More on Ferrites


        Radio on FacebookJoin us on Facebook

Picking up where we left off last month - how to use ferrites to solve RFI problems, such as those caused by an AM or FM transmitter close to a neighborhood or even businesses - let's look at some practical solutions.

I'll mention ferrite beads first mainly because I think this is the least likely thing you would end up using. Beads of course slip over individual wires, and their use would in almost every case involve unsoldering a wire, slipping a bead over it, and then re-soldering. I doubt your neighbors are going to be very keen on the idea of you opening up their electronics and modifying it, so let's move on.

More likely you'll end up using toroids or split beads, or some sort of in-line filter. Toroids offer perhaps the most practical solution to many cases. The offending cable is literally run right through the toroid, and if possible, wrapped around it. This can be any cable that fits: audio, coax, CAT-5 cables. Remember the idea here is to mitigate common-mode noise (RF) that is basically the same on all the conductors. In the case of shielded cable (or coax) the idea is to suppress RF from flowing on the shield itself. There are at least two good sources of toroids for this purpose: One is Palomar Engineers. Another is Fair-rite. In fact, Fair-rite also has ferrites for RF suppression on flat and other ribbon cables as well.

As with any ferrite purchase, make sure you get the correct mix for the frequencies you are trying to suppress: For AM, use mix 73; for FM, use mix 43.

Impedance, resistance and reactance comparison for mix 73 vs. mix 43 materials.

Impedance, resistance and reactance comparison for mix 73 vs. mix 43 materials.


Split the difference

Split beads are especially useful when the cable is just too thick to wrap through a toroid. These are beads that are split longitudinally, and fit in a plastic housing that snaps the bead halves together snuggly. The diameter of the cable going through has to be smaller than that of the hole so that the bead halves can fit together well; don't worry about the actual cable being smaller than the hole diameter though. Again, both Palomar Engineers and Fair-rite are possible sources. Be careful in buying surplus beads unless the material type is specified.

There are many sources for pre-made filters for RFI suppression. One of the most interesting suppliers is Mike Sandman for anything to do with RFI in telephone systems. Sandman has also published his own RFI analysis troubleshooting tree. I'd also like to point out that Sandman sells an RFI suppression filter for Ethernet as well (10BaseT and 100BaseT).

Another source for telephone RFI filters is K-Y Filter. You can also look at the Web page of Industrial Communications Engineers for its RFI filters. Unfortunately it looks like ICE isn't taking orders (as I write this) but hopefully later on it will resume. It is my understanding that ICE also makes filters for Ethernet.

You are not necessarily on-the-hook to solve RFI problems generated by a transmitter in or near a neighborhood, but often times it's good politics to help out.


Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at doug@dougirwin.net.


Resources

Fair-rite
www.fair-rite.com

Industrial Communications Engineers
www.iceradioproducts.com

K-Y Filter
www.ky-filters.com

Mike Sandman
www.sandman.com

Palomar Engineers
www.palomar-engineers.com

Sandman RFI analysis troubleshooting tree
www.sandman.com/files/RF-flow1.pdf


We need your tips Tech tips may be suitable to earn SBE recertification credits. Send your tips to radio@radiomagonline.com.




Acceptable Use Policy
blog comments powered by Disqus

[an error occurred while processing this directive]

Today in Radio History

Milestones From Radio's Past

The history of radio broadcasting extends beyond the work of a few famous inventors.

EAS Information More on EAS

NWS XML/Atom Feed for CAP Messages

The feed provides feeds for all US states and territories.

Wallpaper Calendar

Radio 2014 Calendar Wallpaper

Need a calendar for your computer desktop? Use one of ours.

The Wire

A virtual press conference

Information from manufacturers and associations about industry news, products, technology and business announcements.

Join Us Facebook Twitter YouTube LinkedIn
Radio magazine cover

Current Issue

“Free Beer” and “Hot Wings” in the Morning

Townsquare Media's WGRD builds a new studio for their syndicated morning show including video

Browse Back Issues

[an error occurred while processing this directive]