Someone "invented" a previously unpublished military design.

Century means there is a factor of a hundred between lower and upper frequency that the antenna efficiently works on.

Decade means a factor of 10.

Bandwidth is typical the range of frequencies where the antenna is efficient.

Typically we categorise this by comparing it's gain at the centre frequency with how much you have to deviate before the gain drops some set amount.

Let's say it's resonant at 10GHz with a gain of 6dB. Typically we care about the range it's within 3dB of that. Gain in this setting is less about amplification and more about concentrating the energy into a region of space rather than out in all directions.

If you go down to 3Ghz (probably 3.16GHz actually)and up to 30GHz( probably 31.6ghz)and it's still withing 3dB of 6dB, you have a decade bandwidth.

If you can do a century you're looking at 1 to 100GHz

An Octave is 1/sqrt(2) to sqrt(2) a decade is 1/sqrt(10) to sqrt(10) and a century is 1/sqrt(100) to sqrt(100) range.

So if you have an A4 note at 440Hz and you want a filter that captures an Octave with that A at its centre you would need a filter from 440/sqrt(2) =311 and 440×sqrt(2) = 622 which are d#4 and d#5. 6 semitones up from D#4 you get A4, another 6 and you have D#5. Musically it sounds like A4 is exactly between D#4 and D#5.