I am always forgetting the rule of thumb for determining when one is in the near-field of a transducer, transducer array or antenna (the math is the same). There is a very nice description of the issue here..
From the text above:
The distances where the planer, parallel ray approximation breaks down is known as the near field. The crossover distance between near and far fields (R) is taken to be where the phase error is 1/16 of a wavelength, or about 22.5 degrees.
R = 2 D^2/ L
where L is the wavelength and D is the largest dimension of the transmit antenna.
So for example, in a project I'm currently working on, we are building buoys to be used in coastal waters with a WiFi link to shore. The shore base station antenna is 1 m long. The WiFi link is 2.6 GHz which is a wave length of 2.00702458e8 / 2.6e9 = .12 (12 cm). So the far-field for this antenna starts at:
2 * 1^2 / .12 = 16 m
I'm troubleshooting our WiFi link in the lab and I'm definitely in the near field. So my results have sometimes been hard to interpret.