A tunable impedance surface, the tunable surface comprising: a plurality of elements disposed in a two dimensional array wherein the tunable surface comprises a plurality of non-Foster circuits for controllably varying a negative inductance or capacitance between at least selected ones of adjacent elements in said two dimensional array; and being characterized in that the non-Foster circuits in said plurality of non-Foster circuits each comprise: two pairs of cross-coupled transistors (Q1, Q2, Q3, Q4); a reactive load coupled to first current carrying electrodes of a first one of the pairs of cross-coupled transistors and a first control resistance coupled to second current carrying electrodes of said first one of the pairs of cross-coupled transistors; a second control resistance coupled to current carrying electrodes of a second one of the pairs of cross-coupled transistors; current sources (CS1, CS2, CS3, CS4) for suppling a flow of current to at least one of the current carrying electrodes of each pair of cross-coupled transistors; two common-mode feedback networks (CMFB1, CMFB2), each common-mode feedback network being coupled to current carrying electrodes of each pair of cross-coupled transistors; and terminals coupled to current carrying electrodes of said second one of the pairs of cross-coupled transistors, a function of the product of the resistances of the first and second control resistances setting a conversion ratio between the reactive load and a load appearing across said terminals, said terminals being coupled to at least selected ones of said adjacent elements in said two dimensional array. a plurality of elements disposed in a two dimensional array wherein the tunable surface comprises a plurality of non-Foster circuits for controllably varying a negative inductance or capacitance between at least selected ones of adjacent elements in said two dimensional array; and being characterized in that the non-Foster circuits in said plurality of non-Foster circuits each comprise: two pairs of cross-coupled transistors (Q1, Q2, Q3, Q4); a reactive load coupled to first current carrying electrodes of a first one of the pairs of cross-coupled transistors and a first control resistance coupled to second current carrying electrodes of said first one of the pairs of cross-coupled transistors; a second control resistance coupled to current carrying electrodes of a second one of the pairs of cross-coupled transistors; current sources (CS1, CS2, CS3, CS4) for suppling a flow of current to at least one of the current carrying electrodes of each pair of cross-coupled transistors; two common-mode feedback networks (CMFB1, CMFB2), each common-mode feedback network being coupled to current carrying electrodes of each pair of cross-coupled transistors; and terminals coupled to current carrying electrodes of said second one of the pairs of cross-coupled transistors, a function of the product of the resistances of the first and second control resistances setting a conversion ratio between the reactive load and a load appearing across said terminals, said terminals being coupled to at least selected ones of said adjacent elements in said two dimensional array. two pairs of cross-coupled transistors (Q1, Q2, Q3, Q4); a reactive load coupled to first current carrying electrodes of a first one of the pairs of cross-coupled transistors and a first control resistance coupled to second current carrying electrodes of said first one of the pairs of cross-coupled transistors; a second control resistance coupled to current carrying electrodes of a second one of the pairs of cross-coupled transistors; current sources (CS1, CS2, CS3, CS4) for suppling a flow of current to at least one of the current carrying electrodes of each pair of cross-coupled transistors; two common-mode feedback networks (CMFB1, CMFB2), each common-mode feedback network being coupled to current carrying electrodes of each pair of cross-coupled transistors; and terminals coupled to current carrying electrodes of said second one of the pairs of cross-coupled transistors, a function of the product of the resistances of the first and second control resistances setting a conversion ratio between the reactive load and a load appearing across said terminals, said terminals being coupled to at least selected ones of said adjacent elements in said two dimensional array.