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In our antennas and fields course, we had one task where we simulated a line antenna in an incident plane wave. There, we specifically looked at the fields produced by scattering from the antenna. When looking at the poynting field of only the incident wave, we saw an energy flow with constant rate and direction everywhere, as expected. When combining incident and scattered fields and calculating the poynting field from the total fields, we saw that the antenna in a way sucked energy out of its surrounding area. The poynting vectors around the antenna where turned towards the antenna. "Behind" the antenna, they even changed direction fully.

Now I came across canonical minimum scattering antennas. The idea is (I think), that an antenna can be loaded with a specific inductive load, resulting in (practically almost) no scattered fields for a given frequency. As a result, the antenna can be modeled as scatteringless.

How does such an antenna extract energy from an incident wave? As far as our task was concerned, the energy was extracted by turning poynting vectors towards the antenna. But this doesn't happen without scattered fields, does it?

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  • $\begingroup$ suggest you try posting this on the amateur radio stack exchange; lots of friendly antenna experts over there. -NN $\endgroup$ Dec 21, 2020 at 3:44

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For further reference, I think I understood the maths behind this model now.

The minimum scattering antenna is modelled to be scatteringless when open-circuited. And that is the important part.

In the unloaded case, the antenna does not exhibit any scattering fields. Therefore, it can't extract any power from an incident wave. This is not a problem, because there is nowhere the power could be fed to.

Now in the loaded case, the antenna does exhibit scattering fields. And those fields again allow "manipulation" of the poynting vector to extract energy from the incident wave into the load (given the load does not reflect all of the energy).

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