Sorry to say, your basic plan is not workable
, i.e., to run coax from the tuner to the top of the mizzen mast and then connect to an insulated triatic. BIG WASTE OF EFFORT AND MONEY
. Signal losses with such an antenna would be horrendous, no matter which coax you choose.
By the way, the antenna starts AT THE TUNER. All that coax running up the mizzen would be part of the antenna....and a very lossy part at that.
The question is not "which coax to run?". Rather, it should be, "what kind of antenna to rig?".
A much better plan is one like my friend, Scott, has on his 60' ketch. He insulated one of the shrouds for the mizzen and used that for the lower part of his antenna, then put a jumper between the top of that shroud
and the insulated triatic stay.
This means that the whole shebang from the chainplates on the mizzen shroud
to the top of the triatic is the actual antenna.
You could then install the tuner belowdecks and feed the chainplate.
This yields a MUCH more efficient multi-band antenna. Scott uses his for both ham and marine
nets, with great success.
HF antennas on a ketch are quite difficult to rig, especially if there are permanent backstays
on the main mast. Too much wire and too much interference
. I have another very active ham friend on a smallish ketch who has that problem. No matter what he tries, his signal strength is always weaker than other similar sized boats with sloop