Select your coordinate system (make sure it points where you want it!)
Select Cartesian coordinates
x = 50 * cos ( t * 360 )
y = 50 * sin ( t * 360 )
z = 25 * sin (t *360 * 4)
Where the number 50 in the x and y equations represent the radius of the circle, 25 in the z equation represents the amplitude of the fluctuation and 4 represents the number of sine waves on the cylinder.