|Losses in a device with two radiating antennas.|
When we were designing the multiport features in Optenni Lab, one of the key questions was “Which cost function is most appropriate for matching circuit optimization”. In other words, “What variables exactly do we want to optimize?” Rather quickly it became clear that the optimization goal should not just be to obtain the best possible impedance match at all ports. This can be easily achieved by just adding enough resistive losses in the matching network, but obviously, adding such large losses reduces the efficiency of the antenna.
Instead, we identified two distinct cases for the multiport matching, each with their own optimization goals: systems with multiple radiating antennas (e.g. MIMO antennas) where the goal is to maximize transmitted power and minimize the coupling between elements, and systems with both radiating and transmitting elements (e.g. wireless power transfer) where the best efficiency is instead achieved by maximizing the coupling between antennas. In this blog entry I’ll be discussing the case where we are designing the matching for multiple radiating antennas in the same device. The other case will be covered in another blog entry.
When designing devices with multiple antennas, the goal of matching circuit design should be to maximize the antenna efficiency. The picture describes the different loss mechanisms affecting antenna efficiency: loss due to impedance mismatch, losses in the matching components, coupling to the other ports and losses in the antenna structure (described by antenna radiation efficiency). The antenna system is supposed to radiate as much of the input power as possible, to be captured by receiving antennas at the other end of the link (not shown). Any energy that is coupled to the other device antenna at right reduces the efficiency of the antenna at left.
Without careful antenna and matching circuit design, you could end up with a nicely matched but inefficient antenna system, in which most of the power is coupled to the other antenna port. When the matching circuits are optimized for maximal total efficiency, we might actually want to have some impedance mismatch because it will reduce the coupling to the other port and thus increase the overall efficiency. Optenni Lab designs matching circuits with optimized balance between the different loss terms, giving optimal total efficiency. Optenni Lab’s simultaneous multi-antenna matching can handle any number of antennas and also supports the optimization of tuning and decoupling components that are placed within the antenna structure.
Of course, in other cases, a strong coupling between elements is actually desired. In these cases, we want a very different sort of matching circuit. Look out for the next blog post, explaining multiport matching for power transfer and explaining how Optenni Lab can fit into the multi-antenna matching design flow with CST STUDIO SUITE®.