A conductor in a laminar structure, such as a printed circuit board or thin-film stack, is closely flanked by at least one open trench filled with an ambient medium (e.g., air, another gas, vacuum) of a lower dielectric loss than the conductor's surrounding dielectric. The trench may be made by any suitably precise method such as laser scribing, chemical etching or mechanical displacement. A thin layer of dielectric may be left on the sides of the conductor to prevent oxidation or other reactions that may reduce conductivity. When the conductor carries a signal, part of an electric and/or magnetic field that would ordinarily travel through the surrounding dielectric encounters the low-loss ambient medium (e.g. air) in the trench. The effective dielectric loss surrounding the conductor is lowered, reducing signal attenuation and crosstalk, particularly at high frequencies.