data are shown for a system performance check. A finite-element method simulation of the adiabatic line is shown. A model of the effect of the adiabatic line on thermopile measurement is introduced. Effective efficiency at selected frequencies is presented for the reference standards.