Question on helix antenna used as a feed

Hi friends
can any one tell me how to decide the proper position of helix feed on parabolic reflector?
I mean I know we place helix at focus, but helix is not single point, rigth?
so how to properly place it at focus to get maximum signal?

I would observe the secondary radiation patten and pay particular attention to the null depths between the sidelobes. As the feed is moved in or out along the vertex line for the parabola. Deep nulls indicate good focusing while indistinct sidelobes indicate that the dish and feed/phase center are not aligned. You could also plot gain vs. feed position and choose the location that provides the highest gain. That is probably what you are looking for anyway.

You could also go through an exercise to determine the phase center location of the helix by measuring the curvature of the phase front and working backward to find the physical location. There are articles in the literature describing that approach.

Thanks azulykit.