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As the schematic clarifies, the TL impedance is 50 ohm.
Apart from this point, it's unclear however, because it doesn't tell about the TL geometry. The coupling of both lines definitely matters, there may be possibly a core not shown in the schematic. From the schematic allone, it can't be determined, if the structure is operating as a resonant (λ/4 or whatever) transmission line transformer or a wideband balun.
to Swild: Thanks, if the length is λ/4, your answer is right, what I really want to know is the wide-band case, which often shows itself as a λ/8 cable (at mid-band)
to FvM: if TL impedance is 50, than the length is irrelevant, the ratio will always be 1:1. As to the coupling effect you mentioned, I think the other cable is used here only to balance the (supposed) balanced port of transformer, and there is no signal at that cable, so in the first order, no coupling will be detected.