Feeding a dish with a helix

[Explodinglemur]

I'm interested in doing some L-band monitoring and was thinking of using a helical antenna as a feed for an offset dish. I'm not sure where to put the helix in relation to the dish's focal point though...should the focus be on the front of the helix, the back (by the ground plane), or somewhere in the middle?
Or, am I better off using a different kind of antenna for a dish feed?
Thanks!