Radio Freuquency Interference

As an amateur radio operator on a small suburban lot, my receiver is under constant assault from a wide variety of RF noise generators. Everything from LED light bulbs to plasma TVs to networking equipment to defective streetlights have caused me all kinds of grief. Further down the page are some favorite references on RFI. I will not attempt to repeat what they say. They cover all the important topics, especially locating noise sources and mitigation procedures.

After a year-long trip to hell and back dealing with my neighbor’s LED light fixtures, I have decided to start characterizing various electrical devices, such as LED lights, and showing you which of them are the good and bad actors. Detailed reports appear below.

References and Articles

The ARRL RFI Book. Comprehensive. Everything you need to know.

RFI, Ferrites, and Common Mode Chokes For Hams by Jim Brown, K9YC. Jim is a leader in the application of ferrite materials and other techniques for interference reduction.

http://k9yc.com/RFI-Ham.pdf

Build Contesting Scores By Killing Receive Noise by Jim Brown, K9YC. This focuses on locating and mitigating noise in receiving systems.

http://k9yc.com/KillingReceiveNoise.pdf

Finding and Killing Receive Noise. A presentation I give at ham clubs: Finding_Noise_NA6O.pdf

Severe RFI: Neighbors, ARRL, FCC, and Me. The complete process from detection to resolution: Severe_RFI_NA6O.pdf

How to make a direction finding antenna: Handheld_DF_Antenna.pdf

And an even simpler DFing antenna from WD8DSB

Homebrew sniffer probes and current probes for measuring common-mode current.

Recommended LED Lights Power Supplies, etc.

This list will grow over time. Obviously there is an almost infinite marketplace full of devices that may generate noise and I can only afford to buy a few here and there. If you have something you want tested, I would be happy to do so; contact me and ship it over. Note that the primary emphasis here is on the HF bands, 1.8-50 MHz, but I do take data up to 150 MHz and note any obvious VHF issues. Click on a link for full info and data, and become a wise consumer. Items showing "Reported by" are from people who I trust 100% for reliable RFI testing. Rumors from Eham do not count!

My experience has shown that virtually every LED light fixture is a serious RFI generator. When in doubt, they should be avoided, but at the very least you should purchase a sample device and test it at your station.

My FCC Part 15 Measurement System

The figure below shows my test system. This follows the standard setup required for FCC and CISPR conducted emissions testing. Please note that I am not running a certified testing laboratory, but my results are reproducible and internally consistent. After 35 years as an instrumentation engineer, I usually get things right...

The heart of the system is a Line Impedance Stabilization Network (LISN) that I built, again based on commercial instruments and standards. Full details are in this writeup. It provides three things: Lowpass filtering to eliminate external noise from the AC line; a stable 50-ohm impedance for the device under test; and outputs for analyzers. I use a spectrum analyzer as well as an oscilloscope to document the RFI, and that is the data that is presented in my reports.

FCC Part 15 calls only for normal-mode (NM), or differential, noise voltage measurements, that is, line to ground and neutral to ground. However, we know from practical experience that this may be insufficient. In fact, common-mode (CM) noise--current flowing in phase on all conductors--is the primary source of radiated energy. In essence, all the wires to the device comprise the antenna, and the common-mode current is what excites that antenna. For that reason, I included a current transformer into my LISN that directly measures CM current. This is data that you will not find reported elsewhere. I use units of dB uA for this current. You can actually convert that to power delivered to an antenna if you like. Knowing the current, just multiply Amps (squared) by a nominal antenna impedance, and you have a (very rough) estimate of power.

NM voltage noise is still important, however, because some devices I have tested have proven to deliver much larger signals on one lead than the other. In that case, application of CM filtering alone (i.e., a CM choke) may be insufficient for mitigation.

A final observation that I make is a qualitative survey of radiated signal taken with a portable shortwave radio and a small direction-finding antenna. I use this rig all the time for locating noise sources and it is a trusted companion. If I can’t hear it on that radio, I know I will not hear it on my station receiver.

This is NOT what you want...

Clamp-on Current Probe

Sniffer Probes