RFI & You

(This article ran in Worldradio, May 1998)

Interference to most home appliances is caused by RF pickup on long wires connected to them. Examples are telephone wiring, power wiring, long speaker wires on stereo systems and coaxial cable shields on TV antenna leads. 

To get rid of the interference we have been recommending ferrite beads and toroids placed on the wires just as they enter the affected appliance. They present an impedance to RF and thus reduce the amount of signal getting into the appliance.

There are other effective methods such as by-pass capacitors and shielding but they are more difficult to use and, in the case of capacitors, require soldering and working on the innards of the appliance. This is definitely not recommended if it is your neighbor's appliance.

Ferrite beads just slip over the wires and are easy to install and to remove. Also it's easy to move them around to different wires when you're not sure just where the RF is getting in.

Ferrites come in different "mixes" or formulations that are frequency sensitive. Some mixes work well as RFI suppressors in the shortwave spectrum; others don't work well at all. So you need to know what mix of ferrites you are using. And you should be careful using those unmarked flea market cores. They might not even be ferrite. Toroid cores are made of many different materials for different purposes. Unlike resistors there is no standard marking or color code; each manufacturer does it his own way.

Test them

So, if you have a box of unmarked toroids, how do you tell if they will work for RFI suppression? One way is to try them by putting them on the cable and seeing if they work. The problem with this approach is that, if they don't work, you don't know whether it's because the toroids are no good, you didn't use enough, the RF is coming in another wire, or what. 

It's better to know if the toroids are good or not before you start. If you have, or can borrow, a Palomar noise bridge or an Autek RF analyst you can test them easily.

Just put a one turn loop of wire through the toroid and measure the RF impedance, Z, at the frequency of the RF you want to suppress. The noise bridge gives you the components of Z: resistance, R, and reactance, X. To calculate Z just square the values of R and X, add them together then take the square root. The analyst gives the combination, Z, directly.

What should you expect in the way of impedance? Table I gives measurement results for three commonly used RFI suppressors, the Radio Shack "Snap-On Choke" (Cat. No. 273-104), and the Palomar FSB-1/4 and FSB-1/2 "split beads." The chokes have a .4" x .9" rectangular hole. FSB-1/4 has a 1/4" diameter round hole, for power cords and RG-58U coax. FSB-1/2 has a 1/2" hole for RG-8U and similar cables. Your toroid should have an impedance in the same ballpark as these.

The higher the impedance of the bead the better the RFI suppression. The Snap-On doesn't look too good in the table but you can increase the impedance by running the cable through the bead more than once. The Snap-On has a large hole just for this purpose. With five turns on this core the impedance on 80 Meters increases to 125 ohms.

Of course you can do this with any core; the more turns the higher the impedance. The practical limit for this is at around ten turns because the capacitance between turns reduces the impedance at the higher frequencies.

Sometimes you can't make more than one turn through the core because there isn't enough extra wire length or you are working with cable that can't be bent into a small turn. In this event just add more beads. Two are twice as good as one; three are three times as good, etc.

A good example of this is the original W2DU balun (QST March 1983 p.38-40). In this article Walt Maxwell describes a balun made by placing 50 ferrite beads over coaxial cable. He used FB-24-77 beads. These measure about 25 ohms on 80 Meters when the wire goes through just once. But with 50 of them over the cable the total impedance is over 1200 ohms. Note: Older literature calls for 73 type ferrite. This is no longer manufactured and has been replaced by type 77 ferrite which has the same permeability.

So you have two methods you can use to increase the RF impedance: Wind several turns through the bead or use more beads. Use whichever method suits your particular application. Maybe you can use them both. 

But before you start, measure your beads to see if they are going to work. You'll save time in the long run and you'll have the confidence you need to dig in and solve the problem.