How Much Choking Do You Really Need — for RX and TX?
Many hams quote “high dB CMRR” numbers for their chokes or baluns — but that number alone rarely tells the truth. What matters physically is the common-mode impedance in ohms, not just a dB figure. The dB number only has meaning if you know the impedance of the common-mode source it’s working against. Let’s unpack how that applies differently for receive and transmit systems — and why measurement is where most myths begin.
Why Ohms Matter More Than dB
A choke is a series element in the common-mode path. The current reduction you achieve is proportional to the ratio between the choke’s impedance (ZCM) and the common-mode source resistance (RCM):
Attenuation (dB) = 20 × log₁₀(1 + ZCM / RCM)
That means the same 30 dB “reduction” can represent vastly different realities depending on RCM. Assuming RCM ≈ 100 Ω, you’ll need roughly:
- 20 dB → ~ 900 Ω
- 30 dB → ~ 3.1 kΩ
- 40 dB → ~ 9.9 kΩ
So a “30 dB” choke might not mean much if your coax shield happens to be resonant and RCM is several hundred ohms. That’s why serious designers prefer to quote impedance curves in ohms rather than “CMRR dB.”
However, there is one legitimate exception: in receive-only systems, CMRR dB figures can be meaningful because the front-end operates on a defined impedance standard — 50 Ω, 75 Ω, or 100 Ω. In that specific context, the dB figure directly represents the expected current attenuation for that known load. But in transmit and broadband antenna environments, where RCM can vary wildly with feedline geometry, quoting only “CMRR dB” becomes meaningless.
Receive-Side Choking (Passive and Active Antennas)
On the low HF bands (160–40 m), proper choking provides one of the largest SNR improvements you can get. The coax shield easily acts as a second antenna, coupling man-made noise (MMN) from the house or nearby wiring into your receiver. A high-Z, resistive choke isolates the antenna element from that noisy environment, letting the intended signal dominate.
Recommended targets for RX antennas:
- 160–40 m: ≥ 5 kΩ (better 8–10 kΩ) — typically ≈ 30–40 dB current suppression.
- 30–20 m: 3–5 kΩ (≈ 25–35 dB CMRR).
- 17–10 m: 2–3 kΩ (≈ 20–30 dB CMRR).
For active antennas such as E-probes, MiniWhips, or active loops, strong choking within one meter of the antenna prevents the coax braid from acting as part of the sensor. A second choke near the receiver helps isolate the system from the shack’s ground potential and mains noise.
Tip — for both receive and transmit systems, 5–10 kΩ of resistive impedance (≈ 30–40 dB CMRR) is where the improvement becomes tangible. Chasing 40 dB+ only helps when you’re fighting extreme local noise or poor geometry.
Understanding Choking Impedance — and Why So Many Measure It Wrong
One of the most persistent myths in ham radio is how to “measure” choke performance. Most amateurs simply insert the choke between Port 1 and Port 2 of a VNA and read the S21 magnitude. That’s differential transmission loss — not common-mode impedance. It tells you almost nothing about how well the choke suppresses shield current.
The only scientifically valid way to measure common-mode impedance is the EMC method used in industry: drive the device in common mode with a current insertion fixture and measure the resulting current with a current probe. This setup quantifies the choke’s true series impedance to common-mode current, exactly as defined in EMC standards. The result is complex (resistive + reactive), and only the resistive part gives broadband suppression — it converts RF into harmless heat rather than storing it reactively.
Any “measurement” based purely on VNA S21 is at best incomplete, and at worst meaningless. If you ever see a choke specified only by “S21 dB,” you’re not looking at a scientific measurement — you’re looking at marketing … or ignorance.
Transmit-Side Choking (Feedline Isolation and Heat)
On transmit, two aspects matter: isolation and heat handling.
- Isolation: You still want the feedline removed from the antenna circuit, keeping the pattern, SWR, and shack RFI stable. The same Z-targets as receive apply — just scaled for power.
- Thermal stress: The choke must safely dissipate whatever current remains. Because heating ≈ I² R, a high resistive component reduces current and spreads loss harmlessly through the ferrite volume.
| Band(s) | 100 W | 500–800 W | 1–1.5 kW + (FT8/RTTY) | Notes |
|---|---|---|---|---|
| 160–80 m | ≥ 5 kΩ | ≥ 8 kΩ | ≥ 10 kΩ | Mix 31; stacked cores, spaced turns for cooling. |
| 40 m | ≥ 5 kΩ | ≥ 6–8 kΩ | ≥ 8–10 kΩ | Ensure enough ferrite mass for duty cycle. |
| 30–20 m | ≥ 3–5 kΩ | ≥ 5 kΩ | ≥ 6–8 kΩ | Mix 31 or 43 depending on range. |
| 17–10 m | ≥ 2–3 kΩ | ≥ 4–5 kΩ | ≥ 6 kΩ | Mix 43 preferred. |
Placement — Where It Actually Matters
- Feedpoint choke: Always first. This is where the antenna and feedline separate electrically. It determines pattern stability, RFI immunity, and SWR repeatability.
- Shack-entry choke: Stops noise from entering the feedline via house wiring or network cables and keeps digital noise out of the antenna system.
- PA or transceiver-side choke: Useful as a “last barrier.” It prevents residual RF from flowing on power and control lines, especially with high-power or multi-band amps sharing grounds with computer interfaces.
Each location serves a different purpose: feedpoint for pattern control, shack entry for noise hygiene, and PA / rig side for safety and immunity under high field levels.
Measuring Real Performance in TX Systems
Once installed, the only meaningful diagnostic for a transmitting station is a common-mode current meter. This clamp-on probe measures the actual current on the outside of the coax. It immediately tells you whether your choke is doing its job:
- < 50 mA common-mode current → Excellent isolation.
- 100–200 mA → Marginal, expect minor pattern skew or RF feedback.
- > 300 mA → Problematic, likely to cause hot chokes and RFI in the shack.
The meter shows dynamic behavior in real time — something a VNA never can. It lets you experiment: move the choke, add a second one, or modify grounding and watch the current drop. In practice, this single tool provides more actionable insight than any laboratory impedance plot when fine-tuning a station.
Bottom Line
- Think in ohms — not in headline “CMRR dB.”
- For receive or transmit, 5–10 kΩ (≈ 30–40 dB CMRR) is a realistic high-performance target.
- Feedpoint → shack entry → PA side — each choke protects a different interface.
- Use a CM current meter on TX — it’s the only scientific way to verify effectiveness in your own station.
More ohms nearly always improve stability and immunity, even when extra dB gains appear small.
Mini-FAQ
- Do I need a choke on every antenna? — If it’s coax-fed, yes. Even “balanced” antennas often aren’t truly balanced once you include ground coupling, surroundings, or humidity variations. There’s no such thing as an “average ground” or “average air.” Real-world asymmetry always generates some common-mode current — and a choke fixes it.
- Do active antennas benefit from strong choking? — Absolutely. It’s the only effective way to stop house noise and ground loops from coupling into the receive signal path.
- Is 20 dB CMRR enough? — Only if your RCM is very low. In practice, 3–5 kΩ (≈ 30 dB) is the safe minimum across HF.
- Can I stack chokes? — Yes. Two properly spaced chokes often outperform one large unit and spread dissipation safely.
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