A method of designing an antenna (106, 806) for an implant device (102, 802) for implant in a human or animal body, the method comprising: generating an initial design, for use as a current design of said antenna (106, 806), targeted at achieving at least one predetermined design goal, said current design of said antenna (106, 806) comprising a magnetic field radiator portion (710, 810) and an electric field radiator portion (708, 814) coupled to the magnetic field radiator portion (710, 810) at a current location on the magnetic field radiator portion (710, 810); generating a body model for modelling lossy body tissue of said human or animal body; simulating performance of an antenna (106, 806) formed in accordance with said current design, and located within a body corresponding to said body model; comparing simulated performance of said antenna (106, 806) with said at least one predetermined design goal to determine if said current design meets said at least one predetermined design goal; modifying said current design, by modifying said current location where the electric field radiator portion (78, 814) is coupled to the magnetic field radiator portion (710, 810), when the current design does not meet said at least one predetermined design goal, and repeating said simulating and said comparing for said current design so modified; and confirming said current design as the basis for fabrication of an antenna (106, 806) when the current design meets said at least one predetermined design goal; wherein said confirming confirms a current design as the basis for fabrication when, based on the comparison of simulated performance of said antenna (106, 806) with said at least one predetermined design goal, it is determined that an antenna (106, 806) fabricated to that design will excite, when in operation and located in said human or animal body, at least one of a transverse electric leaky wave and a transverse magnetic leaky wave in lossy body tissue of said human or animal body to cause said lossy body tissue to act as a waveguide for said at least one of a transverse electric leaky wave and a transverse magnetic leaky wave and thereby cause at least one of the efficiency of the antenna (106, 806) and the far field gain of the antenna (106, 806) meets a predetermined criterion. generating an initial design, for use as a current design of said antenna (106, 806), targeted at achieving at least one predetermined design goal, said current design of said antenna (106, 806) comprising a magnetic field radiator portion (710, 810) and an electric field radiator portion (708, 814) coupled to the magnetic field radiator portion (710, 810) at a current location on the magnetic field radiator portion (710, 810); generating a body model for modelling lossy body tissue of said human or animal body; simulating performance of an antenna (106, 806) formed in accordance with said current design, and located within a body corresponding to said body model; comparing simulated performance of said antenna (106, 806) with said at least one predetermined design goal to determine if said current design meets said at least one predetermined design goal; modifying said current design, by modifying said current location where the electric field radiator portion (78, 814) is coupled to the magnetic field radiator portion (710, 810), when the current design does not meet said at least one predetermined design goal, and repeating said simulating and said comparing for said current design so modified; and confirming said current design as the basis for fabrication of an antenna (106, 806) when the current design meets said at least one predetermined design goal; wherein said confirming confirms a current design as the basis for fabrication when, based on the comparison of simulated performance of said antenna (106, 806) with said at least one predetermined design goal, it is determined that an antenna (106, 806) fabricated to that design will excite, when in operation and located in said human or animal body, at least one of a transverse electric leaky wave and a transverse magnetic leaky wave in lossy body tissue of said human or animal body to cause said lossy body tissue to act as a waveguide for said at least one of a transverse electric leaky wave and a transverse magnetic leaky wave and thereby cause at least one of the efficiency of the antenna (106, 806) and the far field gain of the antenna (106, 806) meets a predetermined criterion.