Here are my pro tips to eliminate RFI (Radio Freqiency Interference) in recording equipment. For fast reference, actions you can take are bold below.
If the problem is intermittent, this may seem obvious, but the first tip is don’t try to fix a RFI problem at a time when you aren’t having RFI. You need to be able to hear it to fix it.
Radio waves are part of the same electromagnetic spectrum as light and as such, they have both a frequency and a power (amplitude). FM radio spans frequencies from 87.5 MHz to 108 MHz and AM radio spans 525Khz to 1625 Khz.
The web site radio-locator.com tells me there are 73 stations that may be within listening range. It also shows their broadcast power. There are 15 very high power AM transmitters in that group.
Because radio waves travel line of sight, like light, the curvature of the earth usually prevents ground-based transmissions from going much further than 40 miles (64 km).
At night, however, AM and SW (short wave) signals may travel hundreds of miles because the ionosphere, a layer of our atmosphere, reflects radio waves differently in the absence of the sun.
You will pick up some radio stations better at night because the reflection characteristics of the ionosphere are better at night, causing radio waves to bounce off and back down to earth beyond the horizon. Wait for daylight. Is the RFI gone? If so, only work when the sun is out, your problem is solved.
Identify sources of interference. In our case, it was clearly an AM radio station. Note that although radio waves travel line of sight they don’t need an unobstructed view. They can travel through foliage and some buildings.
You could at one point buy an AM antenna that was a simple mental loop 25 inches in diameter. Look for coils or loops your cables, uncoil them and lay them out straight. Don’t make antennas with your audio cables.
AM reception will get better when you turn your antenna a certain way. AM radio waves can travel through the air, but they can also ride on power lines. Go outside your building and observe which way the power lines are running, then back inside, lay your audio cables perpendicular to power lines. If your power cables run east to west, arrange your cables north to south.
Unfortunately you may have powerlines running in all different directions around your building. Move your cables around, listening carefully to see if the noise goes away.
Check the grounding in your outlets. Grounding improves suppression of radio waves. Without a connected ground pin — the third, rounded pin on an electrical plug — any electronic device can act more efficiently as an antenna. Grounding issues will most often cause static-like noise and in some cases, low-level radio signals can be discerned. You can get a circuit tester for around $10. If your outlets fail a test or you have outlets (such as in an old house) with only two prongs, have an electrician install properly grounded outlets.
Since radio waves can piggyback on AC power, make sure all power cables for your devices cross your audio cables perpendicularly, if at all.
Try to locate the problem: Turn things off one at a time, starting with lights and non-essential equipment.
A frequent source is a poorly made or improperly shielded microphone cable. Upgrade or replace your microphone cables with the highest quality cables possible. We still had RFI with a new 20 foot Mogami gold stage microphone cable.
Make sure you are matching impedances. Just because a connector fits does not mean it should be plugged in where it is. Check switches and settings in your equipment. Mismatched impedences can cause RFI to be noticed.
Ferrites can suppress RFI by converting EMI to heat within the ferrite core and dissipating it. Ferrites are molded metal powders cast into various shapes and sizes. They are usually made of Iron Oxide (Fe2O3) with Zinc, Copper, Zinc, and other metal oxides. Get some snap-on ferrite chokes, open them and snap them around wires or cables that you have determined are transmitting RFI. Try different places on the cable. Nearest the amplifier may work best. Stack as many as you want and stack different types if you have severe problems.
Usually the above tips will be enough. Steel-framed structures, stucco with embedded mesh-like wire, concrete walls with rebar, foil-backed insulation and aluminum siding can block AM signals. If you check all of the above and still have RFI, especially in a wooden structure, you may need a building modification.
If something else has worked for you, leave a comment.