Splash Plate Feed
Splash Plate Feed
The term splash plate feed is used colloquially to describe the complete subreflector of Cassegrain antennas. However, this subreflector usually has an additional shape or additional elements to prevent the reflection of part of the transmitted energy back into the waveguide. All in all, this subreflector has a very complex structure and cannot be compared to the classic subreflector of a Cassegrain antenna.
The apex plate is a simple way to reduce the feedback effects on the antenna feed. An apex plate is a mostly circular plate in the apex of a parabolic reflector. It has a precisely calculated diameter and distance before the apex of the parabolic reflector. A reflection in the opposite phase to the forward wave is caused by tuning the geometrical length between the plate and the antenna feed. This effect slightly reduces the efficiency of the antenna and thus its antenna gain, but the standing wave ratio on the antenna feed line improves considerably. The size of the plate is limited to the angles at which feedback to the horn radiator is possible. This part of the energy can also be distributed in other directions by a different shape (e.g. a convex shape). However, this causes disturbances in the radiation pattern because the radiation density is largest at the apex of a parabolic antenna.
Figure 2: Weather radar GMWR-400-SST using Splash Plate Feed (Courtesy of GAMIC mbH)
Apex Plate in Subreflector
The subreflector of the Cassegrain antenna should theoretically have a hyperbolic shape. The principle of an apex plate could also be used here. With a subreflector, however, the transmitter's energy can easily be distributed by other shapes than the hyperbolic one. This change of direction can be compensated by the parabolic reflector as long as no significant phase differences occur.
In the simplest case, this can be again only a small conductive plate in front of the horn radiator, which has only a small metal cone in the middle to prevent reflections in the direction of the waveguide. This component is then called Splash Plate Feed because the appearance is closer to a “splash plate” than to a hyperbolic mirror.
Disturbances in the radiation pattern would be caused by necessary brackets for the subreflector. By using dielectric materials the disturbances in the radiation pattern can be reduced. However, the horn radiator can also be replaced by a dielectric antenna, which makes such brackets obsolete. Very complex solutions with much better properties use step-shaped slit (“corrugated”) dielectric materials for impedance matching and for the phase-correct distribution of the transmitter's energy.
Hat Feed Antenna
If the subreflector has an edge that encloses the dielectric antenna like a hat on top of it, antennas with this radiator shape are also called hat feed antennas. Such an edge can also have a corrugated shape and is intended to attenuate waves circulating the subreflector.
- Patent von Siemens zur Scheitelplatte aus dem Jahre 1987 mit Beschreibung der Funktion. (online)