Discussion
A uniformly illuminated aperture is 20 wavelengths across. What is its approximate half power beamwidth?
Answer: A
Using the usual approximation of about 51 degrees divided by the aperture in wavelengths:
\[ \theta \approx \tfrac{51}{20} \approx 2.6 \]
giving about 3 degrees. The precise coefficient depends on the illumination taper, running from about 51 for uniform to 70 for a heavily tapered aperture, which is one more expression of the trade between beamwidth and side lobe level. An order of magnitude estimate of this kind is usually enough to decide whether a design is feasible.
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