My experience with an EFHW is not quite what is here being described. A wire that is resonant on 40M wasn’t resonant on any of the other bands without some adjustment. I think the reason why is what is known as “end effect.” The wire is two halfwaves on 20M which would want four ends to be “right on” but there are only two.
I got a 40M EFHW to be exactly resonant on 20M by dropping a short stub from the middle of the wire. This stub was about 18 inches in length. It is ignored on 40M, and its length can be adjusted for perfect resonance.
Incidentally,the wire can be deployed in a shape called a half square which leads to a 40M dipole behavior, and a two element bi-direction vertical beam on 20M. While not perfectly resonant on 15M, it IS perfectly resonant on 12M, and with a little help from a tuner, will be usable on 15M.
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73, fred KT5X ( WS0TA )