  The extremely low efficiency is regarded as the bottleneck of Wireless Power
Transfer (WPT) technology. To tackle this problem, either enlarging the
transfer power or changing the infrastructure of WPT system could be an
intuitively proposed way. However, the drastically important issue on the user
exposure of electromagnetic radiation is rarely considered while we try to
improve the efficiency of WPT. In this paper, a Distributed Antenna Power
Beacon (DA-PB) based WPT system where these antennas are uniformly distributed
on a circle is analyzed and optimized with the safety electromagnetic radiation
level (SERL) requirement. In this model, three key questions are intended to be
answered: 1) With the SERL, what is the performance of the harvested power at
the users ? 2) How do we configure the parameters to maximize the efficiency of
WPT? 3) Under the same constraints, does the DA-PB still have performance gain
than the Co-located Antenna PB (CA-PB)? First, the minimum antenna height of
DA-PB is derived to make the radio frequency (RF) electromagnetic radiation
power density at any location of the charging cell lower than the SERL
published by the Federal Communications Commission (FCC). Second, the
closed-form expressions of average harvested Direct Current (DC) power per user
in the charging cell for pass-loss exponent 2 and 4 are also provided. In order
to maximize the average efficiency of WPT, the optimal radii for distributed
antennas elements (DAEs) are derived when the pass-loss exponent takes the
typical value $2$ and $4$. For comparison, the CA-PB is also analyzed as a
benchmark. Simulation results verify our derived theoretical results. And it is
shown that the proposed DA-PB indeed achieves larger average harvested DC power
than CA-PB and can improve the efficiency of WPT.
