A field displacement isolator design in finline technique is described using a lamella structure including a ferrite layer, energy absorbing layers and a dielectric spacer layer. An isolator with a forward insertion loss of 1.3 dB and a reverse loss higher than 35 dB at 28.5 GHz was developed. The design was based on a field theory for the calculation of the power-density distribution inside the isolator, exact measured material parameters of the ferrite material and experimental optimisation of the lamella structure. The design procedure and the results are described in detail.