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The aperture efficiency of an antenna is defined as the ratio of the effective aperture divided by the physical aperture. The typical values are between 0.35 to more then 0.70. Wikipedia

What is the maximum realizable aperture efficiency? What drive this upper limit?

Ed Tate
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Theoretically there is no limit. Consider a hertzian dipole, this has a constant gain, hence constant effective aperture, but it can be made as small as you like.

As the size shrinks, the bandwidth necessarily shrinks as well (the Q increases), look up a paper by Wheeler "Fundamental limitations of small antennas"

With large antennas (e.g. dishes), it is hard to utilise all the area (and it is usually a complicated tradeoff with spillover), and you can't do better than 1 in this case.

With small antennas, the situation is different, and a well matched, low loss small antenna may well exceed 1.

Tesla23
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That should be 1.

Example: imagine a plane wave front. A parabolic dish antenna focuses all the energy hitting its physical area on a feed. Effects that reduce its aperture efficiency stem from diffraction on its edge.

As you increase diameter of the dish, the edge effects contribute less and less to the overall result, and the dish can be considered more and more uniformly illuminated.

In practice, that won't happen for size, precision and feed shape problems, but there's no fundamental law that says that the aperture efficiency can't approach 1 with increasing size.

Marcus Müller
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    I don't disagree — I don't see any physical reason why you shouldn't be able to get as close as you like to 1 — but in real life NASA seems to consider it a [pretty good accomplishment](https://www.sciencedirect.com/science/article/abs/pii/S009457651731679X) to get 66% aperture efficiency on a 34m dish with a 9mm wavelength, which certainly counts as large. – hobbs May 03 '23 at 18:18
  • hm that's true, but I guess once you're in that size region, making the dish very exact and high-conductivity might be harder than making it larger, so it's possible that aperture efficiency isn't the primary optimization objective. – Marcus Müller May 03 '23 at 18:52
  • Since the scattering cross section can be larger than the physical cross section for a ~wavelength scale object, I'd guess you can probably get a little above 1.0 in some circumstances with a wavelength-scale antenna but I'm just speculating. – user1850479 May 03 '23 at 22:26