Antenna Trap Calculator

Design precise LC traps for multi‑band antennas (dipoles, verticals, Yagis). Compute inductance or capacitance from resonant frequency — includes interactive LC circuit visualization, trap Q estimation, and practical build notes.

MHz
pF
µH
Enter any two values (Frequency + Capacitance or Frequency + Inductance). The third field auto‑updates for parallel resonant trap design.
? 40m Band (7.15 MHz, 100 pF)
? 20m Band (14.2 MHz, 68 pF)
⚡ 15m Band (21.2 MHz, 47 pF)
? 10m Band (28.5 MHz, 33 pF)
? 80m Trap (3.8 MHz, 220 pF)
Local & Private: All calculations run in your browser – no RF data leaves your device. Perfect for field day planning.

Understanding Antenna Traps: Theory & Practical Design

An antenna trap is a parallel resonant LC circuit inserted into an antenna element to electrically “trap” or block currents at a specific frequency, while appearing nearly invisible at other frequencies. This allows a single physical antenna (dipole, vertical) to operate efficiently on multiple amateur radio bands. The trap acts as a high impedance at its resonant frequency, effectively shortening the antenna for higher bands while leaving lower band currents unaffected.

Thomson resonance formula: f0 = 1 / (2π √(LC))

Where f0 in Hz, L in Henry, C in Farad. For practical RF design: fMHz = 1 / (2π √(LµH · CpF · 10-12 · 10-6)).

Why Use Traps? Advantages & Bandwidth Considerations

Traps reduce the need for multiple dedicated antennas. A classic trapped dipole (e.g., 40/20/15/10m) uses two or three traps per leg. However, traps introduce small power loss (typically <5% when well‑built) and reduce bandwidth slightly. This calculator helps you select standard capacitor values (silver mica, high‑voltage NP0/C0G) and compute the required air‑core or toroidal inductor. High Q traps (Q > 100) improve efficiency and reduce heating. Typical unloaded Q for air‑coil traps: 120–200 at 7 MHz.

Design Methodology

  1. Choose target frequency (e.g., 7.15 MHz for 40m).
  2. Select a practical capacitor (common values: 47, 68, 100, 150 pF). Higher C yields lower L and physically smaller coil.
  3. Compute required inductance using the formula L = 25330 / (fMHz² · CpF) [µH].
  4. Construct inductor using enameled copper wire on a rigid former (PVC, fiberglass).
  5. Verify resonance with antenna analyzer or VNA.

Practical Trap Examples & Reference Table

Band (m) Frequency (MHz) Typical C (pF) L (µH) Coil turns (approx, 25mm dia)
80m 3.8 220 7.96 28–32
40m 7.15 100 4.96 20–22
20m 14.2 68 1.84 13–15
15m 21.2 47 1.20 10–12
10m 28.5 33 0.92 8–10
Case Study: 40/20m Trapped Dipole

A radio amateur builds a trapped dipole for 40m and 20m bands. Using our calculator with f = 7.15 MHz and C = 100 pF yields L ≈ 4.96 µH. The trap is placed 6.5m from the feedpoint on each leg. On 40m the trap is below resonance, presenting low impedance and allowing the full antenna to radiate. On 20m (14.2 MHz) the trap resonates, creating a high impedance that electrically isolates the outer section, shortening the antenna for 20m operation. Field measurements show SWR < 1.5:1 on both bands. This tool simplifies component selection and reduces cut‑and‑try iterations.

How to Use the Calculator & Interpretation

  • Enter desired resonant frequency (MHz). Then input either a capacitor value (pF) to solve for inductance, or an inductance (µH) to solve for capacitance.
  • The third field updates instantly. All results follow the parallel resonant trap formula.
  • Estimated Q factor uses typical inductor loss: Q ≈ 0.5 * sqrt(L/C) / R_coil approx, practical estimator Q ≈ 0.5 * (XL / ESR). Default ESR assumption 1.5Ω for air coil. For precise designs, use Q meter.
  • The LC product displays the product (L × C) which is constant for a given frequency: f²·L·C = 25330 (f in MHz, L in µH, C in pF).

Expert Tips for Building Reliable Traps

  • Voltage rating: Use capacitors rated for at least 500V (or 1kV for QRO). Silver mica or high‑Q ceramic are optimal.
  • Self‑resonance: Ensure inductor self‑resonant frequency is >2× trap frequency. Keep leads short.
  • Moisture protection: Seal traps inside UV‑resistant enclosures (PVC pipe).
  • Temperature stability: NP0/C0G capacitors minimize frequency drift.

Derivation of the LC Formula for Traps

From the basic resonance condition: XL = XC → 2πfL = 1/(2πfC) → f² = 1/(4π²LC). For engineering units: fMHz = 10⁶ Hz, LµH = 10⁻⁶ H, CpF = 10⁻¹² F. Substituting: (f·10⁶)² = 1/(4π² · L·10⁻⁶ · C·10⁻¹²) → f²·L·C = 1/(4π²·10⁻¹²·10⁶·10⁻⁶) → simplifies to f²·L·C = 25330. This constant is widely used by RF designers. Hence L (µH) = 25330 / (fMHz² · CpF) and C (pF) = 25330 / (fMHz² · LµH).

Antenna Trap Calculator Limitations & Real‑World Factors

While the LC formula is exact, stray capacitance and inductor parasitic capacitance will shift resonance by 1–3%. Always verify with an antenna analyzer. Additionally, trap environment (nearby metal, proximity to other traps) alters effective L and C. This calculator provides an ideal starting point; final trimming using a VNA or grid dip meter is recommended. The estimated Q offers relative performance insight; higher Q yields sharper rejection and lower loss.

Frequently Asked Questions

A parallel LC trap (this calculator) exhibits high impedance at resonance and is inserted in series with the antenna element. A series trap (low impedance at resonance) is rarely used for antenna traps because it would short the signal. Our tool focuses on the parallel configuration (standard for multi‑band wire antennas).

Absolutely. The formula works for any frequency. However, for VHF/UHF, parasitic effects become critical; microstrip or helical resonators may replace discrete LC. For HF (1–30 MHz), this design remains gold standard.

With 500V capacitors and #18 AWG enameled wire, traps easily handle 100W SSB/CW, 200W with careful construction. For 1.5kW, use vacuum capacitors or heavy‑duty high‑voltage ceramics.

The ideal formula neglects self‑capacitance. For accurate builds, keep coil length/diameter ratio low, and add ~1–2 pF stray to your capacitor value. We provide advanced notes for fine tuning.

Use a NanoVNA or antenna analyzer: connect the trap in series between two 50Ω resistors and observe the notch in S21 transmission (high impedance). Peak in S11 reflection also works.
References: ARRL Antenna Handbook (2024), “Traps for Multiband Antennas” – J. Hallas, W1ZR; R. Dean Straw, “LC Trap Design”; IEEE Standard 145‑2013. All formulas double‑checked with classic RF design theory. GetZenQuery tech team, May 2026.