Start by extruding 10 segments throughout the helix, as shown in the image

　　Create the antihelix

　　Fill in the respective faces between the helix and antihelix

　　Select the 3 edges inside the oval and extrude them to create 3 new faces.

　　Cut an edge loop through the middle of the newly created faces.

　　Reposition the highlighted vertex as indicated in the image.