This article is an expose of the internals of a 60dbm.com 0.1-3000MHz Return Loss Bridge available on eBay for around $17 incl post.
Above is the published directivity characteristic for the RLB. Giving directivity at -ve dB is an error, and the frequency scale marks just don’t seem to reconcile with the grid… it is best discarded and nonsense.
Above is the Return Loss of two very good 50Ω loads measured using a Rigol DSA815-TG. The Rigol has a firmware defect (Chinese quality) and halves the values plotted, so Return Loss is double that shown on the display.
These are cloned by several makers it would seem, indeed this may be a clone. Chines quality (bit of an oxymoron) varies and I have received reports that the 4 x 100R at the input end of the bridge may not be all properly connected due to PCB track errors… so check.