Harmonics occur at integer multiples of the fundamental. If a body resonates at F, you will see harmonics at F, 2F, 3F...

You will not see harmonics at fractional (1/2 F, etc...) frequencies of the fundamental, though you will likely still see some backscatter.

Retroreflectors[0], to include the Luneburg lenses[1] mentioned in the article, function by directing radar energy parallel to its incidence direction. These reflectors will not significantly increase the plane's RCS for passive/bistatic receivers. Even if these reflectors did increase RCS omni-directionally, it is unlikely that a reflector designed to operate at 3 Ghz (10 cm) would also be designed to operate at 187.5 MHz (1.6 meters).

[0]https://en.wikipedia.org/wiki/Retroreflector [1]https://en.wikipedia.org/wiki/Luneburg_lens#Radar_reflector