About CMC

Common mode choke

Your ability to hear weak MF and HF signals is limited by noise, generated mostly by solid-state electronic switches within your own house, conducted via the 50/60-Hz power line to your shack, and from there to your antenna by common-mode current on the feed­line.  Putting common-mode chokes on your feedline, power, and other cables will sub­stantially reduce your received noise level.

What Is a Common-Mode Choke?

First, what is a choke?  By a choke I mean a radio frequency (RF) choke — a discrete device that you can insert or connect in series with a wire or a cable to reduce substan­tially the RF current flowing along the wire or cable at the insertion point.  The ideal choke has infinite impedance for RF; inserting it would be equivalent, for RF, to cutting the wire or cable.  How much impedance is sufficient in practice, I discuss below.

To define “common mode,” I must define “mode.”  The concept involves nothing beyond electricity and eighth-grade algebra.  Consider a cable of two insulated wires, like zip cord.  Imagine this cable delivering power to a 12-VDC lamp.  The current flowing in one wire of this cable has the same magnitude as the current in the other wire, but the currents flow in opposite directions.  For a 24-W lamp the magnitude of the current in either wire would be 2 A.  The net current in the cable (i.e., the algebraic sum, consider­ing both the magnitudes and the directions, or signs, of the currents in the individual wires) would be zero.

We have just discussed the two possible modes of current flow in a two-conductor cable.  These are the “differential” mode and the “common” mode.  The differential mode is also known as the “transmission-line” mode.  In the example, 2 A flows in the differential mode, and 0 (zero) A flows in the common mode.  The differential or transmission-line current is equal to the algebraic or “signed” difference between the currents in the two wires, divided by two.  The common-mode current is the algebraic sum, or net current in the cable.

In another example, imagine connecting the two wires of a piece of zip cord together at each end, in other words connecting the wires in parallel, to obtain a single conductor able to carry twice as much current as either wire could carry by itself.  In this zip cord, zero current will flow in transmission-line mode; whatever current flows (depending on the application) will be common-mode current.

In different situations (which you can imagine), non-zero values of current may flow in both modes: the transmission-line mode and the common mode.

RF currents (unlike DC) vary with position along a cable, and their amplitudes are com­plex (having magnitudes and angles, or real and imaginary parts); however, the concepts of transmission-line and common-mode apply at any cross-sectional plane; and the com­plex amplitudes are added and subtracted algebraically.

A cable may have more than two conductors.  An N-conductor cable has N independent modes of current flow.  Of these modes, the only one that interests us here is the common mode.  For any number of conductors in a cable, the common-mode current is the algebraic (signed) sum of the currents in all the conductors.

A common-mode choke for use in an N-conductor cable has N insulated conductors, one for each conductor of the cable.  An ideal common-mode choke is perfectly transparent in every mode except the common mode.  It offers no resistance (or reactance) to any differential or transmission-line current; but, for the common mode, it looks like an open circuit.  In other words, the perfect common-mode choke has infinite impedance in the common mode.

To be perfectly transparent in every mode except the common mode, the common-mode choke must be like the cable in such respects as conductor size and current rating, insula­tion thickness and voltage rating, and — for an RF transmission line — characteristic impedance.  The simplest way to make a common-mode choke transparent is to make it from a piece of the same type of cable.  Then, to give the choke a high common-mode impedance, you surround this piece of cable with ferrite.

Why Use Common-Mode Chokes?

The most common reasons for using common-mode chokes are:

(1) to reduce the fraction of the RF power that is fed to your antenna from your trans­mitter, but then is conducted back to your shack via common-mode current on your feedline, causing RFI trouble in the shack or elsewhere in your house;

(2) to keep the transmitted RF power that 60-Hz power, telephone, TV, and other cables in the field of your antenna pick up, from bothering susceptible devices connected to these cables in your own and neighbors’ houses; and

(3) to keep the RF noise that all the electronic devices in your house generate, from being conducted via 60-Hz power, telephone and other cables to the outer shield of your radio, and from there along your feedline(s) to your antenna(s), in common-mode.

