Novel noise suppression sheet exhibiting high decoupling effect up to 6 GHz

With the forthcoming introduction of the 5th generation mobile communication system, the carrier frequency band is expected to be expanded to a 5 GHz band (up to 5.7 GHz) as for lower Super High Frequency (SHF) band. In order to suppress electromagnetic interferences (EMIs) inside compact radio communication terminals such as smartphones often followed by receiving sensitivity degradation problems, a noise suppression sheet (NSS) is used, having the structure in which magnetic flake particles are dispersed in a polymer to be aligned in the in-plane direction. According to the trends for communication system described above, it is necessary to enhance the effective frequency band of NSS up to around 6 GHz. In this research, aiming for finding out a design guideline to obtain an NSS effective in the low SHF band (up to 6 GHz) and confirming the validity of it, an evaluation system was fabricated which simulated inside a smartphone, and an EMI suppression effect of the NSS prepared with the design guideline was evaluated. An electromagnetic field simulation showed that it was indispensable for an NSS for the low SHF band to reduce complex relative permittivity while enhancing complex relative permeability in the frequency band. Based on this design guideline, an NSS was prepared, which contained a flake having higher saturation magnetization and finer particle size than the conventional NSS with an enhanced filling rate by 16%. It was confirmed that complex permeability spectrum of the NSS extended to tens of times higher frequency than the conventional NSS and that complex relative permittivity was suppressed to be equivalent to the conventional NSS. The novel NSS exhibited a large electromagnetic decoupling effect up to 6 GHz in an actual measurement using the near field space coupling evaluation system that simulated inside a smartphone. Consequently, it was clarified that permittivity index was indispensable in addition to the conventionally referred permeability index for the design of the NSS usable in the low SHF band.