This　paper　presents　a　visualized　approach　to　efficiently　designing　an　antenna　decoupling　network　constructed　by　cascaded　coupled　transmission　lines　(CTLs).　The　cascaded　CTL　architecture　is　explored　to　decouple　tightly-packed　antenna　pairs　with　extremely　small　feed　point　spacing,　circumventing　the　narrow　bandwidth　and　loss　issues　associated　with　resonant　decoupling　structures.　To　facilitate　the　fast　CTL　design,　we　propose　to　analytically　map　the　CTLs’　odd-mode　(OM)　and　even-mode　(EM)　characteristic　impedances　onto　the　Smith　charts　of　the　antenna’s　OM　and　EM　input　impedances.　By　leveraging　this　approach,　the　required　CTL’s　OM　and　EM　parameters　for　antenna　decoupling　can　be　directly　determined　without　relying　on　intricate　optimization　or　try-and-error　processes.　To　demonstrate　the　effectiveness　of　our　methodology,　a　printed　monopole　antenna　pair　with　an　element　center-to-center　spacing　of　merely　0.1λ0　(at　3.5　GHz)　is　prototyped,　which　presents　a　15-dB　isolation　bandwidth　of　18%　and　a　total　efficiency　exceeding　85%　over　this　bandwidth.　The　proposed　decoupling　approach　substantially　simplifies　the　design　of　compact　high-isolation　antenna　pairs　which　would　have　important　applications　in　MIMO　communication　systems.　IEEE