The conversion efficiency of electric microwave signals into surface acousticwaves in different types of superconducting transducers is studied with the aimof quantum applications. We compare delay lines containing either conventionalsymmetric transducers (IDTs) or unidirectional transducers (UDTs) at 2.3 GHzand 10 mK. The UDT delay lines improve the insertion loss with 4.7 dB and adirectivity of 22 dB is found for each UDT, indicating that 99.4 % of theacoustic power goes in the desired direction. The power lost in the undesireddirection accounts for more than 90 % of the total loss in IDT delay lines, butonly ~3 % percent of the total loss in the FEUDT delay lines.
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