Calculating some worst-case values for angles in an interval
Your code already seems to do only the steps necessary, slow performance seems to be the limitation of VBA.

You may do some minor improvements however:

Select Case i

You don't need to check conditions here, it's a simple Mod operation:
l = Application.Evaluate("=MOD(" &amp; i &amp; ",360)")

Formulas

Your calculations contains a lot of duplicates, you can use temporary variables to reduce number of calculations:

t1 = AmplCo(FileCounter-1, AktBeam,l,k) * AmplCo(FileCounter, AktBeam,l,k)
t2 = PhaseCo(FileCounter-1, AktBeam,l,k) - PhaseCo(FileCounter, AktBeam,l,k)
t3 = AmplCr(FileCounter-1, AktBeam,l,k) * AmplCr(FileCounter, AktBeam,l,k)
t4 = PhaseCr(FileCounter-1, AktBeam,l,k) - PhaseCr(FileCounter, AktBeam,l,k)

A = A + t1 * Cos(t2) + t3 * Cos(t4)
B = B + t1 * Sin(t2) + t3 * Sin(t4)

t5 = (AmplCo(FileCounter-1, AktBeam,l,k)^2 + AmplCr(FileCounter-1, AktBeam,l,k)^2)
t6 = (AmplCo(FileCounter, AktBeam,l,k)^2 + AmplCr(FileCounter, AktBeam,l,k)^2)

C = C + t5
D = D + t6

BT_Worst = BT_Worst + t5 / t6


This halves the number of array accesses in your code (so best case you can get double speed).



As I've written already, there are limitations in VBA too. Further improvement could be done implementing your code in e.g. Visual Studio and compile as an Excel add-in. (That case you could avoid run time compilation and debugging which are the biggest issue in VBA).