The behaviour and structure of waves launched from a novel double-loop antenna in the SHEILA heliac are presented. Radial wave field and azimuthal phase measurements are compared with cylindrical helicon wave theory after an appropriate magnetic coordinate transformation. This comparison provides strong evidence for the involvement of helicon waves. Damping of the waves is investigated by comparing measured spatial damping rates with theoretical estimates. Comparison of measured density with predictions from the helicon dispersion relation reveals how geometry influences propagation of the waves in SHEILA.