  Covert communication hides the transmission of a message from a watchful
adversary while ensuring reliable information decoding at the receiver,
providing enhanced security in wireless communications. In this letter, covert
communication in the presence of a multi-antenna adversary and under delay
constraints is considered. Under the assumption of quasi-static wireless fading
channels, we analyze the effect of increasing the number of antennas employed
at the adversary on the achievable throughput of covert communication. It is
shown that in contrast to a single-antenna adversary, a slight increase in the
number of adversary's antennas drastically reduces the covert throughput, even
for relaxed covertness requirements.
