  The conversion efficiency of electric microwave signals into surface acoustic
waves in different types of superconducting transducers is studied with the aim
of quantum applications. We compare delay lines containing either conventional
symmetric transducers (IDTs) or unidirectional transducers (UDTs) at 2.3 GHz
and 10 mK. The UDT delay lines improve the insertion loss with 4.7 dB and a
directivity of 22 dB is found for each UDT, indicating that 99.4 % of the
acoustic power goes in the desired direction. The power lost in the undesired
direction accounts for more than 90 % of the total loss in IDT delay lines, but
only ~3 % percent of the total loss in the FEUDT delay lines.
