A common solution for shifting the 80m
resonance up to 75m is to add a capacitor
in the center of the wire, typically 100 to
470pF or so.
Like this.That can be applied to either an OCFD or EFHW.
Again, you have to pay attention to which bands
have a relatively low SWR, rather than assuming
you can force it to work on the others with a
tuner in the shack. Some traditional dimensions
for OCFDs, for example, present a very high
impedance on 15m, and trying to force it can
damage the balun (depending on power level,
design, etc.). There are ways around that, but
sometimes they just shift the problem to another
band instead.
Manufacturers often want to sell you a single
antenna by claiming it will work on all bands,
but you have to read the fine print. In one
case, an antenna claimed to “work” on 80m
was measured to be 1% efficient on that band,
which might not meet your definition of “work”.
In some cases, the limitations may be acceptable
to your operating preferences, and other times
they wont be.