Abstract: The outer conductor of a high frequency electric cable of the kind employed as a stationary elongate element for transmitting high frequency signals to, or receiving high frequency signals from a receiving or transmitting device carried by a mobile body, has extending along its length at least one row of mutually spaced apertures, the mutual spacing between adjacent apertures of the or each row decreasing along the length of the row, being a maximum value at one end of the row and a minimum value at the other end of the row. The or each row of apertures may be sub-divided into a plurality of sub-groups of apertures, the mutual spacing between adjacent apertures of each sub-group being constant, and the mutual spacing between adjacent sub-groups decreasing along the length of the cable.