CMC Is the Biggest Problem in Ham Radio

Updated December 26, 2025.

Most “ham radio mysteries” aren’t mysteries at all. They’re uncontrolled RF current flowing where you didn’t intend — especially on the outside of the coax shield.

That single oversight can create the whole nightmare package: SWR that changes when you reroute the feedline, patterns that drift, noise that comes and goes, RF in the shack, hot mics, USB/audio/CAT weirdness, and “this antenna works great… until it doesn’t.”

And the reason it stays unsolved is simple: we use “common-mode” as one label for different mechanisms… then we chase the wrong fix.

Common-Mode Is Two Different Mechanisms

1) True common-mode current

This is the classic case: the outside of the coax becomes an unintended antenna. On receive, it often drags man-made noise into your station. On transmit, it can radiate and distort your pattern. A choke is designed to build a “wall” for that unwanted current on the coax outside.

2) Stray return current from an incomplete antenna system

A lot of what hams call “CMC on transmit” is not “noise riding on the coax.” It’s the antenna system missing a clean return path, so the station grabs the easiest conductor available: coax outside, mast, shack wiring, mains earth, USB cables, and anything bonded or nearby.

Same outside surface, totally different root cause. Which means the solution depends on why the current is there — not just that it exists.

Why a Big Choke Can Make Things Worse

Here’s the uncomfortable truth: chokes do not “complete” an antenna. They suppress unwanted current on the outside of the coax. That’s all.

So if your coax outside is acting as your “missing half” (missing counterpoise / missing second conductor), choking it hard causes one of two outcomes:

• The system reroutes the return current onto shack wiring, equipment, and everything attached to it.
• The antenna detunes or becomes unpredictable because you removed a piece it was secretly using.

This is why people sometimes say: “I added a choke and my antenna stopped working!”

What happened is usually simpler: the antenna was never a standalone antenna. It was radiator + feedline + station. The choke removed the feedline from the circuit, and the antenna was left without a defined return path.

Every Antenna Needs a Return Path

RF is a loop. Current must come back — always.

• Center-fed dipole: the return path is the other leg (equal and opposite currents).
• Vertical: the return path is the radial/ground system (or whatever it recruits when radials are weak).
• End-fed / asymmetric systems: you need a deliberate counterpoise/return conductor, or the station will become it.

If you don’t intentionally design the return path, the station will design it for you — and that “design” changes with frequency, routing, surroundings, and whatever cables happen to be connected today.

The Dirty Secret of Real HF Installations: “Balanced” Is an Approximation

At HF, wavelengths are huge and our environments are not electrically small. A dipole that looks perfectly symmetric on paper almost never “sees” a symmetric world in practice:

• Ground is part of the antenna system. Unless you’re up around ~0.5 λ with lots of clearance, the two legs rarely couple to ground identically (different soil conductivity/moisture, slope, buried metal, fences, etc.).
• Nearby objects break symmetry fast. One leg closer to a tree line, a roof edge, gutters, a wall, or even a different height above shrubs changes capacitance and induced currents, so the current distribution won’t mirror.
• Feedline routing creates asymmetry. A coax leaving at an angle, dropping along one leg, or running near one side becomes “the third conductor” and can pull current off-balance unless you choke it well.
• Weather adds small but real perturbations. Wet foliage, rain on one side, wind moving one leg closer to branches, ice loading, etc. can shift coupling and imbalance. (“Moisture in the air” itself is usually a tiny effect; wet objects and changing geometry are the bigger culprits.)

So in HF ham radio, it’s more honest to say: there are no perfectly symmetric antennas — only antennas with good enough balance for the job.

The practical goal isn’t purity; it’s controlling what happens when symmetry inevitably breaks: good feedpoint choking, sane feedline routing, and measuring common-mode current so you know when you’re actually done.

On end-fed antennas (like the classic EFHW), it’s easy to accidentally mix up the antenna feedpoint with the feedline. The antenna needs a return path, and if you don’t provide an explicit counterpoise (wire, radials, a defined return conductor at the transformer), the system will happily “borrow” the outside of the coax shield as that return path for some distance. In that context, “hard choking the coax” means placing a strong 1:1 current choke right at (or immediately below) the transformer so the antenna system is forced to close its current loop locally (on the counterpoise you provided) instead of using the feedline. Putting the choke ~0.05λ down the coax can still “work,” but it deliberately allows that first section of coax to act as part of the antenna/counterpoise — which makes performance and noise pickup far more sensitive to routing, height, and nearby objects.

