There’s a very good application note done by u-blox, about general rules for GPS/GNSS antennas:

https://www.u-blox.com/sites/default/files/products/documents/GPS-Antenna_AppNote_%28GPS-X-08014%29.pdf

And a more specific one explaining best practices with GPS/GNSS RTK antennas:

https://cdn.sparkfun.com/assets/0/c/0/1/c/AntennasForRTK_WhitePaper__UBX-16010559_.pdf