  Techniques have been proposed to estimate unknown antenna impedance due to
time-varying near-field loading conditions at multiple-input single-output
(MISO) receivers. However, it remains unclear when a change occurs and
impedance estimation becomes necessary. In this letter, we address this problem
by formulating it as a hypothesis test. Our contributions include deriving a
generalized likelihood-ratio test (GLRT) detector to decide if the antenna
impedance has changed over two groups of packets. This GLRT formulation leads
to a novel optimization problem, but we propose a binary search based algorithm
to solve it efficiently. Our derived GLRT detector enjoys a better detection
and false alarm trade-off when compared with a well-known, reference detector
in simulations. As one result, more transmit diversity significantly improves
detection accuracy at a given false alarm rate, especially in slow fading
channels.
