sirch:
Grumpy_Mike - Thanks for the reply but I would need some convincing. As I see it the thing is that a radio wave is polarised and so a Yagi only needs to work in the direction of the polarisation but the ultrasonic is not polarised and so needs to be 2 dimentional.
Sirch, I agree with you. The array you are talking about is more like a diffraction grating or a synthetic aperture radar than a yagi array.