Airborne Radio Altimeter Antennas

Abstract

The radio altimeters are considered simple frequency modulated continuous waves (FMCW) radars that operate in C-band. Radio altimeters' primary function is to give aircraft height during the approach, landing, and climb stages of an aircraft. This precise height is critical in autonomous landing. Radio altimeters are also used in manual landings to inform the pilot when it is time to start a "flare maneuver," which is executed immediately before touchdown to minimize the ground impact on the aircraft. The time difference between transmitting an electromagnetic signal by the transmit antenna and receiving it back at the aircraft receive antenna after ground reflection is measured. As long as the wave's trajectory is vertical, down, and up, the duration is a function of aircraft altitude. Two distinct antennas on the aircraft's bottom are always employed to transmit and receive signals at the same time and that is to avoid using an internal circulator which causes misleading altitude data readings due to the internal leakage resulting from the continuous transmission and reception. Although using two separate antennas is a typical solution for the FMCW radar, the separation distance between the two antennas should be decreased as small as possible to avoid measuring the slant distance instead of measuring the vertical distance. However, as the separation distance between the transmitting and receiving antennas decreases, the mutual coupling effect between the two antennas arises. So, both transmit and