I will be building a center fed 80 m dipole, 62.5 feet
each side of center. The ends will be about 23 feet above
ground but the center is not supported to it sags to about
18 feet. I will feed it directly with about 25 feet of
RG-213 50 ohm coax and connect it to an antenna tuner in
the shack that can tune from 50 to 500 ohms.

This should work okay on 80 m, right?

How well will it do on 20 m?


For 80 it will work fine for the heights you have to work with. As most
always the higher the beter. If you want to work 20 meters or other bands
with the same antenna you might want to feed it with open wire or go to an
off center fed or (yuck) maybe the g5rv type.