Weird near-field phenomenon I get in my EM simulation

[Rotem Tsafrir]

TL;DR Summary

I get interesting/weird near field behavior in a EM simulation I made. "Nodes" (closed regions of low intensity E field) seem to be "emitted" from the antenna at a speed higher than speed of light, then slow down as they approach far field.

I recently made a basic simulation of wire antennas and I am not sure if the near field in my simulation is modeled correctly. One of the things that worry me is the fact that sometimes I see in my simulation "movements" in the near field that seems to be faster than the speed of wave propagation I defined (the speed of light in the simulation).

Specifically I see "nodes" of low amplitude in the E field that are quickly "emitted" from the antenna and then slow down as they approach the far field. In the far field I get the expected behavior.

In the picture above I circled in black one of such "nodes" that seems to move faster than light in the near field.

Does anyone know if it makes sense? (I know that transmitting information faster then light is a big no no but maybe what I see is still possible since I doubt you can use this to transmit info faster than light)
Link to GitHub project:

https://github.com/rotemTsafrir/dipole_sim

 

[Swamp Thing]

Could it be something analogous to the "superliminal scissors" thing where the point of intersection moves faster than light?

Say we have two plane waves moving in nearly opposite directions, but with say a 181 degree angle rather than 180. Then the nodes / antinodes might move faster than light? The resolution of the paradox is that the nodes are not being emitted from one place and then propagating through space; the propagation of information is at right angles (approx) to the node movement.

Another analogy: moire patterns seem to move much faster than the grids that produce them.

 

[Baluncore]

The group velocity propagates at the speed of light.
The phase velocity moves at between the speed of light and infinity.

Consider a wave impacting a seawall, at a close to perpendicular angle. The point of wave impact with the wall, will travel at a very high velocity. At more gentle angles, the wave will run along the wall, at a slower speed, that is still greater than the group or wave velocity.

Where two waves are emitted towards each other, from different points, their crests will first meet with an infinite phase velocity, from midway between the two points. I believe the sum of those two wave crests, is what you are seeing, which is a wave phase velocity phenomenon.

Super-luminal scissors, and moiré patterns, are phase velocity phenomena.

 

[Rotem Tsafrir]

That’s seems like a great explanation.
Thanks a lot!

 

[Rotem Tsafrir]

Could it be something analogous to the "superliminal scissors" thing where the point of intersection moves faster than light?

Say we have two plane waves moving in nearly opposite directions, but with say a 181 degree angle rather than 180. Then the nodes / antinodes might move faster than light? The resolution of the paradox is that the nodes are not being emitted from one place and then propagating through space; the propagation of information is at right angles (approx) to the node movement.

Another analogy: moire patterns seem to move much faster than the grids that produce them.

Thanks a lot! That seems to be what am seeing

 

[Borek]

Imagine pointing a laser at wall - there is a red dot visible. Now start to rotate the laser - dot moves. As angles change dot moves faster and faster, at some point it moves faster than light.

But is there anything really moving?

 

[sophiecentaur]

But is there anything really moving?

Nothing with mass is actually moving faster than c.

Take two sensors on the wall with their outputs taken along cables of equal length to a display. The moving spot passes over each sensor and the relative timing is shown on the display. (Clearly, it's a long wall, far away) The speed of the spot will be given by the distance between the two sensors and the time difference on the display. There's no limit to the measured speed of the spot. Neither sensor has any idea (information) about the other; it just records the spot as it goes past so there is no violation of any law.

 

[tech99]

Hertz carried out what was, in effect, a race between a wave on a wire and a wave in free space *. If I am reading his paper correctly, he noticed that, close to the antenna, the electric wave had a velocity much greater then c. So one might ask, are electric waves limited to velocity to c when no net energy is being propagated, as in the induction field?
(* Electric Waves, 1893).

 

[Baluncore]

If I am reading his paper correctly, he noticed that, close to the antenna, the electric wave had a velocity much greater then c.

Hertz observed the difference between group and phase velocity.

The group velocity propagates at the speed of light.
The phase velocity moves at between the speed of light and infinity.

 

[sophiecentaur]

So one might ask, are electric waves limited to velocity to c when no net energy is being propagated, as in the induction field?

Here we go again, people (not you) want to apply a 'rule' when it's not actually relevant.

The apparent speed of the waves along the wire can be anything when a plane wave arrives off-axis. It happens with sea waves breaking on a beach. The group velocity along a long wire antenna can be increased by increasing its height above the ground and more energy can be extracted from an incident tilted plane wave.

 

[tech99]

Here we go again, people (not you) want to apply a 'rule' when it's not actually relevant.

The apparent speed of the waves along the wire can be anything when a plane wave arrives off-axis. It happens with sea waves breaking on a beach. The group velocity along a long wire antenna can be increased by increasing its height above the ground and more energy can be extracted from an incident tilted plane wave.

In the Hertz experiment, he did not rely on the waves impinging on the wire. He arranged for an antenna to sample both the wave on the wire and the wave in free space. The velocity on the wire will of course differ somewhat from c, but only within a small range, and Hertz observed very large values for the free space velocity. I cannot see a mechanism for a huge increase in phase velocity of the radiated wave. I think it is more likely he was seeing the phase velocity of the induction electric field, which would be strong at close distances from the transmitting antenna. Similarly with the present question.

 

[sophiecentaur]

I cannot see a mechanism for a huge increase in phase velocity of the radiated wave.

It's got to be complicated because the only wave that will pass the wire at c would be a plane wave, incident or launched along the axis of the wire. A wave, normal to the wire would have infinite velocity (breakers on a beach). So I think that implies there must be a number of near field waves (or at least the equivalent of that).

Something that always escapes me is how 'reciprocity' is invoked and there's an assumption that the antenna pattern is the same for transmit and receive. Basic theory treats a transmitting wire and a sinusoidal current distribution (part of a standing wave) and that model delivers a convincing VRP and HRP. I guess this is the same as

He arranged for an antenna to sample both the wave on the wire and the wave in free space.

The wave near the wire has H and E in quadrature phase (as in a transmission line) and the distant wave has them in phase. Between them, a miracle must occur.