Truncated grid reflector (gooseneck feed)

Image of the Truncated grid reflector (gooseneck feed).

Truncated versions of the parabolic reflector may be termed 'shaped-beam parabolic antennas' and have the advantage that they may be designed for specific (and different) E- and H-plane beamwidths. This is useful in systems such as mechanically scanned search radars, airport surveillance radars, air traffic control radars and military height finder radars - where fan beam radiation patterns are required. More often than not, the reflector surface is constructed using a grid of wires, in order to reduce wind-resistance and weight. The parallel wires of a grid reflector may also act as a polarising filter, as it only reflects linearly polarized waves with the electric field parallel to the wires. Antenna Magus offers a design choice between a practical or an ideal grid spacing, however, even at the smaller ideal spacing, the gain performance of the reflector antenna is slightly degraded when compared to an equivalently sized solid reflector model.

In order to realise a fan beam, the reflector is truncated (either cut-off horizontally, or using an elliptical intersection) and fed by an on-axis sectoral horn antenna. The antenna is usually focused experimentally by finding the optimal axial position (and transverse position if there are alignment errors) which minimizes the null between the main lobe and first side lobe. This is required, as the reflector's focus is not located at a singular geometrical point, but shifts with frequency.

Typical radiation pattern cuts at the centre frequency.
Typical 3D radiation pattern at the centre frequency.