Antenna Trap Design Calculator

A trap is a parallel LC (inductor-capacitor) resonant circuit spliced into a wire antenna's element to make it electrically shorter on higher bands while leaving the full length available on lower bands — the classic technique behind multi-band trap dipoles and verticals. This calculator starts from the standard resonance relationship, f = 1/(2π√(LC)), and solves it for the capacitance needed to resonate your chosen inductor at the trap frequency you enter. From there it derives the inductive and capacitive reactance at resonance (which are equal by definition at resonance) and estimates the trap's impedance using Z = Q × XL with an assumed Q of 200, a reasonable middle-of-the-road figure for an air-wound coil — real coils can run anywhere from about 100 to 300 depending on wire gauge, spacing, and core material, so treat this as an estimate rather than a guarantee.

The coil turns needed to physically wind that inductance on your chosen form diameter come from Wheeler's approximation for single-layer air-core coils, assuming a square-ish winding where the coil's length roughly equals its diameter; it also reports the wire length and turn spacing you'll need to actually wind it. The 3 dB bandwidth (frequency span over which the trap stays effectively resonant) is estimated as center frequency divided by Q — a higher-Q trap is sharper but also less forgiving of small construction errors, so budget for trimming the capacitor slightly after winding to hit exact resonance.