Effect of Subreflector which is in the near-field of Feed

Hi,

What happens if the sub-relfector is in the near of antenna's main feed in a dual-reflector antennas? As feeds are normally electrically large, their farfield distance is quite large! and What is the solution?

Cheers,
Hadi

The far field is a concept that relates to the radiation (secondary) patterns stabilizing at some distance and appearing much like they appear when the observer is a very long way away. The idea does not really apply to feeds, sub-reflectors and main reflectors.

My point is that since the feed is very close to the sub-reflector (near field region), we have to take their mutual interactions into consideration rather than just exposing the sub-reflector with the feeds radiation pattern in simulation. Now what is the effect of this interaction, and how much deteriorates the main reflector illumination?

I agree. Reactive near field and near field interactions are present in spades. I do not think of these as degrading the interactions, but they are quite complex. Simulators are very capable (e. g. expensive ) today and accurate predictions are often achieved. I am not sure how to answer what you are now asking. Try by reading one of the antenna text books that address the myriad of possible interactions.


A

Let's see a simple qualitative way to see that a reflector can affect the feeder.
Let's consider feeder and a reflector consisting of an infinite perfect conductive plane. Then, by the method of images, the plane can be substituted by the image of the feed. In far or near-field, this image has an interaction with the original feeder, dependent of their distance and position.
Regards

Z

The near-field interaction in antenna optics is known and has been studied in many designs.
Find a good textbook on microwave antennas and follow the experts.