Common-mode choke
Adds a high impedance to unwanted RF current flowing on the outside of a coaxial cable or other conductor.
Ferrite Cores, Chokes, Baluns & Ununs
HFDXARC RF Engineering Guide
Ferrite Cores, Chokes, Baluns & Ununs
A practical guide to choosing ferrite material, winding common-mode chokes and selecting transformer ratios for amateur-radio antennas.
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What ferrite actually does
Transformer core
Couples energy between windings so an impedance ratio can be transformed, for example 50 Ω to approximately 2,450 Ω.
Loss and heat
Ferrite is deliberately lossy at selected frequencies. Too much flux, too few cores or the wrong mix can create excessive heating.
Important distinction: a choke suppresses common-mode current; an impedance transformer changes the impedance presented to the radio. Some assemblies perform both functions, but they are not automatically interchangeable.
Core selection
Useful ferrite mixes for amateur radio
Published material curves, measured impedance and the intended winding style matter more than a single headline frequency range.
Broadband choke
Mix 31
Excellent for broadband common-mode suppression through LF and HF, and often useful into low VHF.
- Feedline chokes
- Mains and control-cable RFI
- 160–10 m broadband choke builds
HF transformer
Mix 43
A widely used general-purpose material for HF impedance transformers and many practical choke designs.
- 49:1 and 64:1 EFHW transformers
- 9:1 ununs
- HF current baluns
Higher frequency
Mix 61
Lower permeability and lower loss at higher frequencies. Commonly selected for VHF transformer work and tuned HF applications.
- VHF/UHF transformer applications
- Higher-frequency broadband networks
- Not the first choice for a general 160–10 m choke
LF/MF
Mix 77
High-permeability material suited to lower-frequency transformer and suppression work.
- LF and MF interference
- Receive transformers
- Low-frequency power conversion
| Application | First core to consider | Practical note |
|---|---|---|
| Broadband HF feedline choke | FT240-31 | Choose turns from measured impedance data; avoid relying on turns alone. |
| 49:1 EFHW transformer | FT240-43 | Use suitable high-voltage insulation and verify loss at the lowest band. |
| 9:1 random-wire unun | FT240-43 | The antenna system still requires a counterpoise or defined return path. |
| VHF transformer | Mix 61 | Lead length and winding capacitance become increasingly important. |
| LF/MF receive transformer | Mix 77 | High permeability helps obtain useful inductance with fewer turns. |
Common-mode control
Why a feedline choke works
The wanted differential-mode RF travels inside the coaxial transmission line. Unwanted common-mode current flows on the outside surface of the shield. A well-designed choke presents a high impedance to that unwanted current without disturbing the wanted feedline current.
- Reduces feedline radiation
- Helps stabilise the antenna pattern and feedpoint impedance
- Reduces RF in the shack
- Can reduce noise coupled onto the outside of the coax
Illustrative coax-through-toroid arrangement. Real turn count depends on core mix, cable diameter and target band.
Transformer ratios
Turns ratio and impedance ratio
The impedance ratio is the square of the turns ratio. A 7:1 turns ratio therefore gives approximately 49:1 impedance transformation.
Zsecondary / Zprimary=(Nsecondary / Nprimary)2
49:1
EFHW unun
Nominally transforms 50 Ω to about 2,450 Ω. The actual EFHW feedpoint impedance varies with installation, band and counterpoise arrangement.
9:1
Random-wire unun
Nominally transforms 50 Ω to about 450 Ω. It does not make an arbitrary wire resonant and normally works with an ATU.
4:1
Balanced transformer
Nominally transforms 50 Ω to 200 Ω. Current-balun topology is usually preferred where equal and opposite currents are required.
1:1
Current balun / choke
No intended impedance transformation. Its job is to impede common-mode current and maintain balanced antenna currents.
Single FT240-43 core shown clearly. For higher-power designs, two or more identical cores may be physically stacked to increase core area and reduce flux density.
Power handling
Why cores overheat
Heating can result from core loss, winding resistance, excessive voltage per turn, poor matching, or operation outside the intended frequency range.
Use more core area
Stacked identical cores share magnetic flux and are common in high-power transformers.
Use enough turns
Too few turns can produce excessive magnetising current and low-band heating.
Control voltage
High-impedance EFHW feedpoints can produce substantial RF voltage, even at modest transmitter power.
Ventilate and test
Enclosures should tolerate heat and moisture. Check temperature during a sustained carrier test at reduced power first.
Workshop procedure
Test before installing
- Inspect the winding.Check insulation, spacing, sharp bends, connector strain relief and enclosure clearances.
- Measure transformation.Use suitable non-inductive resistor loads and check the expected impedance transformation across the intended band.
- Measure choking impedance.A VNA fixture or established published design is preferable to guessing from core size and turn count.
- Apply power gradually.Start at low power, use a stable load, then monitor SWR and core temperature during a sustained transmission.
- Retest in the real installation.Coax length, counterpoise, antenna geometry and nearby conductors all affect the completed system.
Safety: Core choice, winding count and power rating must be verified across the intended operating range. Test the finished unit before applying high power.
Quick reference
Common build choices
| Device | Typical job | Common first-choice material | Key caution |
|---|---|---|---|
| 1:1 current choke | Suppress common-mode feedline current | Mix 31 | Turn count must suit the target bands |
| 4:1 current balun | Balanced 200 Ω to 50 Ω transformation | Mix 43 or 31 | Topology matters; avoid assuming any 4:1 winding is a current balun |
| 9:1 unun | High-impedance random wire to 50 Ω system | Mix 43 | Usually requires an ATU and return path |
| 49:1 unun | EFHW feedpoint transformation | Mix 43 | High RF voltage and low-band core loss |
| 64:1 unun | Higher-ratio EFHW transformation | Mix 43 | May not be superior in every installation; verify by measurement |