My question is: before this 120GHz project, did you try to improve the bandwidth of other microstrip antenna array, let's say working on 2.4GHz?
Going on 120GHz without very very deep knowledge of microwave circuits, is like stepping on the driver seat of a Bugatti Chiron without a driver license.
I already improved the bandwidth of a simple microstrip patch antenna at 120GHz by parasitic element to 14%.
The bandwidth of serial elements microstrip array antenna is about of (1-3)% and I need at least 6% at a frequency of 120 GHz.
Can someone help me?
Assuming you want to stick with single layer, so no aperture coupling/etc.
1) do not use patch antenna, use other antenna elements, for example ME-dipole (using vias).
2) if you want to use patches, first try patch arrangement described in patent US20130321214 (pdf link).
3) use differential-fed patches in your array