I'm wanting to build an antenna system utilizing twin lead feedline running to an 80 meter doublet. I'd like to avoid the typical hassles that accompany 450 or 600 ohm ladder line so I'm considering using simple 2 conductor wire as the feedline. I found an online impedance calculator that suggests that regular 2-conductor 14 gauge lamp wire would have a close-to-acceptable impedance, but unfortunately lamp wire typically isn't rated to 600 volts (I'm planning on running several hundred watts of power to the antenna), nor would I expect it to survive long-term outdoors. So I'm thinking a good 14/2 UF-B cable might be a workable solution, except for the presence of a ground wire between the 2 conductors in the cable.
My question is this: what effect if any would the presence of the ground wire have on the HF transmission characteristics of the cable, as compared to an identical transmission line that didn't have the ground wire?
It seems to me (at least in theory) that the simple presence of the ground wire itself should have no effect at all, because of the way that a twin lead feedline behaves in conjunction with a doublet at any given frequency: at any particular point along the length of the feedline, the voltage and/or current in either of the conductors exactly matches but is 180 degrees out of phase with the energy in the other conductor. So the net effect that the near-field electromagnetic wave emanating from each conductor has on any nearby metal object (including a ground wire inside the cable) is ZERO. In fact, consider why a properly setup twin lead feedline presents no discernible RF issues in the shack; the RF energy radiating from either conductor is effectively and exactly cancelled by the opposite-phase energy radiating from the other conductor.
I'm not concerned at this time with any considerations of cost, etc., so please refrain from such comments as "Jeez, that's expensive cable, you can build a ladder line much cheaper!". My main concern is whether the presence of a third wire in the jacket would present any problems with the feedline function of the cable.
Please advise, and kind thanks in advance.