Chapter 1 — Introduction and Scope
This manual covers common-mode choke (CMC) construction for suppressing RF on coaxial feedline shields. Common-mode current — current flowing on the outside of the coax shield — causes RFI, pattern distortion, and RF in the shack. Four CMC types are covered: toroidal coaxial choke (FT-240 ferrite), snap-on ferrite bead choke, sleeve balun (1–4 passes through a bead string), and the “ugly balun” (coax coil).
Chapter 2 — Theory of Operation
2-1 Differential vs. Common Mode
In a coaxial feedline, the desired signal travels as a differential mode: equal and opposite currents on center conductor and inner surface of shield. The core sees equal and opposite H-fields; they cancel. The core presents zero impedance to this mode.
Common-mode current flows on the outside of the shield. The core sees an unbalanced H-field and presents the full choking impedance Z_choke (series impedance) in the common-mode path. Result:
CMR (dB) = 20 × log10(1 + Z_choke / Z_path) Z_path = impedance to which common-mode current flows (typically 50–300Ω) For Z_choke = 5000Ω, Z_path = 100Ω: CMR = 34 dB For Z_choke = 1000Ω, Z_path = 100Ω: CMR = 21 dB
2-2 Core Material vs. Frequency
Core material determines the frequency range of effective choking:
| Material | Best range | Peak μ" | Application |
|---|---|---|---|
| #31 | 1–300 MHz | 3000 | HF antenna feedpoints, coax chokes |
| #43 | 1–100 MHz | 750 | HF balun windings, TLTs |
| #61 | 10–300 MHz | 125 | VHF chokes, 6M and 2M feedpoints |
| #73 | 1–40 MHz | 2500 | HF snap-on beads, power line chokes |
| #75 | 0.5–10 MHz | 5000 | Audio and low-HF chokes |
Chapter 3 — Equipment and Materials
| Design | Core | Passes | Z_choke at 14 MHz |
|---|---|---|---|
| Toroidal coax choke | FT-240-31, ×2 stacked | 12 passes RG-303 | ≥5000Ω |
| Snap-on bead string | Mix #31, 31 mm OD (×6) | 1 pass RG-58 through all 6 | ≥2000Ω |
| Sleeve balun | FT-50-61 (×6 stacked) | 4 passes RG-58 | ≥2000Ω at 50–150 MHz |
| Ugly balun (coax coil) | Air core | 8 turns RG-58, 100 mm dia | ≥1000Ω at 14 MHz |
Chapter 4 — Construction
4-1 Toroidal Coaxial Choke
- Stack two FT-240-31 toroids (same orientation). Secure with a cable tie around the outside.
- Thread RG-303 (or RG-58) coax through the stacked cores 12 times. Keep passes parallel and tight; each pass reduces the impedance uniformity slightly, so 12 is the practical maximum.
- Mount in a weatherproof enclosure at the antenna feedpoint. Connect input coax to radio-side SO-239; output coax to antenna.
4-2 Snap-On Bead Choke
- Open 6 type #31 snap-on ferrite beads (Laird 28A0300-0A2 or equivalent).
- Snap all 6 beads onto the coax at the feedpoint, positioned within 100 mm of the antenna connection. Close each bead securely.
- Secure the bead cluster with a cable tie or wrap of self-amalgamating tape.
Chapter 5 — Operating Procedures
A CMC is a passive device; there is no operational procedure other than installation. Install CMCs at:
- Antenna feedpoint (highest priority)
- Entry panel where coax enters the shack
- Any point where the feedline changes direction sharply
- Audio and control cables if RF on those cables is suspected
Signs of inadequate common-mode chocking: RF in the shack (microphone pickup, TVI), pattern distortion on receive, SWR that changes when hands are placed near the feedline.
Chapter 6 — Calibration
- Measure choking impedance |Z_choke| with a NanoVNA: connect port 1 to the coax shield at one end; port 2 to the coax shield at the other end (center conductor floating). Measure |Z| across the band.
- Target: |Z_choke| ≥2000Ω at all frequencies in the operating range. For type #31 stacked pair: Z should exceed 5000Ω from 7–30 MHz.
- Verify that differential-mode insertion loss is <0.1 dB: connect NanoVNA port 1 to input center/shield, port 2 to output center/shield. S21 must be >−0.1 dB at all HF frequencies.
Chapter 7 — Verification and Acceptance
- Z_choke ≥2000Ω at all operating frequencies.
- Differential-mode insertion loss <0.1 dB.
- After installation: receive noise floor on the antenna must not increase (some CMC failure modes add noise; verify with SDR noise scan before/after).
- Log: date, core type, design (toroid/bead/sleeve/ugly), measured Z_choke at 7 and 14 MHz, differential insertion loss, operator.
Appendix A — Z_choke vs. Passes on FT-240-31
| Passes | Z_choke at 14 MHz (Ω) | Note |
|---|---|---|
| 6 | ∼1500 | Adequate; marginal for dipoles |
| 9 | ∼3000 | Good for most HF antennas |
| 12 | ∼5000 | Excellent; use for multiband or high-power |
| 15 | ∼7000 | Maximum practical (coax fills toroid bore) |
Appendix B — Worked CMR Example
Toroidal choke at dipole feedpoint: Z_choke = 5000Ω. Path impedance to shack ground: Z_path = 100Ω.
CMR = 20 × log10(1 + 5000/100) = 20 × log10(51) = 20 × 1.708 = 34.2 dB
A 34 dB reduction in common-mode current on the feedline. This should eliminate most feedline radiation and shack RF problems on HF.