If you have a length of twisted pair cable, just untwist an end to get the two arms of a dipole (horizontal, far from house), then run the twisted part to your shack window (at exact right angle to the dipole for as long as possible, to get the best balancing), then use a differential ATU to tune the whole thing to any SW frequency. Typical length of wire (including the dipole and both wires of the line) should be about 3 half wavelengths (for exemple), but the coils in the ATU can change this widely).
In fact, this is the doublet antenna, tunable to almost any frequency by the ATU at your hand near the receiver. Instead, the total length maybe about 5 or 7 half wavelengths, or only 1 wavelength (for example on 75m and close to the house). The twisted line can be coiled near the house entrance and/or near the central pole to better fight the common mode.
Wrong questions: the complex impedance of the doublet, the characteristic impedance of the line. The right question is to tune the full assembly of the doublet + the bifilar line + the ATU with an odd number of half wavelengths (approx.).
I tried the doublet years ago, when my large horizontal loop broke down, just using the remnants, and got immediately much better DX results. Thinking back, maybe a part of the not so good "sky hook" results was due to the line not in the symetry plane of the loop (it was easier with the remnants to place the line just at equal distances from both arms), but the doublet has also a better differential behaviour (the ends are nodes!), and is easier to place far from the house.