0
\$\begingroup\$

This is about confusion of relating antenna radiation theory to power lines. Assuming a straight 100km or even longer possible power transmisson line with 50Hz frequency. What can be said about the ability of 50Hz electromagnetic wave propagation to the space? I found this similar question but didnt find an answer.

I have read part of an answer to a previous question that it wont act as a dipole antenna but rather a transmission line, and the fields will cancel eachother. But I didnt really get what is meant and what is the limitation here. Is that because for radiation to occur the ends of the transmssion lines must be open? An illustrative answer appreciated.

Having asked that I also sometimes read that power lines acting like an antenna when it comes to EMC and radiated noise. So the topic of the power lines radiating “waves” is blur to me. Or when people talk about radiated interference do they also include the fields?

\$\endgroup\$
  • 1
    \$\begingroup\$ I forget the name of the system. It's been 50 years, perhaps? But the United States put in a 7 Hz transmitter in the state of Wisconsin which took up something like 1/3rd of the state for the antenna. This was to communicate with military submarines without requiring them to surface. There must be some published details about the design somewhere. \$\endgroup\$ – jonk Jan 30 at 4:59
  • \$\begingroup\$ 100km is kind of short. But the wires should be able to radiate, if they are excited properly. Under normal circumstances, the fields will cancel as you move away from the wires and they shouldn't radiate too much. But there are ways to make shorter antennas. Maybe by incorporating a coils or something, or some matching elements at the feed point. Note that the near-field of a 50 Hz cable extends for quite some distance due to the long wavelength. \$\endgroup\$ – mkeith Jan 30 at 5:00
  • \$\begingroup\$ The question does answer it. It does not meet the criteria to be a dipole antenna, as a dipole antenna for 50Hz would need to be 3000km long (lambda / 2) to radiate half-decently. I could be wrong, but I don't think there exists any powerline which is an unbroken, untransformed, piece of cable going that far. Since 100km, as in your example, is nowhere near this, it's equivalent to a 1kHz sine wave oscillating on a PCB trace - such little radiation, that even FCC does not require this slow of an oscillator to be tested. \$\endgroup\$ – DSWG Jan 30 at 5:36
  • 1
    \$\begingroup\$ Think in terms of a 1/8th to 1/16th wavelength antenna with a large loading coil to make up most of the resonate length. \$\endgroup\$ – Sparky256 Jan 30 at 5:43
  • \$\begingroup\$ @jonk Project Sanguine \$\endgroup\$ – JonRB Jan 30 at 8:33
2
\$\begingroup\$

A well designed, installed and operated 50Hz transmission line radiates very little EM waves. It’s short for that frequency, and more important the lines are arranged to cancel the fields pretty well compared to the wavelength of 50Hz (about 6000km).

But that doesn’t mean that there are no E and B fields in the few hundred meters around the line. On that scale, the cancellation is not complete. You can detect significant fields out to a few 10’s of times the conductor spacings.

It’s just that those are local fields, not radiating out as waves.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.