Conformal mapping-based method and partial capacitance method are given to accurately calculate inter-digital capacitance and the inter-digital high impedance ground plane (HIGP) design formulas are revised.The models are useful for a wide range of dielectric constants and layer thickness.Every finger length and spacing may be different;moreover,the derived formulas can easily extend to multi-layer dielectric including cover layer.Three samples of the inter-digital high impedance ground plane are manufactured and the surface wave propagating property of the inter-digital HIGP are simulated and tested to demonstrate the potential of the models.