“One FT-240 Core” as the Default Choke — Is It Enough for QRO?

Short answer: usually not. What actually matters for a common-mode choke is its impedance in ohms (Z_CM) against the unknown common-mode source resistance (R_CM), not a headline “CMRR dB.” We use the physically correct relationship at RF.Guru:

Current reduction (dB) = 20 · log₁₀ ( 1 + Z_CM / R_CM )

With a typical R_CM ≈ 100 Ω, you need many kΩ of choking for strong suppression. Our own transmit guidance (based on measured data) recommends:

• ≥ 10 kΩ on 160–80 m
• ≥ 8–10 kΩ on 40 m
• ≥ 6–8 kΩ on 30–20 m
• ≥ 6 kΩ on 17–10 m

In practice, a single FT-240-31 or -43 with 8–12 turns of small coax yields only 1–3 kΩ — well below QRO targets. #31 performs best on low HF; #43 peaks around 10–15 MHz. That’s why serious stations stack cores or use multiple chokes in series (feedpoint, entry, PA side).

How Many “Lost Watts” in the Coax at 1.5 kW?

Using our own definitions:

Not much, really — the actual dissipated power on the coax is usually small. The real issue lies in the excess return currents flowing on the outside of the coax braid — what most people loosely call “common-mode currents,” but they are in fact stray return currents that should never have left the feedpoint path. These unwanted currents modify the radiation pattern of your antenna, increase RFI, and can severely overheat the ferrite core. Once the core temperature exceeds roughly 100 °C, the ferrite structure begins to lose permeability permanently, causing lasting performance degradation or even cracking of the core itself.

• P_TX = 1500 W into 50 Ω ⇒ V_rms = √(P·R) = 273.9 V rms
• R_CM = 100 Ω
• V_CM = k · V_rms with k = 0.25 (moderate) and k = 1.0 (worst case)
• #31 on 160–40 m; #43 on 30–10 m
• Z_single values: 160 m ≈ 2 kΩ … 10 m ≈ 1 kΩ

I_CM = V_CM / ( R_CM + Z_CM ) and P_coax = I_CM² · R_CM

Results for a Single FT-240 “Default” Choke

Even at 1.5 kW, coax heating is modest — but ferrite heating above 100 °C can permanently damage the core and distort antenna patterns.

What If You Meet Our QRO Target Values?

Now coax losses drop to milliwatts, current below 50 mA, and the core remains far below the 100 °C damage limit.

Why a Single FT-240 Core Under-Performs at Legal Limit

• Too little impedance: 1–3 kΩ vs 6–10 kΩ target (≈ 10–20 dB short).
• Thermal margin: Core may exceed 100 °C causing μ′ loss or cracking.
• Band mix: #31 for low HF, #43 for high HF — no single core fits all.
• Geometry resonance: Feedline length affects R_CM; we always think in ohms, not just dB.

What to Do Instead

• Band-split: #31 for 160–40 m (≥ 8–10 kΩ); #43 for 30–10 m (≥ 6 kΩ).
• Stack or series chokes: Impedances add and spread heat — keep each core below 100 °C.
• Monitor temperature: IR gun or thermal tape; if > 90 °C, add more ferrite or cooling.

Key Takeaways

• A single FT-240 core is not enough for legal-limit QRO.
• “Lost watts” are small — but stray return currents and hot ferrite can wreck your pattern and the core itself.
• Use multiple cores, higher Z, and band-split design for safe operation.

Interested in more technical content? Subscribe to our updates for deep-dive RF articles and lab notes.

Questions or experiences to share? Contact RF.Guru.