Depending on your design, loading the antenna with a dielectric results in shorter wavelengths propagating in the substrate.
This usually translates to the following effects:
- decrease in input impedance.
- for the same size, resonance frequency decreases. (this means that you can have a smaller antenna to resonate at the same frequency)
- slightly poorer gain, as more energy is trapped in the substrate and increased losses due to surface waves.
There should be more, but off-hand, this is all i can think of for now. Good luck!