I've seen that there are some planar antennas which are not simply a metal patch above a substrate, but present also a dielectric superstrate.
Some examples of this configurations are:
- This planar biconical (aka bow-tie) antenna (taken from here, page 5)
- This ring antenna (taken from here, page 3)
According to both articles, the superstrate has the effect of alleviating the resistive losses created by a resistive frequency selective surface (RFSS) which is put under the substrate for impedance matching purpose.
I don't need a precise and accurate analysis of this, but only an intuitive description. Basically, the situation described by both articles is this (in which I've represented the resistive FSS with a resistor):
How can a dielectric layer above the antenna help to reduce the dissipated power on the resistor (resistive FSS)?
A professor has told me that the reason may be that the superstrate may attract more of the electromagnetic field toward itself. Less electromagnetic field goes to the resistive FSS below the antenna, more goes above the antenna, so, less resistive losses.
But, why should a dielectric superstrate enhance the electric field going towards itself?