After having some Beer and Obstler I finished the Helix and did some comparisons in different configurations, here are the results:
TX-signal (received with a separate dish):
DVB-S2 MER (measured with Minitiouner):
As expected with the TV offset dish, the TX signal of the Helix is almost 3dB stronger.
However, the helix shades the LNB by 1 or 2 dB, which is not relevant for SSB, but represents a significant attenuation for a DATV signal.
So what to do? Building a PA with 3 dB more power? Or buy a larger dish to compensate for the loss of MER? Or just living with these results the way they are. I think I'll do the latter.
vy 73, Kurt, DJ0ABR