Why Coax Is the Perfect Accomplice

Coax is not “one conductor.” Think of it like a multi-lane highway:

• Intended RF travels inside the coax: center conductor out, inside of shield back.
• The outside of the shield is a separate lane that can carry its own RF current when the system becomes asymmetric.

Some people assume coax “starts radiating” because the antenna isn’t exactly 50 ohms, or because mismatch somehow makes the feedline “unbalanced.” That’s not how it works. Balance has nothing to do with impedance. A perfectly balanced antenna can be 20 Ω, 200 Ω, or 300 Ω — balance is about symmetry and equal-and-opposite currents, not a magic resistance value. Coax is ~50 Ω because of its geometry and dielectric, and it can feed a non-50 Ω load without magically creating common-mode current; you simply get reflections and standing waves inside the line. Coax radiates on transmit (and picks up noise on receive) when the outside of the shield is driven with common-mode current by asymmetry or a missing return path. By contrast, a truly balanced line (open-wire / ladder line) tends to keep equal-and-opposite currents even with severe mismatch, so it can remain “quiet” and non-radiating — as long as the installation is kept symmetric and away from conductive objects that would upset that balance.

And in real ham installs, asymmetry is everywhere: coupling to ground, nearby metal, routing, shack wiring, and antennas that are inherently not balanced across all bands.

That’s also why multiband systems can feel “alive”: the common-mode source impedance changes with frequency, and the coax itself can hit resonant conditions. Move the coax… and the antenna “changes.” That’s not magic — that’s feedline participation.

One Choke Is Rarely Enough

A single choke at one point can help, but it often does not control the whole system — because the coax has multiple places where it can be excited and multiple places where noise/current can enter.

So the right mindset is not “install a choke.” It’s:

Build a choking system.

The three-zone choke habit

If you want a simple rule that works for most real stations (especially multiband): think in three zones — antenna, building entry, station.

(Sometimes you’ll add a mid-run choke too — long coax runs can develop resonant “hot spots,” and the clean fix is to break that path where it’s actually excited.)

Chokes Also Act as Buffers

Most people think of a choke only as an RF cleanliness tool. But in real stations, chokes can also act as buffers — a high-impedance “speed bump” that discourages unwanted energy from continuing indoors.

Important: a choke does not replace proper surge protection, bonding, and arrestors. It’s a complementary layer — not “lightning protection.”

Clip-On Ferrites: Useful, But Often the Wrong Tool

Why clip-ons create false confidence

One or two clip-ons is usually nowhere near enough to be a real HF feedline choke. Effective feedline isolation typically requires thousands of ohms of choking impedance on the bands you actually use — not just “some ferrite somewhere.”

Where clip-ons actually belong

• USB, DC, and control leads (where adding turns is possible)
• Accessory cables and local RFI cleanup
• Low-power suppression tasks where heat is not a concern

Why clip-ons are risky as the primary QRO feedline solution

At higher power and high duty cycle, ferrite heating and property drift can reduce choking impedance and destabilize the system. For primary HF feedline choking at serious power levels, you want purpose-built chokes designed for that job.

QRP, 100W Stations, and QRO

QRP

At low power, you can sometimes “get away with it” without burning ferrite — but that doesn’t mean the system is clean. Common-mode current can still distort patterns and create unstable behavior.

Around 100 watts

This is where the choke must do two jobs: provide meaningful isolation and survive real duty cycles. Thermal stress becomes a design requirement, not an afterthought.

QRO and high-duty digital

This is where “almost works” turns into failure. Long duty cycles (FT8/RTTY and similar) demand more ferrite mass, more robust designs, and often staged/cascaded choking to keep impedance high across bands without cooking the choke.

How to Know You’re Done

The most practical truth in this whole topic is:

“Enough” is not a feeling. It’s a measurement.

• Characterize chokes properly (don’t guess from marketing numbers).
• Verify your station with a common-mode current measurement on the coax shield using an RF ammeter / clamp-on current meter.
• Move/add chokes until current at the station side becomes negligible on the bands you use.

Note: since MFJ no longer produces their RF ammeters/current meters, RF.Guru is developing a practical RF ammeter solution for real-world common-mode current verification.

Bottom Line

CMC is the biggest problem in ham radio because it’s the most common hidden mechanism behind feedline radiation, unstable antennas, noisy receivers, and RF in the shack.

The most common mistake is thinking: “I added a big choke, so I’m done.”

The real order is:

1. Give the antenna a real return path (balance the system).
2. Use a feedpoint choke to keep the feedline out of the antenna.
3. Control every interface (feedpoint, entry, station; sometimes mid-run too).
4. Don’t rely on clip-ons as the primary QRO feedline solution.
5. Measure so “enough” becomes “verified.”

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.