rf probe 20db
At 6 GHz wavelenght is 5 cm.
this menas that, if you don't want any trouble in reading the maximum lenght is about 1/20 of wavelenght (means 2,5 mm ).
Any lenght higher than this will affect your measurement, also considering that the parasitic capacitances make changes in your measurement depending the position of the ground, hands, ecc.
I have something similar but till 2 GHz and the ground connection is made through the screen of the probe; thismeans you have to have a ground ( shield, box, PCB ground plane, ecc ready available.
Mandi