Using the radio altimeter for altitude measurement，some departments have put forward two quality requirements for the altimeter: ① to reduce the flickering movement of the altimeter pointer during the flight. ② to replace the upright beavertail antenna of aircraft altimeter with a conformal or low-profile antenna so as to improve the aerodynamic characteristics of high speed aircraft and make it less liable to damage on the ground. Based on the above-mentioned requirements，We make preliminary theoretical and experimental investigafions about the rectangular microstrip antennas for aircraft altimeter. The main results are presented in this paper. Test f1ights have shown that the pointer f1icker of altimeter is distinctly reduced during level f1ights.