As a rule, more wire is "said" to be better. Theoretically, you can make a
different case but just try transmitting at 160m with a tuner and 80m
dipole. At that MF frequency, your tuner will try to reflect so much energy
that you will probably hear the crackling and may see a corona, no matter if
it is rated at 3KW and your are transmitting at 100W.
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It all depends what feeder you use.
When using twin feeder and a matching unit to suit both such a feeder
and the traditional asymmetric 50 Ohms output of any
transmitter/receiver , an 80m dipole,or a dipole of any length for that
matter, can be readily used on 160 m provided that half the dipole + the
length of the feeder is roughly an odd number of quarter wavelengths at
the operating frequency (assuming the velocity factor of the feeder is 1)
In this situation the current in the twin feeder at the matching unit is
relatively high ,hence the impedance is low ,because the voltage is
relatively low (hence no crackling/corona problems.

Cecil Moore's web site shows how he adds lengths of twin feeder to suit
the different bands.


Frank GM0CSZ / KN6WH