What design feature allows a single Yagi antenna to function on the 20-metre, 15-metre and 10-metre bands?

To make a single antenna element resonant on multiple bands, designers use tuned circuits known as element traps. An element trap consists of a parallel inductor and capacitor designed to block RF energy at a specific resonant frequency while allowing other frequencies to pass through. In a tri-band Yagi for the 20, 15, and 10-metre bands, the traps electrically isolate the outer sections of the elements on the higher frequency bands. For example, on 10 metres, the first set of traps blocks the signal, making the antenna act like a physical 10-metre antenna. On lower frequencies, the signal passes through the traps, utilizing the full length of the element. Options like large diameter elements only increase bandwidth on a single band, and a T-match is used for impedance matching at the feedpoint, not for making elements multi-band resonant.