Find the wire length for an end-fed half-wave antenna and the harmonic bands it also covers, for any frequency.
An end-fed half-wave is a half-wave wire fed at the end through a 49:1 transformer, so it needs little or no counterpoise. Length in feet = 468 / frequency in MHz. Because it also resonates on harmonics (2x, 3x, 4x the frequency), one wire can cover several bands.
Length in feet is 468 divided by the lowest frequency in MHz, the same as a half-wave dipole. For a 40m EFHW at 7.15 MHz that is about 65.5 feet (20.0 m).
A half-wave on the lowest band is a full wave on twice the frequency, one and a half waves on three times, and so on. Those harmonics land near other ham bands, so a 40m EFHW also works on 20m, 15m and 10m with the right transformer.
At the end of a half-wave the impedance is very high (a few thousand ohms). A 49:1 unun transforms that down close to 50 ohms. A short counterpoise of a few feet helps stabilise the match.