I encounter sayings like this everywhere:

RFI interference is unintentional radiation from sources such as electric power transmission lines.

My confusion comes from my following thinking:

1-) First of all, power transmission lines work with 50/60Hz which is not radio frequency.

2-) Electric power transmission lines produce electromagnetic fields not radiation like antennas, so calling it radiofrequency interference doesn't make sense to me. It should be called electromagnetic field interference?

I'm sure I have lack of knowledge somewhere but couldn't figure out.

  • \$\begingroup\$ Note that the radiation from antennas is also an electromagnetic field although usually at much higher frequencies than for power transmission. \$\endgroup\$ – Barry Sep 13 '17 at 22:49
  • \$\begingroup\$ One can make an antenna radiates with huge wavelength at 50Hz yes, but the power lines have no directivity; Im not sure they really radiate the em fields(WAVE PROPAGATION), Do they? Which direction? \$\endgroup\$ – floppy380 Sep 13 '17 at 22:54
  • \$\begingroup\$ You can use your body as an antenna for 50/60Hz and touch the scope probe tip and prove to yourself you are a wireless antenna. But we usually say EMI not RFI anymore. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 13 '17 at 23:49
  • \$\begingroup\$ @TonyStewart.EEsince'75 At 50Hz the wavelength is 6000km. For an antenna to radiate that long wavelength do you think I am tall enough to radiate or receive? In antenna theory there must be a relation between the antenna size and the wavelength for propagation. What do you think? \$\endgroup\$ – floppy380 Sep 14 '17 at 0:03
  • \$\begingroup\$ I think we are not like resonant patch antenna but rather simply a big lump of dielectrics with high permittivity (k>80) for easy capacitive coupling to high impedance electric fields. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 14 '17 at 0:08

Power lines get RF injected onto them

The power lines are a wonderful antenna, actually -- they get RF noises injected onto them by gadgets all the time via said gadgets' power cords (this is why we have conducted interference tests), and then proceed to happily radiate these noises out.

You are right that calling interference at the power-line frequency "radio frequency interference" isn't all that sensible, though -- most power-line coupling is closer to a near-field scenario than the far-field stuff we deal with when dealing with other sorts of RFI problems. The catch-all term you're after is simply "electromagnetic interference", or EMI for short, as EM fields encompass both the near-field (either magnetically dominated or electrostatically dominated) and the far-field (both fields converge into a unified EM field) case.

  • \$\begingroup\$ You mean the powerlines first absorb the noise from gadgets and then radiate them with higher power?? Never heard about this. \$\endgroup\$ – floppy380 Sep 13 '17 at 22:51
  • \$\begingroup\$ @doncarlos -- clarified for you \$\endgroup\$ – ThreePhaseEel Sep 13 '17 at 23:03

Radios work at low frequencies (well below AM band of 0.55 to 1.65Mhz) and motors contribute very sharp-edged spark energy to the power lines. The older fluorescent lights, with that arc re-igniting very 8milliSeconds (120 times a second), have 10 microsecond spikes riding atop the 60Hz waveform.

Thus motors and fluorescent lights provide trash that interfers with radios.

Remember the automobile ignition wires....trashing AM radios.


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