An antenna structure has a rotatable dipole array constituting a driven antenna element, a cup mounted around the dipole, a first conductor connected to and extending from the antenna, and a first dielectric mounted around the first conductor. A second conductor is mounted around the first dielectric, and a second dielectric is mounted around the second conductor. A third conductor is mounted around the second dielectric, and the first and second conductors form a quarter-wavelength transformer. The first dielectric maintains a uniform spacing between the first and second conductors, and the third conductor defines a radio-frequency coupling cavity. The second dielectric electrically enlarges the cavity. Thus the physical dimensions of the structure can be reduced without sacrifice of its electrical properties.