The 18 gauge shouldn't present a problem, for Power, nor Loss.
It , however may present a problem, in the winter, if you get
ice loading (can streatch, and also break, if has enough stress
(extra weight) applied to it) , but, as far as qrp, on the low
bands, you may ve amazed at the dx you can pull in , even with
power in the milliwatt range, and, with a couple of the older
(10 watt) rigs, like the ft301-s, or the ft-7 (and kenwood,
Icom, and Ten-tec made them, too!), can work reliably several
hundred miles on 75-40 meters, and THOUSANDS on 20-15, and 10!
Have fun-- Jim NN7K



Steve Nosko wrote:
Ditto. No and no.


"Ed" wrote in message
. 93.175...

...a dipole antenna for 15 Meters from insulated 18 guage battery
wire ...
But, will I have to upgrade my wire when I go to 50/100 watts output?


NO


...is the insulation an impediment to the functioning


NO

Put it up. For a 40M dipole, I have insulated 20 guage (with the ends
threaded through the end insulators then doubled back and wrapped around the
antenns for about three feet, for easy length adjusting if needed).

73, & Enjoy Steve, K.9,D'C;I