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Near and far field are usually explained for transmitting antennas, but since antennas are reciprocal, I would expect a receiving antenna to have a near-field phenomenon.
1) Does the impinging field (from a far-away transmitting antenna) cause the receiving antenna to develop its own near field, and what is its "shape" or "quality"? (I realize it would depend upon how far apart the antennas are.)

2)Continuing with this idea, is there a simulation anywhere such as this one made at MIT: http://web.mit.edu/8.02t/www/802TEAL3D/visualizations/light/dipoleRadiationReversing/DipoleRadiationReversing.htm except it shows two antennas simultaneously, one receiving and one transmitting? (Preferably, it would be a simulation of a transient broadcast.)

3)If there is a derived near field at the receiver, does the receiving antenna then re-broadcast any energy?

4)Extra credit: Does this touch upon Physics' "Arrow of Time" question?

EEatWork
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  • https://en.wikipedia.org/wiki/Near_and_far_field - there is a reason why this is only explained for a transmitting aerial - the clue is in the words. – JIm Dearden Feb 19 '16 at 22:44
  • Kindly elaborate. – EEatWork Feb 23 '16 at 20:22
  • The near field is generated by the transmitting aerial with the E and H fields essentially out of phase, the receiving aerial re-transmits the signal (and in effect becomes a 'transmitting' aerial). A mismatch in physical wavelengths also re-radiates the signal (see YAGI array or incorrectly terminated transmission line). An aerial is an aerial is an aerial. Dave has given an excellent answer below (+1 from me). As Dave says you can't distinguish the separate fields - they superimpose on each other. – JIm Dearden Feb 24 '16 at 10:52

2 Answers2

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Yes, antennas are reciprocal in every way.

1) Does the impinging field (from a far-away transmitting antenna) cause the receiving antenna to develop its own near field, and what is its "shape" or "quality"?

It's the same as for any antenna.

2)Continuing with this idea, is there a simulation anywhere such as this one made at MIT:

Yes, it's the same simulation.

3)If there is a derived near field at the receiver, does the receiving antenna then re-broadcast any energy?

Yes, but you can't distinguish it from the impinging signal. Its effects are seen as a modification of the field — a reflection or refraction.

4)Extra credit: Does this touch upon Physics' "Arrow of Time" question?

No, not really.

Bascially, any conductive or dielectric object will interact with a propagating EM wave in a way that is defined by Maxwell's equations. You can think of the object as creating a local modification to the effective impedance of the space around it. The field causes charges to move, and moving charges cause propagating fields. It really doesn't matter "which came first".

The Feynman Lectures on Physics, particularly volume II, provide a lot of insight into this topic.

Dave Tweed
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  • It sounds like the asker is unclear on the reciprocality of antennas, and most of his or her confusion stems from that. This is an excellent answer, but it might be worth adding a little bit more about them being reciprocal to drive the point home. Basically, he needs to understand that the antenna is just a thing with charge carriers wiggling around. The thing that is causing the wiggles is irrelevant. Wiggles are wiggles, whether its due to impinging EM waves or a 1000W transmitter dumping out power. – metacollin Feb 19 '16 at 23:27
  • @metacollin not _entirely_ accurate. A 1KW xmitter will cause far more element heating & thus increase effective resistance & alter the tuning of a 5W-20W capable 'whip' antenna... not to mention corona effects being _far_ more likely on TX than RX. :-P - - But I digress; as for near-field E & B fields, incidet or radiated signals should show basically identical behaviour. – Robherc KV5ROB Feb 19 '16 at 23:48
  • Thanks. To be fair, the antennas are reciprocal but but I insist that there is a difference between an outgoing wave and an incoming wave, so the events at each antenna must somehow be different. The wavefront will be concave or convex, depending upon the direction of propagation, relative to the antenna positions. This simulation helps to express my idea more fully: http://web.mit.edu/8.02t/www/802TEAL3D/visualizations/light/CreatingRadiation/creatingRadiation.htm What I'm trying to conceptualize is how does the outgoing wave deform as it couples into the receiving antenna. – EEatWork Feb 23 '16 at 20:24
  • Readers may be interested in these resources which explain the theory and offer the source code behind the cited simulations: http://web.mit.edu/viz/soft/visualizations/DLIC/DLIC.htm http://web.mit.edu/viz/soft/ – EEatWork Feb 29 '16 at 15:36
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A receiving antenna has an effective area (called an aperture) and it's measured in square metres. The far-field EM wave comprises E and H fields and these are expressed as volts per metre and amps per metre respectively. Therefore, the "signal" received by the antenna (when correctly terminated electrically) is

aperture area \$\times\$ volts/m \$\times\$ amps/m = volts \$\times\$ amps i.e. real power.

This has to mean that the receive antenna casts a shadow behind it i.e. all there is no (or very little) power passing beyond the aperture. This in turn means that the antenna MUST create a near field that disrupts the far field power it receives.

It's also interesting to consider that an unterminated and lossless antenna has no effect on the power passing through its aperture i.e the near field it creates is zero.

Andy aka
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