Open the Helix dialog, set the 'Number of Turns' to the number of loops in your Spiral, set the 'Pitch' to ZERO, set the 'Radius Method' to 'User Law' and select the 'Linear' method. Now enter the Minimum and Maximum radius values for your Spiral and hit OK. Now make sure that the direction is what you desire, select a point and hit OK and you have a fully parametric Spiral.
John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA