Understanding Common Mode Buildup and the Need for Multiple Chokes

Understanding Common-Mode Buildup and the Need for Multiple Chokes

Common-mode current buildup is a widespread and often misunderstood phenomenon in RF systems. It arises when unwanted currents appear on the outside of coaxial cables or other transmission lines, flowing in the same direction on all conductors. Unlike differential signals, these currents do not contribute to signal transmission; instead, they often lead to unwanted radiation, RF interference, and degraded antenna performance.

At the root of common-mode buildup is imbalance. When a transmission line system lacks symmetry in its impedance paths, or when antennas are not perfectly balanced, the differential-mode return current can no longer cancel out completely inside the shield. This imbalance results in some of the return current flowing on the outer surface of the coax shield — effectively turning the feedline into an unintended radiator.

Induced Common Mode: A Summation of Couplings

Common-mode current is not a single event; it accumulates along the line. It can be induced by various sources:

• Asymmetry in the antenna (e.g., end-fed, off-center-fed designs, Yagis, Hexbeams, and other asymmetrical multi-element arrays)
• Proximity to metallic structures
• Ground loops
• Electric field coupling from the antenna to the feedline
• Magnetic field coupling from nearby radiating elements

Each of these interactions contributes to the net common-mode current. The problem is cumulative: small induced voltages and currents at different points add up along the cable, particularly in the presence of standing waves on the shield.

A special note should be made for antennas that are inherently unbalanced or asymmetrical by design — such as end-fed half-wave antennas and Yagi/Hexbeam arrays. Both act as efficient generators of common-mode currents. End-feds present a high voltage and asymmetrical feedpoint that easily excites the outside of the coax. Yagis and Hexbeams, due to their asymmetric element geometry and exposed feed arrangements, often pick up and reradiate common-mode noise unless aggressively choked.

Engineering Deep-Dive: Quantifying Common-Mode Behavior

1. Mode decomposition

v_dm = v₁ − v₂
i_dm = (i₁ − i₂)/2
v_cm = (v₁ + v₂)/2
i_cm = (i₁ + i₂)/2

These relations separate differential- and common-mode components for any two-conductor line, showing that the outer surface current (i_cm) is distinct from the desired differential return (i_dm).

2. Skin effect and dual surfaces

δ = √(2 / (ω μ σ)),   with   ω = 2πf

At 14 MHz, δ for copper ≈ 17.7 µm, meaning the inner surface carries the signal return while the outer surface can independently carry common-mode current.

3. Common-mode impedance of the outer surface

Z₀,cm ≈ 60 ln(2h/r) Ω   (natural log)
       ≈ 138 log₁₀(2h/r) Ω   (base-10)

Example: for a coax with 8 mm OD (r ≈ 4 mm) running 2 m above ground, Z₀,cm ≈ 414 Ω. This sets the typical environmental impedance against which a choke works.

4. Short-antenna radiation resistance

R_rad (dipole) ≈ 80 π² (ℓ/λ)² Ω
R_rad (monopole) ≈ 40 π² (ℓ/λ)² Ω

As the outer surface approaches λ/4, R_rad rises ≈ 36 Ω — the coax begins to radiate effectively, turning your feedline into part of the antenna.

5. Suppression math: Thevenin model

I_cm,no = V_th / Z_env
I_cm,with = V_th / (Z_env + Z_choke)
Suppression = 20 log₁₀(1 + |Z_choke| / |Z_env|)

If Z_env ≈ 400 Ω and Z_choke = 5 kΩ, suppression ≈ 22.6 dB. A second choke (3 kΩ) near the building entry adds ≈ 18.6 dB more, totaling ≈ 41 dB under weak coupling — explaining why multiple chokes outperform one.

6. Ferrite choke behavior

L(ω) ≈ μ₀ μ′ N² Aₑ / lₑ
R_core(ω) ≈ ω μ₀ μ″ N² Aₑ / lₑ
Z_choke(ω) ≈ R_core(ω) + j ω L(ω)

The resistive term (μ″) provides broadband attenuation. Type-31 ferrite (μᵢ ≈ 1500) suits 1–50 MHz, while type-43/61 fits upper HF and low VHF. Each turn multiplies both inductance and loss by N².

7. Quarter-wave sleeve (bazooka) balun

Z_in = j Z₀ tan(β l)  for l = λ/4 → |Z_in| → large

A simple resonant barrier for narrow-band operation; ferrites offer broadband performance.

Why One Choke Is Rarely Enough

A single common-mode choke can provide significant suppression, but in most installations, it’s not sufficient. Common-mode current is distributed and re-induced along the line, so placing one choke at the feedpoint suppresses the current there — but not new currents excited further down the run.

Multiple Chokes: Strategic Placement

• At the feedpoint — block the first entry of CM current.
• At bends or transitions — where coupling is strongest.
• At building entry — prevent RF from coupling into house wiring.
• At the transceiver — remove residual CM and protect equipment.

A spacing of roughly λ/8–λ/6 along long runs avoids creating standing-wave maxima between chokes.

Measurement & Verification

• Clamp-on RF ammeter to directly measure I_cm on the shield.
• VNA mixed-mode S-parameters: S_CC shows CM blockage; S_DC or S_CD indicates DM↔CM conversion.
• Transfer-impedance fixtures (IEC 62153) for precise cable-shield evaluation.

Practical Design Rules

1. Use lossy ferrite (high μ″) for broadband suppression.
2. Aim for |Z_choke| ≫ |Z_env| → multi-kΩ for strong HF reduction.
3. Combine λ/4 sleeves and ferrite chokes when both bandwidth and high attenuation are needed.
4. Segment long coax runs with 2–3 chokes; the dB suppression adds cumulatively.

Formula Reference

δ = √(2 / (ω μ σ))
Z₀,cm ≈ 60 ln(2h/r) Ω
R_rad ≈ 40 π² (ℓ/λ)² Ω
Suppression_dB = 20 log₁₀(1 + |Z_choke| / |Z_env|)
L = A_L · N²
Z_in = j Z₀ tan(β l)

Summary

Common-mode current is not a single defect but a distributed effect. Treat your coax as a potential radiator. Segment it with multiple broadband chokes sized for multi-kΩ impedance, and place them at key locations — feedpoint, transitions, entry, and equipment. This approach restores system balance and dramatically reduces both noise pickup and unwanted radiation.

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