How do I test the shielding effectiveness of a cable assembly using the transfer impedance method?

Testing the shielding effectiveness of a cable assembly using the transfer impedance (Z_t) method quantifies the cable shield's ability to prevent external electromagnetic fields from inducing signals on the internal conductors, and vice versa. The transfer impedance is defined as: Z_t = V_inner / (I_outer x L) [ohm/meter], where V_inner is the voltage induced on the inner conductor per unit length when a current I_outer flows on the outer shield. Lower Z_t means better shielding. The test method (per IEC 62153-4-3 or MIL-STD-1344A Method 3008) uses a triaxial fixture: the cable under test is mounted inside a larger outer tube (the triax), forming a two-conductor structure; a current is injected on the cable's outer shield and the voltage appearing on the inner conductor is measured. The test procedure is: mount the cable sample (typically 0.5-1 m length) in the triaxial fixture with the cable shield connected to the fixture's inner adapter at both ends, inject a known current on the cable shield (using a signal generator and a known source impedance), measure the voltage on the inner conductor at the far end (using a VNA or spectrum analyzer), and calculate Z_t = V_inner / (I_shield x L_cable). Results interpretation: a solid copper tube has Z_t approximately = R_DC at low frequencies (dominated by DC resistance of the shield) and Z_t that decreases at high frequencies as the skin effect forces current to the outside of the shield. A braided shield has Z_t that increases at high frequencies due to the porosity of the braid allowing field penetration. Typical values: solid copper (semi-rigid coax): Z_t < 1 mohm/m. Single braid: 5-50 mohm/m at 100 MHz. Double braid: 1-10 mohm/m at 100 MHz.