Building a sparse antenna array
One solution may be to use sparse array architectures, in which the distances between the antenna elements are much larger than the transmitted wavelength, but the array still should not generate unwanted side lobes or grating lobes next to the transmitted beam to avoid interference.
Antenna arrays with inter-element spacing equal to or larger than the transmitted wavelength λ give rise to unwanted grating lobes - if the elements are placed uniformly in a grid. If the elements are not spaced uniformly, as in the 150 element ‘sunflower array’ shown in Figure 4, the average inter-element spacing can be larger, in this case 5λ, without suffering from grating lobes [1].
To design a sunflower array, the elements are placed along a Fermat spiral (see Figure 5) so that with each turn, an equal amount of area is enclosed. The individual elements are positions on the spiral at multiples of the angle χ, which is 4π/(3+√5), the so-called Golden Angle.
The sunflower array has an almost uniform power density, which simplifies cooling and makes effective use of the total aperture if all elements are excited to the same level. This arrangement does not generate unwanted grating lobes, and its beam angle can be steered as well as that of a dense array. This makes the sunflower array a good candidate for the kind of sparse arrays that will be needed in passively cooled 5G mm-wave antenna panels.