Here we present the magnetic properties and upper critical field (B C2 ) of polycrystalline Y(Ba 1-x Sr x ) 2 Cu 3 O 7- superconductors, which are being determined through detailed ac/dc susceptibility and resistivity under magnetic field (RTH) study. All the samples are synthesized through solid state reaction route. Reduction in Meissner fraction (the ratio of field cooled to zero field cooled magnetization) is observed with increasing Sr content, suggesting occurrence of flux pining in the doped samples. The ac susceptibility and resistivity measurements reveal improved grain couplings in Sr substituted samples. Consequently the inter-grain critical current density (J c ), upturn curvature near the T c in temperature dependence of upper critical field [B C2 (T)], and B C2 are enhanced. Both J c and B C2 increase in lower Sr substitution (up to x = 0.10) samples followed by decrease in higher doping due to degradation in effective pining and grain coupling. PACS: 74.25.Wx, 74.25.fc, 74.25.Sv, 74.25.Op

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We report the structural, superconducting, magnetic and granular properties of Y 1-x Ca x Ba 2 Cu 3 O 7-δ; x = 0.0, 0.1, 0.2 and 0.3. Rietveld fitted X-ray diffraction data confirms the single phase formation for all the samples. The orthorhombicity of parent compound (x = 0.0) decreases and becomes closer to the tetragonal structure for higher Ca concentration. The superconducting transition temperature (T c ) decreases with increasing Ca content due to both over-doping and ensuing disorder in the superconducting CuO 2 planes with Ca doping. DC susceptibility measurements reveal reduction of Meissner fraction with Ca doping, suggesting the flux pining effect. The AC susceptibility measurements reveal the enhancement of grain coupling with increasing Ca content in the system. The average grain size is found to increase with Ca doping.The Scanning Electron Microscopy (SEM) observations indicate better grain connectivity in terms of narrow grain boundaries for Ca doped samples. It is concluded that limited Ca doping enhances the superconducting performance of YBa 2 Cu 3 O 7-δ system.