The effect of burial depth depends on your soil. For example, in average soil (.005 S/m, 13) the skin depth at 1.8 MHz is about 6 m (20'). Even at 3' you will still keep current out of much of the soil column but there will be some loss in the top layer of soil. Burial up to 1' is quite common and considered effective. 73, Rudy N6LF