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RFI Transmission Paths – How Does It Get Into My Receiver?

rfi transmission paths

Your house and neighborhood are full of RFI transmission paths. It’s time to understand these better as we fight the effects of interference on our radio hobbies. 

Over the years, I have written a lot about fighting radio frequency interference around my house. Most sources of my RFI are switched mode power supplies. either stand alone or incorporated into consumer electronics. But I have never talked much about how RFI travels.

Recently, one of my readers asked me about the different between conducted versus radiated interference. So, I thought it may be time to explore RFI transmission paths. There is a ton of information available online about RFI in general and SMPS interference in particular. A lot of it is pretty advanced and rarely provides a beginners with an overall framework.

Most of the literature on RFI/EMI presents the basic model of source-path-victim. A source obviously generates an interference. For most of us who listen to MF and HF, interference is a narrowband or broadband undesired signal that interferes with signals we seek to listen to on our victim – the receiver. Because our receivers are so sensitive, even weak interference can have a big impact. Typical sources contain some sort of oscillators that generate interfering signals.

There are three basic paths taken by RFI from source to victim: conduction, radiation and near field coupling. Conduction is the most common path, at least in the first instance. It is very easy for EMI from a power supply to link into household wiring and travel a great distance. Power lines serve as radio transmission lines and antennas. RFI from a SMPS typically starts out as low-level conducted interference and then migrates onto the second path: radiation. Finally, over very short distances, electrical signals will couple between devices by capacitance or inductance effects. This is the third path – near field coupling.

Try this with your portable AM receiver. Turn it on and walk around your house. You can actually hold it against the wall and use EMI to trace hidden wiring in your house. Or, you might hold the radio near a plug across the room from a noisy power supply and hear the RFI. The ability of RFI to travel from one power plug to another is conductance. Finally, you can pick up radiated RFI 10-20 feet (or more) from power lines – often stretching out into the back yard.

Most RFI uses at least two paths in combination. It might start out as conducted noise, and end up being radiated. Or near-field coupling can spread conducted noise along multiple cables, something often called cross-talk.

RFI Transmission Paths – Unintended Consequences

We never designed our world with RFI transmission paths in mind. Hams first discovered this when regulators licensed the original TV channel 1 right in the middle of a ham band.

AC household wiring was never designed to be balanced or shielded. Once EMI escapes into the wiring, it is easily conducted and radiated. Yes, interfering signals attenuate over distance and travel less distance at higher frequencies.

In the old days, inefficient linear power supplies rarely caused interference, except for some low frequency hum in audio equipment. But while modern SMPS raise efficiency from 30 to 80%, they are also near perfect MF/HF jamming devices even if compliant. And incorporating computers with ham radio and networking has made the problem worse.

In the next few articles we will talk more about RFI transmission paths and mitigation techniques.

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