Re: Farfield of antenna array
Hello
It is the size of the antenne where there is significant current. for your array it will be the size of the complete array.
The Far field distance relates to the distance where the point source approximation is valid. That is that each doubling of distance gives 6 dB reduction in power flux density (or E-field). If no reflections are present, in the far field region the Friis formula is valid. Note that you need to sum the far field distances for the transmit and receive antenna.
You are right several antennas can have reasonable plane wave field well within the far field distance, but the point source approximation doesn't hold. This is used in compact antenna measuring ranges. You are also right that the far field distance can be large.
With some degradition in accuracy, one can reduce the measuring distance to D^2/lambda. In reality some reduction of measuring distance may even enhance accuracy as the influence of unwanted reflections becomes less.
When you use field taper (to reduce side lobes), there is less current at the extremeties of the antenna hence far field conditions occur at even shorter distance.
If you have something in simulation, you may simulate this by measuring S21 for increasing distance. You can use a simple dipole as receiving antenna in your simulation.