It seems that most feedlines used for HF dipole antennas are 50 ohm impedance such as RG8 and RG58. Assuming the dipole is 1/4 wavelength. above ground, its characteristic impedance should be around 75 ohms according to my ARRL handbook. I know that a 50 ohm line is not a huge mismatch, but wouldn't it be better to use a 75 ohm feedline and use an ATU in the shack to match to 50 ohms? Along those lines, I have a good source of foil-shielded RG6 CATV cable, the only issue I see is connecting to the aluminum shield wires. I think that a crimp-on connector and sufficient waterproofing should work for the antenna side. The lighter weight of RG6 vs. RG7 seems like a big advantage when constructing a dipole that's hung from the ends. I did find a good article on using RG6 for transmission lines here: http://vk1od.net/transmissionline/RG6/ That says RG6 can handle 500W at 30Mhz and more at lower frequencies.