Regarding the recent return loss bridge posts and the RLB Kit being offered by Amtronix and other sources.
These bridges are all variations on a Wheatstone bridge. In a classic Wheatstone bridge, you can ground two of the three ports. In the Agilent S-parameter test sets, they place the balun configuration in the RF source, and then the DUT and detector ports can both be grounded. In the Wetterlin design, he's chosen to float the detector side. A Wheatstone bridge for 50 ohms is two legs of 100 ohms in parallel, with the detector at the midway tap point on both sides. But, there's no reason it has to be that way. One leg has the unknown DUT in it. If you reduce the value of the top resistor on that side from 50 ohms down to, say, unity, then the resistors on the other leg now have to rise, and maintain the 50:1 R ratio that is required for balance. Ultimately, the whole thing devolves into what is known as the Breune Bridge, which is a Wheatstone bridge with very little loss between source and DUT and is what a high power SWR meter is made with. cheers, s.
