After looking back at the original Elektor project, I became not so satisfied with my results, seeing the original specs claiming a bandwidth of 100 kHz to 1.5 GHz +-2.5dB, which I was not getting even close to. Since I still had 2 unused PCBs from OSHpark, I decided to build another one, keeping as close to the original design as I could. But that did not bring any significant improvement. The Bode plot I saw on the spectrum analyzer suggested to me that the problem part was the FET, so I tried replacin the originally specified BF998 with a CF739, which had better specs in the datasheet as the BF998. That proved to be the right move, giving me a bandwidth of 95 kHz to 1.94 GHz at +-2.5 dB. NOW I am satisfied! Oh, yeah, I also used an SMA connector instead of the pogo pins, allowing me to easily configure it with any probe tip I might end up needing.