  Joint communications and sensing (JCAS) is potentially a hallmark technology
for the sixth generation mobile network (6G). Most existing JCAS designs are
based on digital arrays, analog arrays with tunable phase shifters, or hybrid
arrays, which are effective but are generally complicated to design and power
inefficient. This article introduces the energy-efficient and easy-to-design
multi-beam antenna arrays (MBAAs) for JCAS. Using pre-designed and fixed analog
devices, such as lens or Butler matrix, MBAA can simultaneously steer multiple
beams yet with negligible power consumption compared with other techniques.
Moreover, MBAAs enable flexible beam synthesis, accurate angle-of-arrival
estimation, and easy handling/utilization of the beam squint effect. All these
features have not been well captured by the JACS community yet. To promote the
awareness of them, we intuitively illustrate them and also exploit them for
constructing a multi-beam JCAS framework. Finally, the challenges and
opportunities are discussed to foster the development of green JCAS systems.
