It is said that 21MHz, the third harmonic, is carried on a 7MHz full-size dipole However, if you actually try it, you will need to reduce the SWR by using the radio's internal tuner, etc. However, it is not a good idea to use Samway's DXV500ZS linear amplifier with an antenna that has a 22.140MHz SWR drop!
I was looking for information on the Internet and found a Youtube channel called "Momo Channel! on Youtube. https://youtu.be/LbjRYRG9x9I?t=487
It is recommended to put a 4-turn coil 3.5m from the end of the element of a 7MHz dipole. By extending the element end by 4 turns of element winding, the SWR of 7MHz also drops at the target frequency.
However, when I actually tried it, the center frequency of 21MHz was lowered by setting the coil to 6 turns.
There is no capacitor in this coil, just wind the coil.
Screen capture of NanoVNA's nanovna-severs software The center frequency that was 22.140MHz is now 21.220MHz and SWR is 1.2