An adjustable constant impedance phase shifter comprising: a circuit element comprising a conducting circuit path, a conductive sheet, a dielectric plate and a conductive plate, characterized in that: an outer edge point on the circuit element is steadily positioned over an outer edge of the conductive plate; an outer edge point on the circuit element is steadily positioned over an outer edge of the dielectric plate; the conductive circuit path is defined as a curve between the said outer edge points; the conductive circuit path is arranged between the conductive sheet and a parallel plane parallel to the conductive sheet; a spacing between the conductive plate and the conductive sheet is narrower than a spacing between said conductive plate and the circuit element; an edge of the dielectric plate and an edge of the conductive plate are adjoined such that the dielectric plate and the conductive plate form a slide member; and the slide member is movably arranged along a slide path between the circuit element and the conductive sheet so that any point of the conducting circuit path is consistently enclosed between the slide member and the parallel plane, and so that if a point on the conducting circuit path leaves the immediate vicinity of the dielectric plate, it enters the vicinity of the conductive plate such that the relative permittivity of a medium adjacent to the said point is simultaneously changed as the slide member is moved; and the circuit element and the slide member are arranged such that no part of a transverse, relative the slide path, portion of the circuit element can be transgressed by an intermediate boundary between the adjoined edges (131, 141) of the dielectric plate and the conductive plate. a circuit element comprising a conducting circuit path, a conductive sheet, a dielectric plate and a conductive plate, characterized in that: an outer edge point on the circuit element is steadily positioned over an outer edge of the conductive plate; an outer edge point on the circuit element is steadily positioned over an outer edge of the dielectric plate; the conductive circuit path is defined as a curve between the said outer edge points; the conductive circuit path is arranged between the conductive sheet and a parallel plane parallel to the conductive sheet; a spacing between the conductive plate and the conductive sheet is narrower than a spacing between said conductive plate and the circuit element; an edge of the dielectric plate and an edge of the conductive plate are adjoined such that the dielectric plate and the conductive plate form a slide member; and the slide member is movably arranged along a slide path between the circuit element and the conductive sheet so that any point of the conducting circuit path is consistently enclosed between the slide member and the parallel plane, and so that if a point on the conducting circuit path leaves the immediate vicinity of the dielectric plate, it enters the vicinity of the conductive plate such that the relative permittivity of a medium adjacent to the said point is simultaneously changed as the slide member is moved; and the circuit element and the slide member are arranged such that no part of a transverse, relative the slide path, portion of the circuit element can be transgressed by an intermediate boundary between the adjoined edges (131, 141) of the dielectric plate and the conductive plate. an outer edge point on the circuit element is steadily positioned over an outer edge of the conductive plate; an outer edge point on the circuit element is steadily positioned over an outer edge of the dielectric plate; the conductive circuit path is defined as a curve between the said outer edge points; the conductive circuit path is arranged between the conductive sheet and a parallel plane parallel to the conductive sheet; a spacing between the conductive plate and the conductive sheet is narrower than a spacing between said conductive plate and the circuit element; an edge of the dielectric plate and an edge of the conductive plate are adjoined such that the dielectric plate and the conductive plate form a slide member; and the slide member is movably arranged along a slide path between the circuit element and the conductive sheet so that any point of the conducting circuit path is consistently enclosed between the slide member and the parallel plane, and so that if a point on the conducting circuit path leaves the immediate vicinity of the dielectric plate, it enters the vicinity of the conductive plate such that the relative permittivity of a medium adjacent to the said point is simultaneously changed as the slide member is moved; and the circuit element and the slide member are arranged such that no part of a transverse, relative the slide path, portion of the circuit element can be transgressed by an intermediate boundary between the adjoined edges (131, 141) of the dielectric plate and the conductive plate.