You have drawn a taper. Matching with a simple taper needs to be several wavelengths long because it depends on the reflections from the changing impedance to be small at any point, so the taper needs to be gradual. If you use a quarter wave transformer to get the right impedance then that will a quarter wave long. It is possible to shorten a taper if the taper is profiled so that the reflections from different parts of it cancel.

The Tapered microstrip line denoted by 'Lt' will affect the impedance matching and initial length you can choose as a quarter of operating wavelength. furthermore, you can optimize for better matching.

The length of the microstrip line (shown in RED) you can choose any length it will not influence much except PHASE.

try from this paper:

[24]Zamzam Kordiboroujeni, Jens Bornemann, “New Wideband Transition from microstrip line to substrate integrated waveguide”, IEEE transactions on Microwave theory and techniques, pp 2983-2989, Vol. 62, No. 12, December 2014

Nitin Muchhal I have read this paper. It does not mention anything about microstrip line length

Not sure if this is the same paper, but it does give a transformer length.

http://www.ursi.org/proceedings/procGA14/papers/ursi_paper1784.pdf

Choose quarter wavelength as microstrip wavelength or odd number multiples of quarter wavelength

Tapering will influence the impedance matching and better to choose quarter wavelength and finally optimize as per your application...

Tapered transitions are not usually a quarter wavelength long or any multiple - it is step transformers that have those lengths.

I don't have any problem with tapered length. My concern is microstrip line length. RED part in the attached image

You can make the microstrip any length you like. It needs to be about 2 mesh cells long in CST because CST wants a uniform line that close to the port, but it can be as long as you like. If you make it too long a simulation will take too long and there will be attenuation.

Malcolm White Thanks a lot