E-N voltage below 2V

Hi Friends,

I need help. I have done earthing for one of my dish. Earth resistance
is 1.5 ohm.

Requirement :- The voltage difference between E-N ports should be
below 2V so as to safe working of equipment.

Present method of earthing :- Earthing depth is 9ft, at base Copper
plate of 30cm x 30cm, from copper plate copper strip of 19mm width up
to the dish base structure.

With this set up required voltage difference is not up to the mark.

If any one know how to reduce this E-N voltage below 2V please let me
know.

Thanks

Gururaj