Retrodirective
Antenna Arrays
WAYNE SHIROMA & RYAN MIYAMOTO
A retrodirective array transmits a
signal back to the interrogator.s position without
any a priori knowledge of the incoming angle, without
relying on sophisticated digital signal processing
algorithms. The most popular technique to achieve
retrodirectivity is phase conjugation. Phase conjugating circuitry can
be realized by simple heterodyne technique, in which
the received RF signal is mixed with a local oscillator
(LO) at twice the RF. In this case, the lower sideband
product has the same frequency as the RF, but with
conjugated phase. When combined with an antenna and
placed in an array, the phase-conjugated signal from
each antenna element will be reradiated towards the
original source direction. This self-steering feature
of the retrodirective array can play an important
role in mobile communications or satellite-to-satellite
communications where high link gain and self-beam-tracking
are desired.
Remote
information retrieval-on-demand or Radio Frequency
Identification (RFID) systems have been
reported as practical applications using the
retrodirective array. These systems require modulation
function
for data transfer in addition to phase conjugating
service. The compact phase conjugator proposed
in can modulate the received signal while
phase
conjugating it. A prototype system using a four-element
retrodirective array was constructed and has
shown excellent retrodirective performance. The
recent
report of a retrodirective array using self-oscillation
mixers shows further integration of the circuitry,
eliminating the need for external local oscillators.
For the next step, the University of Hawaii currently
plans to investigate the implementation of the retrodirective
array system onto nano-satellites, where low power
consumption and ease of deployment are required.
The self-steering feature of the retrodirective array
is very attractive to satellite-to-satellite communications
where it is difficult to fix the communication link
as satellites are floating in space.
