Increasing the impedance of an antenna

A method of increasing the impedance of an antenna having a number of radiating elements comprises removing conductive material from at least one of the elements so as to increase the impedance of the element. The method preferably involves coupling the antenna 40 to a radio frequency source 48, whose frequency is swept across a band including the desired operating frequency of the antenna, and capacitatively coupling probes 42A-D to various positions on the elements so as to monitor the voltage and relative phases of the induced signals. Conductive material can then be removed by laser etching so as to satisfy various monitored conditions e.g. so that the resonance frequency of the antenna is closer to the desired frequency (Figure 4). In the embodiments of the invention this method is applied to a helical quadrifilar antenna (Figure 1) whose elements are mounted on a cylindrical dielectric core. Preferably the elements run along the sides of the cylinder and also across one of its flat ends, so as to be connected to a feed which passes through the centre of the core, and the conductive material is removed from the portions of the elements running across the flat end of the core to produce a rectangular aperture 26A-D in each of the elements.