Usually a Radio Altimeter transmits in pulses to the ground and back to determine the time it takes for the cycle . This in turn gives you distance in time and is translated as height. Usually 1 beam from 1 antennae. The time between the pulses is determined by the manufacturer and can vary.
Doppler Radar measures frequency change between Tx and Rx and this continuous wave transmission usually used in an Aircraft for drift and ground speed. and also uses 4 beams from 4 antennae.