TE21 monopulse coupler

How does it work? I found a paper(http://ieeexplore.ieee.org/document/7068002/) and a company that explains it vaguely(https://docs.wixstatic.com/ugd/bdfa9...ce45656ab3.pdf) but is there any book or formula you can point how we get the signals and how do we seperate the azimuth and elevation errors after we get them as shown in second link? Thanks in advance.

it, unfortunately, uses magic. Complicated antenna manifolds like this are more art than science. you are not going to find a simple paper to explain all the little twerks they added to their cavities to make it work.

I particularly liked Biff44's answer.

I do not have a specific answer but I would look carefully at a number of references. Skolnik, Radar Handbook; Stimson, Introduction to Airborne Radar: Volakis (air any of the earlier editions) Antenna Engineering Handbook; I suspect that D. Pozar might also be useful (Microwave Engineering) for more specifics. Basically a monopulse converter is a collection of hybrids and a corporate feed behind four horns that produces a main beam and difference patterns in elevation and azimuth. It can be used to guide an antenna based on a single pulse.

The references should provide you with a reasonably good starting point.