1. First you have to understand, that, if you want to varify the radius of a swept surface over the splinelength in this sinusodial manner, you should use an advanced law.
2. The parameter for the radius in this example is called "FormalLength.1" and you can define it for yourself. In the example "20 mm" was defined.
3. The parameter of the splinelength is 1 and is called "FormalAngle.1" in this example.
4. In the formula of the law "l=20mm+1mm*sin(2*PI*FormalAngle.1*20)" the variation of the radius is 1mm and you get 20 sinusodial waves.
5. If you want to create a swept surface, by center and radius, choose your "Splinesketch", select "law" beside the radius box and don`t forget to choose "advanced" for the law type. There are also "constant", "linear" and "S type" laws possible with this formula.
6. By modifying the values in the formula you can easily create 40 waves or a different radius or a higher radius amplitude.