antenna-theory.com

[Rfdesign]

suppose we have a microstrip antenna like this:



then we add an additional layer exactly same with the substrate on the patch radiator,so what's the effect of this layer in the performance of the antenna?

[bigSteve]

A superstate on a microstrip antenna would be equivalent to a slight increase in the substrate permittivity.

Basically, when you surround an antenna with a dielectric (known as dielectric loading), the antenna radiates at a lower frequency than it otherwise would. This is because the antenna is "effectively longer" because the speed of electromagnetic waves is slower in dielectrics, so that the wavelength is smaller (hence the size of the antenna, in wavelengths is bigger).

The supersaturate is not nearly as important as the substrate for the radiated fields. However the effect is a slightly lower resonant frequency (depending on the permittivity), with a corresponding decrease in bandwidth.

[Rfdesign]