Calculating Total Exposure Ratios for Co Located Transmitters in Handheld Equipment
Multi-radio handheld exposure compliance requires summing normalized SAR values across concurrent transmitters to verify the total exposure ratio stays below unity.
Antenna Mutual Coupling
Meaning
Electromagnetic interaction between adjacent radiating structures represents a major physical constraint in multi-antenna compact designs. When antennas sit in close proximity, antenna mutual coupling occurs as energy from one transmitter is received by the other instead of radiating into space. This undesirable energy transfer alters the impedance matching and degrades the radiation patterns of the individual elements.
System Degradation
The physical consequences of this coupling include decreased total radiated power and increased correlation between channels. High correlation impairs the performance of multiple-input multiple-output architectures by reducing the spatial diversity that these systems rely upon to multiply throughput. Overheating of the inactive transceiver port can also happen if the coupled energy exceeds the isolation rating of the rf switch.
This degradation directly affects the overall power efficiency, forcing the system to draw more current from the battery to maintain a stable link.
Mitigation Technique
Engineers employ several physical approaches to decouple the co-located radiating elements. Inserting ground-plane stubs or using defective ground structures creates a stopband that interrupts the surface currents between the antennas. Other methods involve utilizing neutralisation lines or selecting orthogonal polarisations for the adjacent elements.
Measurement Protocol
Quantification of this interaction relies on scattering parameters measured during the device design validation phase. The s-parameter matrix reveals the transmission coefficient between the ports, with values below fifteen decibels usually indicating acceptable isolation for cellular and wifi co-existence. These measurements must be conducted with the enclosure fully assembled to capture the impact of nearby plastic and metallic housing parts.