the open-circuited stubs design in MMIC

I've read several research papers about full waveguide band MMIC tripler for W-band, recently. Such as 《A W-band GCPW MMIC Diode tripler》、《A Full Waveguide Band tripler for 75-110GHz》??

In these papers, the simple open-circuited stubs were used to present an approximate short-circuit at the diodes from 75-110GHz, and provides frequency isolation as well as a ground-return for the output.

In my opinion, the quarter wavelength open stub can present short-circui in a narrow band, since the tripler is for full waveguide band, that is to say, it is broadband, so why the quarter wavelength open stubs were chosen?

There is what is called a "radial stub" which looks like a slice of a cake or pie. It has wider bandwidth. It was invented or discovered around 1943 and used in microwave valves/tubes in the US.

yeah,I know the "radial stub",　I just wonder why they did not use the "radial stub" but "open stub" in a broadband system?

Is about occupied space on PCB, and also the open stub is not narrow bandwidth as should be according theory. This due to material loses or thickness (both affect the bandwidth).