Detachable mobile radio units, which can be used not only in a car but also as a portable radio unit outside the car, have been developed by the Electrical Communication Laboratories, NTT, for use in the existing 800 MHz mobile radio telephone system in Japan. This paper describes the circuit configuration and principal techniques to reduce volume, weight and power consumption for these detachable mobile radio units. The results of field tests carried out in the existing 800 MHz land mobile telephone system to evaluate effective antenna gain and practical service coverage are also presented.