Why do aerials have direction?

The pictures come from the museum at Croome. I remember studying this for my degree but I've lost the detail now. So I was wondering why you get the lobes for electromagnetic waves and don't get equal intensity all the way round. It turns out that it is to do with interference. There are angles where there is destructive interference. I don't yet fully understand why. But notice that the smaller wavelength is more uni-directional. That means that the lobe doesn't touch the ground. In the top picture the radar can only detect planes in front that are as far horizontally as the plane is vertically above the ground, because the ground reflects waves and it detects the ground as well.