Horn fed Gregorian Axisymmetrical dual reflector

Image of the Horn fed Gregorian Axisymmetrical dual reflector.

Dual-reflector antennas are based on principles that have been used in optical telescopes for centuries. The Gregorian telescope was the first dual lens optical telescope. It provided a revolutionary advantage, in that the observer could look through the telescope while standing behind the main reflector, without getting in the way of the light from the observed object (an effective way of reducing aperture blockage!).

The Gregorian dual-reflector antenna has a number of advantages when compared to the prime focus reflector. In cases where the housing for the feed electronics is electrically large, the feed can be positioned behind the primary reflector, reducing unwanted aperture blockage (provided, of course, that the sub reflector is smaller than the feed electronics). A second advantage is that for a given feed beamwidth the dual reflector configuration can reduce the antenna length and require shorter, less sturdy struts.

The Gregorian dual-reflector is a very popular antenna for very high gain applications. Similar to the Cassegrain reflector (already included in Antenna Magus), it transforms the low to medium gain radiation of the feed horn to a high-gain pencil beam.

Difference between the Gregorian and Cassegrain configurations.

The above image shows the difference between the Cassegrain and Gregorian configurations. The main difference is the position and shape of the sub-reflector. There are various factors (like main reflector size, feed angle, strut size, feed blockage etc.) that influence which one of these reflector configurations are preferable for a specific application.

The image below shows a comparison between three different reflector configurations, designed using Antenna Magus. Each of these are designed at 20 GHz for 40 dBi gain using a horn feed with a 10 dB beamwidth of 45 degrees.

Comparing length and width dimentions between the Cassegrain, Gregorian and prime-focus reflectors for a 40 dB gain design in Antenna Magus using a 45 degree 10 dB beamwidth horn feed in all three cases.

Note that the dual-reflector configurations both have much shorter physical lengths when compared to the single parabolic reflector. In the case shown, The Gregorian requires a primary reflector that is 11% wider than the Cassegrain to achieve the required gain. This is due to the aperture blockage.

The Gregorian reflector can be designed in Antenna Magus for gains from 30 to 50 dBi, and the positioning and dimensions of the feed horn are included. The radiation patterns shown below are for a Gregorian designed for an overall gain of 40 dBi, using a 10 dB feed beamwidth of 60 degrees.

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