An antenna design problem

Hi, everyone

Here is my antennas design problem.

I want to design an antenna, which can be used to communicate within 60 feet.
This antenna is inside the cavity on the infinite ground plane.
The size of cavity is 16" (long) X 1.7" (width) X 0.85" (depth).There exists the cover on the the cavity. The size of the cover is 16" (long) X 1.7" (width) X 1" (depth). The material of the cover are FR4. There exists two layers on the infinite ground plane. One layer is the mud liquid (er=81, conductivity = 0.01 S/m), whose thickness is 1.75" . Another layer is the rock (er=8, conductivity=1 S/m), whose thickness is infinite.

Because two antenna are inside the different cavities on the infinite ground plane, the desired antenna should be end-fire antenna. The maximum value of E field should be on the long direction of the cavity.

Right now, I want to know it is possibe to design this antenna, or not.

In my knowlegde, the needed frequency will be above 10GHz due to the limitation of the size of the antenna.

On other hand, the used frequency is only 100MHz in order to penetrate 60 feet under the earth ground.

Is it the contradiction?

Please provide me your opinion.

Thanks