Reasons (1) and (2) are obvious and compelling.  When your logging computer crashes or your spouse is screaming, you have to do something.  Reason (3) is more subtle and ignorable.  Even when QRM and QRN are obliterating half the stations on the band — hell, 90% of the signals — you can continue operating and having fun.  There are still plenty of stations strong enough to work.  Reason (3) matters only to a serious DXer or contester, but it is one of the most economical of all ways of improving receiving performance.

A significant fraction, typically -15 dB, of the noise power arriving at an antenna feed­point via common-mode current on the feedline is coupled into the antenna’s receiving mode, because a typical balun (adequate for transmitting purposes) has this much residual imbalance, and also because a nominally balanced antenna is never perfectly balanced.

A common-mode choke is reciprocal.  It reflects and absorbs transmitted power that would otherwise be conducted from your antenna back to your shack and onto your 60-Hz power and other circuits to bother, say, your telephone, by the same factors or numbers of dB as it does for QRN going in the opposite direction.

If your antenna is highly directional, as a Yagi or a Beverage is, then you have another reason to use a common-mode choke: to prevent reception of QRM and QRN by your feedline as opposed to your antenna.  Without a good common-mode choke in the feed­line at its feedpoint, your potentially excellent antenna’s 25- or 30-dB front-to-back or front-to-side ratio could be reduced to 15 or even 10 dB.

In the HF hamshacks that I’ve visited, the background noise level heard on most HF bands (especially the low bands) could be reduced by more than an S-unit by means of common-mode choking.  In some cases (which I could name but won’t, to avoid embar­rassing my friends) I was able to reduce the received noise level by four or five S-units.  I reduced my own received noise level on the low bands by even more.

For years I’ve been a regular participant in CW nets on the 80- and 40-m ham bands, and in SSB nets on MARS frequencies near these bands.  I hear better than anyone else in these nets.  I am often the only participant able to copy every one of the dozen or two dozen stations in a net.  Why can I hear so exceptionally well?  My receiver is nothing special, nor is my antenna; it’s a wire just 23 to 40 feet high.  Nor is my QTH very quiet.  I have a small lot in a dense suburb, just two miles from Cambridge and three miles from Boston.  I hear exceptionally well because I have good common-mode chokes in my antenna feedline, in the other cables connected to my radio, and also in the cables con­nected to the worst of the QRN sources in my house.

A typical American household contains more than a hundred significant QRN sources.  Some of these sources, e.g., incandescent light dimmers, fill the MF and HF spectrum with noise in periodic bursts or impulses at the 60-Hz power-line frequency (or at 120 Hz).  In a SSB receiver (even more in an AM receiver) this noise sounds like a steady buzz, and its strength doesn’t change if you tune a few tens of kHz.

Switching power supplies, which are in all kinds of electronic appliances, in the battery chargers of portable devices, in the solid-state electronic “ballasts” of fluorescent lamps, and in the “solid-state transformers” of low-voltage incandescent lighting systems, gener­ate relatively narrow-bandwidth, hum-modulated, QRN at harmonics of their switching frequencies, usually in the 15-25 kHz range.  These frequencies are not very stable, but drift and fluctuate with temperature, power-line voltage, and load current.

Digital-electronic circuitry is ubiquitous (not just in computers) and usually switches at stable frequencies.  Digital electronics generates QRN most often with discrete spectra and quasi-periodic, often complex, but regular temporal structure.  However, some digital-electronic sources of QRN have very broad spectra, or spectra with such broad peaks, that the QRN can be mistaken for natural “white” noise in a communications receiver.  Also, the typical house contains so many independent sources of QRN that, although their individual spectra may be peaky, the composite spectrum can sound pretty flat.

Many or most of these sources continue to generate QRN even when the appliance or device is switched “off.”  Many of them, e.g., video and audio entertainment devices, computers and related devices, telephones and related devices, clocks and timers in all sorts of devices, and (probably worst of all) alarm systems, contain batteries or super-capacitors and continue to generate QRN even after AC power is disconnected by unplug­ ging the device/system or flipping a circuit-breaker.

Read more in article Common-Mode Chokes by Chuck Counselman, W1HIS