How to design a Directional BPF?

I have a butterworth bandpass filter (cutoff frequencies: 20 Hz and 200 Hz). Using cos^2 dependence I have created a directional component. See the attached screenshots for the frequency response of the BPF and the directional component oriented toward +ve 90 degree.

My question is how can we achieve a directional BPF using these two?

I am actually trying to implement the filter from this paper: http://link.springer.com/chapter/10.1007%2F3-540-45787-9_37

Answer 1

[Honglei Chen]

To achieve spatial component, you need to have the measurement in the spatial direction too. So it's actually filters in two dimensions, one in spatial domain and one in frequency domain. You can pass the signal through these two filters in sequence. Theoretically the order doesn't matter.

HTH