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As per the title, I'm interested to know if it's possible, either theoretically or practically, to have an antenna design which has a constant effective aperture (antenna aperture - these are the same thing right?) over some (non-infinitesimal) frequency range. If so, what are the conditions required?

For context, I have a hunch that for any antenna if the frequency is below a certain cutoff point it will have a constant effective aperture with frequency, but I've been unable to find anything to prove this hunch.

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  • \$\begingroup\$ For what it's worth, I don't think so. \$\endgroup\$
    – Andy aka
    Commented Jun 22, 2023 at 14:36
  • \$\begingroup\$ The aperture size is fixed by the physical size of the antenna, at least for antenna arrays. \$\endgroup\$
    – SteveSh
    Commented Jun 22, 2023 at 15:55
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    \$\begingroup\$ @SteveSh I'm referring to the effective aperture, which typically varies with wavelength. It's not a fixed quantity \$\endgroup\$
    – Christian
    Commented Jun 22, 2023 at 15:57
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    \$\begingroup\$ @Christian - We build antenna arrays using notch or patch elements. For those antennas the effective aperture (the capture area) is the same as the physical aperture. \$\endgroup\$
    – SteveSh
    Commented Jun 22, 2023 at 16:27
  • \$\begingroup\$ Acoustic horns provide constant directivity by using shapes that open up fast enough to let frequencies leave the boundary before they get too focused. I would imagine that an antenna where you don't have as localized an emitter as with a horn driver but can better distribute active elements and guides would be able to use similar techniques. Acoustical horns of good quality tend to cover about a decade of frequency. \$\endgroup\$
    – user107063
    Commented Jun 22, 2023 at 23:36

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Your hunch that there is a "cutoff point" in the relationship between frequency and aperture is correct, but you have the consequences reversed.

For antennas that are large compared to the wavelength -- such as a horn, dish, or large array -- the effective aperture is roughly the physical size of the antenna, and can be fairly constant over a wide range of wavelength (or frequencies).

For antennas that are on the order of a wavelength or smaller -- which you could consider as being below your "cutoff point" in frequency -- it's practically very difficult to design an antenna with a constant effective aperture over a wide range of frequencies. As the antenna becomes small with regard to wavelength, the radiation resistance drops and the reactive impedance rises, leading to losses that reduce the effective aperture.

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  • \$\begingroup\$ But you can design antenna (elements) that have very little insertion loss (less than 0.5 dB) over a 10:1 or 20:1 frequency range. \$\endgroup\$
    – SteveSh
    Commented Jun 22, 2023 at 19:38
  • \$\begingroup\$ I think my hunch came from seeing a graph of the correction factor for a magnetic loop antenna, which had a negative gradient on a log-log graph up to a certain frequency, at which point the graph became flat. Baring in mind the relationship between effect aperture and wavelength, it suggested to me that the antenna may have a constant effective aperture in the region of negative gradient (though this reasoning doesn't consider varying values of gain). \$\endgroup\$
    – Christian
    Commented Jun 26, 2023 at 8:11
  • \$\begingroup\$ Example graph I was referring to in my above comment: researchgate.net/profile/Michal-Ulvr/publication/262308078/… \$\endgroup\$
    – Christian
    Commented Jun 26, 2023 at 8:12

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