You’re lucky if your 40m dipole tunes well on 15m. There is an end effect capacitance on the dipole ends and the result of the capacitance is to lengthen the effective length. Or when you have trimmed for the antenna and measure it, it will be a bit shorter than you would expect, the end effect capacitance adds the missing length.
When you go to 15m, 3x the frequency, the antenna is quite a bit too short as you have an antenna 3x the actual length. In my case my 40m dipole is well matched on 40m but the match was too high for comfort on 15m. 1.1 : 1 was at 21.6MHz. I have some end danglers, two pieces of wire I attached to the 40/60m links that add the missing length so use on the 3rd harmonic falls where you want. In my case there are 2 pieces of the same antenna wire 20cms long (8in in old money). They’re not accurately trimmed, adding 20cms to the ends makes the 40m antenna have nice figures upto 21.325, the SWR starts to rise quickly above there. But 2x wire pieces adds a DX band to a workhorse “local” antenna. 