Enhanced radiation properties of spiral lens antennas

Spiral antennas have been widely used in the THz regime as ultra wide band-width feeds for dielectric lenses. Indeed, the impedance behavior of such antennas is very flat over the frequency band, especially when logarithm spirals are used. Even so, these feeds generate a relatively wide field inside the lens and this results into a poor illumination of the lens. In this contribution, we will demonstrate how the performances of these lens antennas can be considerably improved by using a small air gap between the spiral and the dielectric material.