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Superconductive analog devices are being developed which utilize long electromagnetic delay lines, typically 2 to 50 m long on a single planar substrate Criteria for the choice of substrates for these devices are established. Desirable characteristics are low dielectric loss, low dispersion, high dielectric constant, dielectric isotropy and compatibility with fine-line lithography of robust superconductive films such as niobium. A number of different substrates have been examined including crystalline and vitreous quartz, sapphire of various orientations, alumina, calcium fluoride, and silicon. Silicon is shown to be an excellent substrate for most envisioned devices. Experimental results indicate that a 100-m long line with a total loss of 8 dB at 5 GHz is feasible. A technique to allow the use of a 25-μm-thick, 5-cm-diameter silicon substrate to realize an 18-m-long niobibm line is discussed.