The radiation from the open-circuited end of an externally dielectriccoated metallic waveguide can be controlled by varying the size of the guide, the thickness and/or the dielectric constant of the coating. A new type of aerial designed around this principle is given the name ‘surface-wave aerial’ and radiation-pattern measurements are used to confirm the theory underlying this type of radiator.According to its mode of operation it belongs to the category of end-fire aerials, which are briefly reviewed to show that surface-wave aerials occupy a place in their own right amongst the many possible arrangements that utilize the end-fire effect to produce a directional radiation pattern.The theory of the surface-wave aerial is developed in detail, a necessary preliminary step being a full theoretical analysis of the properties of the first-order cylindrical surface wave. It is shown that a dielectric-coated cylinder which is approximately a wavelength in circumference can act as a waveguide for higher-order surface waves, of which the first order is an example.The ‘characteristic equation’ is determined for the general case from which the cut-off frequency, propagation coefficient and conditions under which propagation can take place are derived in turn.Two specific cases are evaluated numerically and the results are used to calculate the polar diagrams of surface-wave aerials operating at 9Gc/s.The wavelength constant as well as the continuous radiation loss of a surface waveguide for which λg/λ0≃1 are measured directly and the results obtained are used to account for the radiation pattern of very long aerials. In conclusion, an outline of future work is given.