Tech Tips: Using Ferrites
I have worked on some tough RFI problems in my career. In one particular, an AM station was causing problems in an enormous warehouse that stored fireworks; it was getting in to their telephone system as well as an IP-based inventory system (all done via Wi-fi). In the end, judicious use of ferrites solved those problems. That's our topic this time around.
First, let's first review how these RFI problems come about. Any wire in the presence of an alternating magnetic field is by nature an antenna. In the vast majority of cases, the signals induced in those wires are just too weak to matter; however, if the wire is near a strong alternating field (like an AM or FM transmitter) enough current can be induced to flow in the wire to cause problems with whatever circuitry happens to also be connected to that same wire. The 'other circuitry' could be a telephone, an alarm system, someone's home entertainment system, a computer, or my personal favorite - an oven. (I was an active ham as a teenager - when I got on 40 meters, our house's oven thermostat used to go on and off.)
If you have just moved your transmitter site you have a year to resolve interference complaints inside the 1V/m contour for AM and the 115dBu contour for FM (see 73.88 and 73.318 respectively). Some devices are specifically excluded in 73.318; and perhaps your transmitter site has been in place for way more than a year. Still, it is probably wise to help your neighbors out in the mitigation of RF interference. Just keep in mind that there are limits to your liability as far as the FCC is concerned anyway.
A possible answer
Obviously when someone or something is near the transmitter site, there is little if anything you can do to move it farther away. Your neighbors probably aren't going to move just to get away from RFI. Likewise you certainly aren't moving the transmitter. So what has to happen then, to make this problem go away? Well, the installation of ferrites could be the answer. Ferrites work by greatly reducing the current flow, at the interfering frequency, in the 'antenna' that is the root of the problem. The 'wire' is made to look like a high-impedance at the interfering frequency, thus severely reducing the current flow, and likely curing the problem.
Ferrites come in different shapes, sizes, and formulas: there are ferrite beads, toroids, and split beads. Perhaps most importantly though, is the fact that different formulas are used for different interfering frequencies: mixes 73, 31, 43, 44, 61 and 64 are a few of those. Mix 73 is effective at AM frequencies; Mix 43 would be better for FM frequencies.
Beads are small cylinders through which you can run the wires that are picking up the unwanted RF. Of course to install those, you may have to un-solder, and then re-solder wires in the equipment you are trying to eliminate RF from. That, of course, isn't always practical. Toroids are used to mitigate common mode pickup; in other words, the same amount of RF on both leads (like with twin-lead - you can wrap that around the toroid) or coax - you can wrap that around the toroid to reject RF induced in to the shield. If that isn't practical, you can use split-beads - two pieces that come apart, fit around the offending cable, and then snap together. Likely you've seen these on computer cables before.
Next month we'll delve in to specific situations and how they can be solved.
Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at firstname.lastname@example.org.
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