RX Chokes: Why You’re Still Thinking Like a Transmitter

Still winding fat coax chokes for your active RX antenna? Stop. Put the toroid down. You're solving a problem you don't even have.

The Wrong War: TX Thinking in RX Territory

Most hams and engineers approach receive chokes as if they’re still in the TX world. Big toroids, coax loops, ferrite clamp bracelets — all designed to quench high-power return currents on the outside of the coax.

Makes sense when you're pushing kilowatts. Makes zero sense when you're receiving microvolts.

Active RX antennas are a different beast entirely — and they deserve a different approach.

What’s Actually the Problem in RX Systems?

• You're not dealing with kilowatts — you’re dealing with signal levels in the microvolt to millivolt range.
• Impedance mismatch is irrelevant because most active antennas present a high input impedance (often >1 MΩ) and act as voltage probes.
• The real enemy is common-mode voltage, often coupled capacitively via long coax runs, especially at low HF and LF.

Measured Example: ARRL and RSGB lab tests with active RX antennas show common-mode voltages induced on coax shields can exceed 1 V peak-to-peak in suburban RF environments — enough to drive active front ends into non-linear behavior.

The Real Enemy in RX Chains

• Active antennas have high input impedance and very low signal levels.
• They almost always include an unbalanced amplifier at the front end.
• The real coupling path is shield-to-ground common-mode voltage — not bulk shield current.

So, What Should You Do?

Forget the "coax-as-waveguide" fantasy. You're in the land of E-field probes, whip antennas, and noise sniffers. You need voltage-mode common-mode suppression — not brute-force current baluns.

Recommended RX Choke Strategy:

1. Treat both signal and shield paths as equal suspects. The shield will carry shack noise back into your front end.
2. Use small, wideband SMD common-mode chokes. They’re cheap, light, and designed for the right frequency range.
3. Integrate the chokes at the antenna PCB, before the amplifier input — not 20 m downstream after noise has already coupled in.
4. Break ground loops completely. Use isolated power or differential outputs. Where possible, magnetically decouple the signal.

Why TX-Style Coax Chokes Don’t Work for RX

Toroidal or bead-based chokes (e.g. 10 turns of RG-58 on an FT240-43) are designed to present high impedance — typically 1–10 kΩ from 1–30 MHz. That’s fine in TX, where:

• A low source impedance ensures voltage develops across the choke.
• Significant return current exists to be choked.

But RX-only systems are different: common-mode voltages dominate, and current is vanishingly small. You don’t need to block current — you need to isolate grounds.

Reference: Fair-Rite 43 material plots show impedance plummets below 1 MHz — exactly where E-field probes are most sensitive to common-mode voltage.

Use SMD Chokes — and Use Them Correctly

Surface-mount CMCs are not magic — they must be selected for high impedance across your band of interest, and placed at the amplifier input, not meters away at the shack wall.

Ground Loops: Why Magnetics Beat Resistance

Common-mode voltage arises when the antenna ground floats relative to receiver ground. Hard-bonding coax braids at both ends forms a ground loop antenna. Breaking the loop with transformer coupling or isolated DC-DC supplies suppresses LF hash far more effectively than resistive methods.

Reference: Henry Ott, Electromagnetic Compatibility Engineering, Chapter 8 – "Grounding Myths and Misconceptions".

Mistakes to Avoid

• Using TX-style coaxial chokes — they suppress current, not voltage.
• Skipping a shield choke — the braid imports Ethernet and SMPS noise straight into the RX chain.
• Placing chokes far from the antenna — noise is already inside by then.
• Trusting USB cable grounds on SDRs — they’re notorious noise bridges.

Test It Like a Pro

Want proof? Build a common-mode injection jig:

• Drive a 1–30 MHz signal between the coax shield and chassis ground.
• Measure what appears at the RX input.
• Add SMD chokes to shield and signal.
• Rerun the test. The noise floor drop is unmistakable.

This is how EMC labs characterize cable-borne noise coupling — and why they use clamp probes, not TX baluns.

Final Takeaway

RX antennas need a different mindset. Forget TX-grade brute-force chokes. Think clean, localized, voltage-mode filtering. Use SMD chokes, symmetrical paths, and proximity suppression.

TX-style chokes suppress common-mode current. Active RX systems are plagued by common-mode voltage. They are not the same.

Modern RF front ends demand SMD-level, proximity-based, voltage-oriented suppression — not blind replication of TX baluns.

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

Questions or experiences to share? Feel free to contact RF.Guru.