According to the definition of FourierF the parameter "a" contains the coefficients of a Fourier-Sine Series. Therefore to get "a" for some arbitrary function, you need to calculate the Sine Series coefficients. This implies that your function has odd symetrie around the origin. Therefore, you need to setup the triangular wave so that the zero is at the origin before you calulate the Sine Series. Also, take care about conventions used for Sine Series coefficients, they must match your base functions Sin[2Pi i t].
Further, by using the Sine Series coefficients as a radius, you imply that they are positive. This is easily fixed by taking the absolute value for the radius.
Here are the first few coefficients:
{0.202642,0,-0.0225158,0,0.00810569,0,-0.00413556,0,0.00250176}
As the first coefficient is much bigger than the rest, we will have one big circle and very tiny ones.
