The　tunnel-diode　resonator　(TDR)　technique　for　accurate　measurements　of　the　magnetic　penetration　depth　is　used　to　measure　the　London　and　Campbell　penetration　depths　of　polycrystalline　SiC　doped　(10wt.%)　MgB2.　The　Campbell　length　was　used　to　investigate　the　field　and　temperature　dependence　of　the　critical　current　density.　The　as　determined　critical　current　density　provides　values　as　high　as　6x10(6)　A/cm(2)　at　4.2K,　1T,　which　is　higher　than　values　estimated　by　Bean　method.