Why start long
It is far easier to prune wire than to add it once the antenna is up. Leave 2–3 % extra on both the inner and outer sections and trim after the first measurements at operating height.
Trap Dipole Calculator
Starting dimensions for a classic two-band (or three-band) trap dipole. The traps isolate the outer wires on the higher band and let the whole length work on the lower band.
How a trap dipole behaves in practice
A trap is a parallel-resonant circuit that looks like a high impedance near its design frequency. On the higher band the traps effectively open the circuit, so the inner sections alone form a half-wave dipole. On the lower band the traps look more like a small series inductance and the outer wires come into play, lengthening the antenna to resonance.
Real traps are never perfect. Component Q, lead length, and the physical spacing of the trap affect both the isolation on the high band and the exact loading on the low band. That is why every trap dipole still needs careful trimming once it is in the air. Start long on both the inner and outer sections, raise the antenna, measure both frequencies, and trim in small equal steps while watching SWR.
Coaxial traps (a few turns of coax wound into a coil) are popular for home construction because the same coax provides both the inductance and the distributed capacitance. Commercial traps or discrete L-C traps also work. Whatever you use, keep the voltage rating well above the expected RF voltage at the trap location.
Two-band trap dipole
Inner half (each side)—
Outer extension (each side)—
Total length each side—
Overall tip-to-tip—
Practical tip: Place the traps at the calculated inner-half points. Cut the outer wires a little long. After raising, first trim the outer sections for the lower band, then fine-tune the inner sections (or trap capacitance) for the higher band if needed. Expect interaction — small changes on one band usually move the other.
Trap design notes
Inner half-wave ≈ 468 / fhigh × vf factor
The traps sit at the ends of the higher-band half-wave. Outer length is chosen so that the whole (inner + trap + outer) is approximately a half-wave on the lower band, with a small allowance for the residual reactance of the traps.
- Coaxial traps: 6–12 turns of RG-58 or similar on a short former is a common starting point for 20 m / 40 m.
- Keep trap Q reasonably high; lossy traps reduce efficiency and heat under power.
- Weather-proof the traps thoroughly — moisture shifts the resonant frequency and can cause arcing.
- A 1:1 current balun at the centre feedpoint is still recommended.
Three-band versions
A third pair of traps can be added for a third band (classically 80/40/20). The same principle applies: each successive outer section brings the next lower band into resonance. Complexity and interaction increase quickly, so many builders prefer a fan dipole or separate antennas beyond two bands.
Next measurements
After the lengths look close, check SWR and resonance on both bands with an analyser. The Antenna Trimmer tool helps translate a measured frequency offset into a length change.
Assumptions & limitations
- Trap isolation is idealised. Real traps have finite Q and residual reactance that shifts both the high-band and low-band resonances.
- Outer-section length is approximate; interaction between bands means both sections usually need iterative trimming.
- No account is taken of trap physical size, lead inductance or weather sealing effects.
Things that bite builders
- Traps that heat under power are lossy — check after a few minutes key-down at your operating power.
- Moisture inside a trap moves the resonant frequency and can cause arcing. Seal thoroughly.
- Trimming the outer wires for the lower band almost always moves the higher-band resonance a little; expect to re-check both bands.
Next step in the workflow
Use the calculated lengths as a build starting point, then move to the tools that answer the next practical questions.