No expert here, but seems like I recall that the purpose of the scalar ring is to prevent entry of stray c-band frequency signals that emanate from the earth itself. I forget what the proper term for that is. "Ground clutter" comes to mind, but that might not be correct.
The most common type of feedhorn manufactured today is called a scalar feedhorn. This type of feed has a large circular plate with a series of three or four concentric rings attached to its surface. The scalar rings conduct the incoming signal from the outer edges of the focal cloud to the large waveguide opening located at feed center. The scalar feedhorn primarily sees or illuminates the inner portion of the antenna's surface area, while attenuating the signal contribution from the outer portion of the dish by 8 to 22 dB, depending on whether the dish is deep or shallow in its construction.
Molecular motion within the Earth itself generates random noise which permeates the entire electromagnetic spectrum used for the transmission of satellite signals. This random noise is many times stronger than the satellite signals reaching any location. The attenuation or illumination taper provided by the feed sharply reduces the reception of the Earth noise which lies just beyond the antenna's rim. The outer area of the antenna's surface therefore acts more as an Earth shield for the feedhorn than as a contributor to the overall signal gain of the receiving antenna.