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Ferrite rod coil calculator

When you need this

A receiver's loopstick antenna, or a choke on a ferrite rod. The model accounts not only for the permeability of the ferrite but also for the length of the winding and its offset from the middle of the rod.

Coil on a ferrite rod: rod length lf, diameter df, winding length lc
lf — rod length, df — rod diameter, dc — former diameter, s — offset of the coil from the middle
Required inductance L (µH)
Ferrite rod
Rod length lf (mm)
Rod diameter df (mm)
Initial ferrite permeability μr
Coil
Former diameter dc (mm)
Offset of the coil centre from the rod centre s (mm)
Wire diameter dw (mm)
Winding pitch p (mm)
Estimate the pitch automatically
Result
Number of turns N
Gain from the rod, Lf/Lair
Winding length lc

📐 Formulas​

A rod is an open magnetic circuit: the flux closes through the air, so the real gain is set by the geometry, not by the permeability of the material. For a rod with μi = 600 the gain comes out at 40…70 — an order of magnitude less.

Step 1. The inductance of the coil «in air» — Lundin's formula minus Rosa's correction, plus a correction for a spaced winding:

ΔL_pitch = −10⁻⁴ · Dk · (p/dw − 1) · (12 − p/dw)/4 · N      for N greater than 4

Step 2. The gain from the rod:

free    = lf − lc
stray   = 5·10⁻⁴·π·free / (ln(2(lf+df)/df) − 1)
damping = 1 / (1 + (free/df)^1.4 / (5·μr))
n       = (damping·stray + 0.002·df) / (0.002·dc)

reach   = lc + 0.45·dc
r       = (reach/dc) · 5.1 / (1 + (dc/reach)·2.8)
μfe     = (μr − 1)·(df/dc)² + 1

Lf/Lair = (1 + r) / (1/n + r/μfe)

Step 3. The correction for the offset of the coil from the centre, x = 2s/lf:

kx = 0.995 + 0.3013663x − 7.6027344x² + 71.6178135x³ − 363.8218957x⁴
     + 1021.078226x⁵ − 1604.5491034x⁶ + 1318.707293x⁷ − 440.9943706x⁸

Together: L = L_air · (Lf/Lair) · kx

Notation:​

  • L — inductance [µH]
  • lf — rod length [mm]
  • df — rod diameter [mm]
  • μr — initial permeability of the ferrite
  • dc — diameter of the coil former [mm]
  • s — offset of the coil from the centre of the rod [mm]
  • lc — winding length [mm]
Limits of the calculator:
  • The winding pitch must lie between dw and 2·dw
  • The former diameter between df and 2·df
  • The winding shorter than 3/4 of the rod
  • The end of the coil no closer to the end of the rod than 1/8 of its length
  • This is the most empirical model in the section: expect errors of tens of per cent, especially on short rods

🎯 Where it is used​

  • Loopstick antennas for long-, medium- and short-wave receivers.
  • Chokes on ferrite rods.
  • Estimating the effective permeability of a particular rod.
Practical notes:
  1. The «offset» field is not a formality: sliding the coil along the rod is exactly how a loopstick is trimmed to the inductance you want.
  2. The more of the rod the winding covers, the lower the gain: a short free end closes the flux less well.
  3. The model makes sense for permeabilities above roughly 100. For low-permeability rods the result will be far too optimistic.

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

How to cite this calculator
UR3PKI. «Ferrite rod coil calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/coils/ferrite_rod (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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