The characteristics of new microwave integrated circuits called earthed and isolated finlines, namely the cutoff wavelength, the equivalent dielectric constant, attenuation constant, and characteristic impedance based on power-voltage and power-current definitions are calculated. The ridged waveguide characteristics are found direct from the results of the earthed finline setting εr = 1. For some symmetrical configurations the transcendental equations resulting from the calculations can be solved approximately; the results being represented in a closed form approximate to those arising from numerical calculations to a high degree. Some of the results are confirmed by measurements.