The ELECRAFT BL2 BALUN Return Loss and Balance were measured as shown in the referenced links. The Balance measurements were performed according to W7EL's referenced document. Refer to pg 163 paragraph named 'Current Balun'.
Elecraft BL2 BALUN Specifications & Operating Manual
BALUNS. What they do and how they do it. W7EL
Test Configurations and BALUN Schematic
Return Loss Measurements
Test Fixture 1
Test Fixture 2
Test Fixture 3