Admittance parameter matrix.
The y-matrix  is a 3-dimensional
numpy.ndarraywhich has shape fxnxn, where f is frequency axis and n is number of ports. Note that indexing starts at 0, so y11 can be accessed by taking the slice y[:,0,0].
Returns: y – the admittance parameter matrix. Return type: complex
numpy.ndarrayof shape fxnxn