  This work proposes an energy-efficient Directional Modulation (DM) scheme for
on-body Internet of Things (IoT) devices. DM performance is tested using a
5-port stacked-patch MIMO antenna under two scenarios: a free space case and
using a four-layer human forearm phantom to simulate the user's wrist. It is
demonstrated that the scheme achieves steerable secure transmissions across the
entire horizontal plane. With a low Bit Error Rate (BER) of
${1.5\times10^{-5}}$ at the desired directions, eavesdroppers experience a high
error rate of up to ${0.498}$. Furthermore, this work investigates the DM
performance using a subset of the stacked patches in the MIMO antenna,
revealing that some combinations achieve a low BER performance using a lower
antenna profile, albeit high side-lobes of BER${<10^{-2}}$ seen outside the
desired region. Overall, the solution is proposed as a good candidate to enable
secure wireless communications in emerging wearable IoT devices that are
subject to size and energy constraints.
