Some antennas are constructed with traps. What is a trap?

A trap in an antenna system is a resonant circuit made of an inductor (coil) and a capacitor connected in parallel. This combination forms what is known as a parallel LC circuit. At its specific resonant frequency, a parallel LC circuit presents a very high impedance, effectively acting as an insulator or an open circuit. This isolates the outer sections of the antenna, allowing the inner section to resonate at the higher frequency. At lower frequencies, the trap acts more like an inductor, loading the antenna and allowing the entire length to radiate. Distractors like wire-wound resistors or empty metal cans are incorrect because they do not provide this frequency-dependent isolation required to make a multi-band antenna function properly.