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How do we rigorously classify antenna profiles based on their electrical height, and what design rules apply to each category?

 I'm analyzing a variety of small antennas for IoT applications. I see terms like "electrically small" (ka < 0.5), "moderate size," and "resonant" used loosely. Is there a standardized classification scheme that links an antenna's physical height (or ka value) to its expected pattern type (e.g., omnidirectional, figure-eight), impedance behavior, and fundamental limitations like Q and bandwidth? This would help me set realistic expectations during the initial design phase.

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By Nkumardo Answered 1 year ago

Yes, classification by ka (k=2π/λ, a=radius of smallest enclosing sphere) is standard. Electrically Small (ka < 0.5): These are dominated by stored energy, resulting in a high Q, very narrow bandwidth (< a few %), and a simple, often omnidirectional pattern. They are notoriously difficult to match. The Chu-Harrington limit defines their best possible bandwidth-efficiency trade-off. Moderate/Resonant (0.5 < ka < 3): This is the common design space. Antennas can be resonant, with manageable Q, practical bandwidths (5-20%), and patterns that develop nulls and lobes (e.g., a dipole's figure-eight). Electrically Large (ka > 3): Here, you achieve high directivity, complex multi-lobe patterns, and very wide bandwidths. Understanding your ka tells you what's physically possible before you start designing.

 

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