Currently, relay approach is a feasible scheme in solving the communication blackout problem when spacecraft reentries. This paper builds up a model of plasma sheath at leeward, analyzes influence of reentry spacecraft pitch angle on signal transmission in sheath, and contrasts the transmission of Ka-band signals with S-band signals in the leeward plasma sheath under initial pitch angle of the spacecraft reentry atmosphere. The result shows that the attenuation of signal electromagnetic wave in plasma sheath decreases with increase of pitch angle; under condition of the same pitch angle, the transmission of Ka-band signal is significantly superior to the transmission of S-band signal. Compared to S-band signal, Ka-band signal's the transmissivity on leeward increases by at least 55%, with decrease on the attenuation by at least 33.69 dB. It's suggested that Ka-band signal can be chosen as the communication frequency between reentry spacecraft and relay satellite. Relay approach is Proposed by relay optimum proposal, providing ideas to solve the reentry spacecraft communication blackout problems.