How to express a helix using math equation?

how to express a helix using math equation?I find a paper "An efficient curved-wire integral equation solution technique" which can be used to solve the current distributing for helix antenna,but i need know the helix curve equation.anybody can help me? anybody know how to use MOM to solve helix antenna current distributing ?

Hi. I don't have the equation. But can you let me to see this paper?