Discussion
For an antenna of largest dimension D, the far field begins at about \( 2D^{2}/\lambda \). What is this for D = 1 m at 3 GHz?
Answer: C
At 3 GHz the wavelength is 0.1 m, so:
\[ \tfrac{2 \times 1^{2}}{0.1} = 20 \]
metres, giving 20 m as the start of the far field. Pattern measurements made closer than this will not show the true far field pattern, which is why antenna ranges are long and why compact and near field ranges exist at all. The distance grows as the square of the aperture, so a large reflector at high frequency can need kilometres of range.
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