A robust and efficient method is presented for computing the Q limits of arbitrary shape electrically small antennas (ESAs). These limits are based on the theory of characteristic modes (CMs). In contrast to previous techniques, the CM method does not imply assumptions on the antenna shape, its transmit/receive properties and polarization or absorption efficiency. By finding the lowest-order ESA CMs, the lower bound of Q is readily obtained. We can also extract the exact antenna surface currents that minimizes Q. Examples are given for a variety of relevant antenna geometries.
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