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Far field zone of a single antenna can be determined by using the equation below.
$$R>\dfrac{2D^2}{\lambda}$$
Where D is the biggest dimension of the antenna.
This equaiton is suitable for single antenna. But what if there are more antennas than one?
How can we determine rayleigh distance, far-field distance, of an array antenna? For example, how can I determine far-field distance of an two element (aperture type antennas) linear array antenna as shown below?
The answer is still $$2D^2/\lambda$$ but we need to define D value correctly.
Rayleigh distance of a single horn antenna can be calculated as shown below. $$R=\dfrac{4A^2}{\lambda}$$
We can define D of array antenna as shown below.
Rayleigh distance of an array antenna can be calculated as shown below. $$R=\dfrac{2(4A+A)^2}{\lambda}=\dfrac{50A^2}{\lambda}$$
