By principle the helix antenna will blank out a certain amount of signal as it is mechanically located in front of the receiving element. A geometrical analysis illustrates why and gives a good indication of the attenuation order of magnitude.
Assume an opening angle of the LNB of 50 deg. The helix shade from the LNB phase center looks like the image below:

Within a 50 deg cone (dotted circle) there is 62% of white pixels (no shading), which relates to about

2dB attenuation.

That pretty much coincides with the measurements I did receiving QO100 with and without the helix.