A method to reduce self-interference in a single antenna frequency modulated continuous wave (FMCW) altimeter system , the method comprising: setting a length of a transmission line to a selected length to ensure a composite-leakage signal at the antenna-input of a receiver mixer has a linear phase across an operating bandwidth of the altimeter system and to ensure composite-leakage-amplitude nulls are outside the operating bandwidth, wherein the composite-leakage signal is a coherent superposition of an antenna-reflection-leakage signal and a circulator-leakage signal at an antenna input of the receiver mixer , wherein the transmission line communicatively couples an antenna to a transceiver circulator coupled to the antenna-input of the receiver mixer ; tuning an input impedance of the antenna so: a variation of the amplitude across frequency of an antenna-return signal reflected from the antenna is relatively flat and thus does not have any significant antenna-return-loss amplitude nulls; phase response of the composite leakage signal input at the antenna-input of the receiver mixer is linear across a sweep-frequency range of the altimeter system ; and an output voltage of a receiver mixer is a direct current (DC) voltage with reduced alternating current (AC) voltage produced by internal leakage across the operating bandwidth; and adjusting a phase of a local oscillator signal input to a local-oscillator-input of the receiver mixer by adjusting a phase length of a local oscillator (LO) delay line to maintain a quadrature relationship to a phase of the composite-leakage signal input to an antenna-input of the receiver mixer across the operating bandwidth of the altimeter system . setting a length of a transmission line to a selected length to ensure a composite-leakage signal at the antenna-input of a receiver mixer has a linear phase across an operating bandwidth of the altimeter system and to ensure composite-leakage-amplitude nulls are outside the operating bandwidth, wherein the composite-leakage signal is a coherent superposition of an antenna-reflection-leakage signal and a circulator-leakage signal at an antenna input of the receiver mixer , wherein the transmission line communicatively couples an antenna to a transceiver circulator coupled to the antenna-input of the receiver mixer ; tuning an input impedance of the antenna so: a variation of the amplitude across frequency of an antenna-return signal reflected from the antenna is relatively flat and thus does not have any significant antenna-return-loss amplitude nulls; phase response of the composite leakage signal input at the antenna-input of the receiver mixer is linear across a sweep-frequency range of the altimeter system ; and an output voltage of a receiver mixer is a direct current (DC) voltage with reduced alternating current (AC) voltage produced by internal leakage across the operating bandwidth; and adjusting a phase of a local oscillator signal input to a local-oscillator-input of the receiver mixer by adjusting a phase length of a local oscillator (LO) delay line to maintain a quadrature relationship to a phase of the composite-leakage signal input to an antenna-input of the receiver mixer across the operating bandwidth of the altimeter system . <EOS>