AFAIK radio waves can be influenced by magnetic fields (when not traveling through a vacuum). Electrical wires create a magnetic field perpendicular to the length of the wire, as a function of the current running through it.

In air, water, or other medium, would radio waves be influenced by this magnetic field (or conversely by the electric field)?

Say for this purpose I have a radar which scan through walls, and I am interested in knowing wether the wires in the wall are currently "on" or "off". Would this be possible? How weak, or strong magnetic field (conversley electric field) is needed to influence radio waves (when not in a vacuum)?

Or have I completely misunderstood something here :) ?

Thanks alot!

  • $\begingroup$ Well, if you're asking whether photons are affected by a local EM field, the answer is no. See ,e.g., cosmoquest.org/forum/archive/index.php/t-53401.html $\endgroup$ – Carl Witthoft Jul 18 '14 at 11:47
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    $\begingroup$ You know antennas are just wires, right? $\endgroup$ – Phil Frost Jul 18 '14 at 11:50
  • $\begingroup$ And yes I know I can measure the electric or magnetic field of wires by other devices, but Im specifically interested in wether this is possible with a radar. Forgive me if I dont know all the theory around radars and magnetic fields, but thats why Im asking, since no amount of googling and reading can give me the answer Im looking for :) $\endgroup$ – Sevenius Jul 18 '14 at 11:55
  • $\begingroup$ Carl, yes I knew that, but Ive also seen several places indicating that radiowaves can be influenced by external fields, newton.dep.anl.gov/askasci/phy00/phy00804.htm $\endgroup$ – Sevenius Jul 18 '14 at 12:00
  • $\begingroup$ Right, but your link is talking about absorption and re-emission in the material, not free space. $\endgroup$ – Carl Witthoft Jul 18 '14 at 14:43

Radio waves are not perturbed by external magnetic fields. If you put your radio receiver in a magnetic field you may experience some electrical noise which blinds the signal, but this is still there untouched and your neighbour downstream would confirm this.

Even if the generic wire is a correctly matched antenna, the power that is absorbed from the wave is tiny and impossible to detect downstream without being in an extremely controlled environment.

That being said, I guess you have AC current in those wires. That's a good news since they emit EM-radiation by themselves. The bad news is that $50/60~$Hz radiation falls in the Extremely Low Frequencies, it requires huge antennas and is very hard to detect, especially for the tiny powers emitted by common wires. If you can access the wire, then you can use a non invasive tool like an induction clamp, this works very well.

If you have DC current in the wire then things are much more difficult as it only generates a static magnetic field coming from the Biot-Savart Law. You need to go close enough and sense it (it will be tiny and won't be easy).

  • $\begingroup$ You can however, easily pick up the 60hz signal with a scope and yourself as an antenna $\endgroup$ – aPhysicist Dec 10 '14 at 22:33

The EM-fields do not directly influence the radio waves.

The EM-fields can (if they are strong enough) influence the matter. The matter again can influence the radio waves.

So for this to work

  • you would need some matter which is easily influenced by EM, like a plasma
  • or you would need very strong EM-fields and very strong radio waves
  • or an extremely sensitive sensor (and than you will most probably still only measure directly the EM-field in the wall, or if you are far away a lot of other stuff) or a mixture of these above.

So, it is theoretical possible to do that, but if there are a lot of problems, and i doubt that this can be easily achieved.


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