By Danny Horvat, E73M / N4EXA — antenna design engineer and founder of MyAntennas.com.
Over 45 years on the air and decades of transformer and choke design and measurement inform everything on this page.
Few accessories are as misunderstood as the common mode choke. Hams buy them, install them in the wrong place, and conclude they “didn’t do anything” — or skip them entirely and spend years fighting noise and RFI they could have cured in an afternoon. This page explains what a CMC actually does, whether you need one, and — the part almost everyone gets wrong — exactly where it belongs in your station.
What a Common Mode Choke Does
Your coaxial feedline is meant to carry current in one mode: down the inside of the center conductor and back on the inside of the shield, equal and opposite. That is the wanted signal, and a choke does not touch it. The problem is a second, unwanted current that flows on the outside of the shield — the common mode current. That outer-shield current turns your feedline into an antenna in its own right. On transmit it radiates RF into your shack; on receive it collects every noise source between the radio and the entrance to your house and pours it into your receiver.
A common mode choke is simply a high impedance placed in the path of that outer-shield current. It ignores the wanted signal inside the coax and chokes off the unwanted current on the outside. A good one does this with resistive impedance across the whole band — and that distinction is the heart of the matter, as I’ll explain.
Why Your Station Needs One
Common mode current is behind most of the problems hams blame on other things:
RF in the shack. A hot microphone, a burning sensation when you touch the rig, a tuner or amplifier that behaves erratically at full power, USB devices dropping out when you key up — that is common mode current radiating off the feedline right at your operating position.
A high noise floor on receive. This is the one serious operators care about most. A modern house is full of noise generators — switching power supplies, LED drivers, chargers, solar inverters, plasma TVs — and their hash rides the outer shield of your coax straight to your receiver. A properly placed choke can drop your noise floor by one, two, sometimes several S-units on the low bands. Nothing else you can buy for the price improves weak-signal reception as much.
RFI to your neighbors and your own devices. The same radiating feedline couples RF into telephone lines, TV cables, and house wiring, bothering equipment in your home and next door.
Distorted antenna patterns. On a directional antenna, feedline radiation ruins the very front-to-back ratio you paid for. A Yagi with 25 dB of front-to-back can be reduced to 10–15 dB by an unchoked feedline picking up and re-radiating signals the antenna was supposed to reject.
Do You Actually Need One?
Not every station does, and I would rather tell you the truth than sell you a choke you don’t need. If your coax shield is grounded where it enters the house per NEC-810, and most of the feedline run is on or in the ground, your system may already be quiet. I run 1500W on my own EFHW at home with the cable grounded at the entrance and lying on the ground.
You do want a choke if you have any of the RFI or noise symptoms above, if your feedline runs largely through the air, if you use an end-fed or off-center-fed antenna (which inherently put some current on the feedline), or if you are a DXer or contester chasing every last decibel of receive performance. When in doubt, a choke never hurts — the only wrong move is putting it in the wrong place.
Where the Choke Belongs — The Part Everyone Gets Wrong
This is the single most important thing on this page, so I’ll be blunt: the choke belongs where the coax enters your home, before the shield contacts any ground. Not at the radio. Not at the antenna transformer. At the entrance.
Here is why. The job of the choke is to stop noise current that has accumulated along the whole outdoor feedline run from reaching your shack. Place it at the entry point and it does exactly that, while also giving the outer shield a clean, single grounding point right there per NEC-810. Everything upstream of the choke — the long outdoor run — can pick up whatever it likes, and the choke stops it at the door.
Now consider the common mistake: putting the choke inside at the radio, then running a ground wire from the radio chassis back outside to a ground rod. That ground wire bypasses your choke completely. The noise current simply flows around it. You have installed a good choke and then wired a detour around it. I see this constantly, and it is why so many hams believe chokes “don’t work.”
A related point: long ground wires from the shack to an outdoor rod are not the RF safety improvement people imagine. At HF, a wire many feet long is not a ground — it is an impedance, and it can radiate. Ground the coax shield at the home entrance, put your choke there, and keep it simple. (More on this in About Grounding.)
Why a Snap-On Ferrite Isn’t Enough
The clip-on ferrite bead you can buy at any electronics counter is nearly useless at HF. A single pass of coax through one small core provides only a few tens of ohms of choking impedance — nowhere near enough against the thousands of ohms of common mode impedance the outer shield can present. Manufacturers of cheap chokes rely on the buyer not measuring the result.
Effective choking at HF requires many turns of the cable through a substantial ferrite mass — enough to reach several thousand ohms of impedance, and, critically, impedance that stays resistive across the band rather than going reactive at the edges. A reactive choke can actually resonate with the feedline and make matters worse at some frequencies. This is what our Multi-Zs™ design addresses: each MyAntennas choke is designed and measured to hold high resistive choking impedance across its entire rated range, not just at one lucky frequency. We publish the impedance curves because the curve is the product — a choke’s spec sheet without a measured impedance-versus-frequency plot is telling you nothing.
Choosing a MyAntennas Choke
For most HF antennas, the broadband CMC-130-3K (1–30 MHz) is the one to reach for — install it where the coax enters the house. If you also work 6m, the CMC-154-3K and compact CMC-154S-3K extend coverage to 54 MHz. High-power stations want the CMC-330-5K. Low-band specialists have the CMC-0510-R (0.5–10 MHz), VHF operators the CMC-VHF-2K, and for lighter cases of feedline noise our RFI-Kits offer an economical start. See them all on the CMC / RF Isolators page.
Further reading: for a thorough academic treatment of common mode chokes, the widely cited paper “Common-Mode Chokes” by Chuck Counselman, W1HIS (2006) is excellent. GM3SEK’s presentation on ferrite chokes and current baluns is also a valuable resource. [DANNY: re-link the GM3SEK .ppsx here if you want to keep it.]
Related: About Grounding · About BALUNs · All About EFHW Antennas
