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Coaxial trap calculator

How it works

A trap is a parallel tuned circuit inserted in the wire. At its own frequency it presents a high impedance and cuts off the rest of the antenna; below resonance it behaves as a loading inductance. In a coaxial trap one and the same piece of cable provides both halves of the circuit: the winding is the inductance, the cable itself is the capacitance.

N turnsresonancewireextensionformer Ø50 mm
The winding gives the inductance, the coax itself the capacitance; together they resonate
What to compute
Cable type
Capacitance per metre (pF/m)
Cable outer diameter (mm)
Former diameter (mm)
Length of wound cable (m)
Transmitter power (W)
Result
Resonant frequency
Cable length
Number of turns
Winding length
Inductance
Capacitance
Reactance at resonance
Estimated voltage across the trap (at Q ≈ 200)

📐 The formulas​

Number of turns:

N = L_cable / ( π × (D_former + d_cable) )

Inductance of the winding (Wheeler's formula, metric form):

L[µH] = r² × N² / ( 25.4 × (9r + 10l) )

where r is the winding radius to the centre of the cable and l is the winding length, both in mm.

Capacitance and resonance:

C = L_cable[m] × C_per_metre[pF/m]
f = 1 / ( 2π × √(L·C) )

Symbols:​

  • C_per_metre — the cable's rated capacitance per metre: 101 pF/m for RG-58, 54 pF/m for RG-6
  • d_cable — the outer diameter of the cable including the jacket
Where this calculator stops:
  • Wheeler's formula is good to a few percent; a trap gets finished on the analyser regardless
  • The radius is measured to the centre of the cable, not to the former — on a thin former the difference shows
  • The self-capacitance of the winding and the effect of the wire alongside are not accounted for
  • The voltage estimate assumes a Q of 200 and depends heavily on the real Q

🎯 In practice​

  • A 40/80 m dipole. Two traps for 7 MHz in the legs, and one antenna covers two bands.
  • A 20/15/10 m vertical. A few traps up the mast instead of three separate antennas.
  • Reworking someone else's design for a different cable or former.
Practical advice:
  1. Tune the trap before it goes into the antenna: on the bench, with an analyser, on the dip or the peak.
  2. Fix the turns with cable ties or heatshrink — a millimetre of movement shifts resonance by tens of kilohertz.
  3. The wire beyond the trap is shorter than a plain dipole calculation gives: below resonance the trap adds inductance.
  4. For 100 W use RG-58 or better; for a kilowatt a coaxial trap is a poor idea.
  5. The trap must be sealed: water inside moves the capacitance and ruins the antenna.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator
UR3PKI. «Coaxial trap calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/antennas/coax_trap (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.

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