A variable capacitor having a groove portion formed in an insulating substrate, two upper portions of the substrate located on either side of the groove portion forming two lateral edges, a conductive layer covering the inside of the groove portion, a flexible conductive membrane, placed above the groove portion by bearing on the edges, a dielectric layer covering the conductive layer or the membrane to insulate the conductive layer and the membrane, and terminals of application of a voltage between the conductive layer and the membrane, and such that the depth of the groove portion continuously increases from one of the edges to the bottom of the groove portion, and that the conductive layer covers the inside of the groove portion at least to reach one of the two edges, that it may cover.