Between sources and victims of received RFI are four primary RFI paths. Each requires understanding and applying different mitigation approaches.
To mitigate radio frequency interference, you need to find the sources and consider each of the primary RFI paths. For the moment, we will treat any sources outside your own home as a “black box” , i.e. your neighbor’s house is simply a source of RFI radiation.
Most people initially think of RFI as radiated interference. Actually, very little interference in MF/HF bands starts off as radiated. Simply put, the components and connections inside a source are not long enough to radiate at these frequencies. Most radiated interference we experience follows the conducted⇒radiated path (#3). EMI gets first conducted into household wiring and then radiated second. Assuming you can’t enter your neighbor’s house to do mitigation, houses all around you are simply radiators.
Once radiated, RFI is simply just another signal picked up by your antenna. Your only tools are directional antennas or spatial filtering, i.e. phasing with two different antennas. This works pretty well if the noise is far enough away to be a point source, e.g. a few hundred feet.
Another way that radiated noise gets into your receiver is by common-mode coupling with the outside shield of your antenna’s coaxial cable. This is shown as the radiated⇒conducted path (#2). You will find this to be a very common path for radiated RFI to couple from household wiring (yours or your near neighbors) into your receiver. Other cables attached to your radio, such as for audio or computer control, can also enable common-mode coupling directly if not properly isolated or shielded.
Within your own home, the main RFI source is switched mode power supplies or appliances that use PWM motor control (including furnaces, washing machines and treadmills). A normal home will have 20 – 40 such sources, a few of which will be major culprits. These conduct⇒radiate RFI through the power cable into household wiring. Also, you will experience radiation from noise coupled to DC power cables for computers, phones, monitors, etc. Path #3 is the worst of the primary RFI paths for most people.
Lastly, paths of pure conductance (#4) often exist between power supplies attached to receiving equipment. These are also the source for ground-loop conductance or near field coupling between cables. Keep in mind, also, that grounds in our homes are designed for safety not RFI mitigation. EMI noise travels well along many common paths, including “grounds”.
Primary RFI Paths and Filter Locations
The best places to filter EMI among the four paths are shown with “¬” symbols above. Purely radiated RFI from a device is tacked by shielding the device. As for the rest, filter conducted noises at the point where they leave the source or enter the victim. Both in the case of the pure conducted path, #4.
There are many articles and discussions on the web about filtering, particularly the use of ferrite cores. If you have tried and failed to reduce RFI with ferrite cores, it might be that you using the wrong ferrite mix (#31 and #43 are right for MF/HF) or that you have not yet figured out the true RFI path.
If you want a deep dive into the technical aspects of EMI and paths, check out Murata’s Noise Suppression Basic Course online.