The term feedhorn was originally derived from uplink antenna jargon. An uplink feedhorn "fed" microwaves onto and "illuminated" a reflective surface below. The terms feedhorn and illumination have persisted even when describing downlink components. A feed thus "illuminates" a dish even though it actually receives reflected microwaves. The detected illumination pattern describes its field of view. A perfectly illuminated feed would collect radiation coming from nowhere but the targeted satellite signal reflected from the dish surface; it would reject microwaves from all other sources. In practice, prime-focus feeds illuminate antenna central regions most strongly and are progressively less able to detect radiation at increasingly greater off-axis angles. Similarly, feeds employed with an offset fed reflector are designed to be most capable of collecting energy from its center.e illumination pattern is an important design criteria. A feed that illuminated just the central portions of a dish would introduce little noise into the system but would miss some of the signal originating from the reflector edges and hence result in lowered system gain. However, a feed which over-illuminated a dish would take advantage of all the available gain but would introduce too much ground noise. Over- illuminating a dish could be a serious problem, especially when receiving signals from spacecraft close to horizontal elevations, because the ground on a typical cool summer day emits noise at a "hot" 290°K. A feed must be properly matched to the f/D ratio of the dish so that illumination is optimized. Using an inappropriate feed results in detection of less signal and more noise. It will also result in poor side lobe performance and thus increased susceptibility to terrestrial interference.sh efficiency can be altered by simply changing feed illumination pattern. An over-illuminated dish would have a very high efficiency but the additional ground noise detected would result in a lower signal-to- noise ratio, the final determinant of system performance. In contrast, under-illuminating would result in substantially lower antenna efficiency as well as detected noise. The end result would still be the same, an undesired reduced signal-to-noise ratio.