Loss Bridge ?
The correct term is Return Loss Bridge (RLB), this is a device used in RF applications for measuring the impedance presented by some item such as an antenna, or the input impedance of something like a receiver or spectrum analyser.
The "Return" part of the description comes from the idea that you are measuring the energy which is reflected back to the source from a termination which is different from the system characteristic impedance, usually 50 Ohms.
Second circuit
The second circuit in post#1 of this thread is a true Return Loss Bridge, it uses a Wheatstone type bridge which is made up from 50 ohm resistors R5, R6 and R8, the bridge is completed by the unknown impedance connected to Z-Port.
R7 forms a termination resistor on the input to the detector U1. (I am not sure if this resistor is strictly necessary here in this circuit.)
In a previous post:
https://www.electro-tech-online.com...capacitance-based-probes.151805/#post-1304218
I commented that the use of a return loss bridge for capacitance measurement seemed rather odd.
Having no experience in this area, I am still of that opinion although I am prepared to be convinced otherwise.
First circuit
If I interpret the first circuit correctly, it is not in the form of a bridge but a series connection of two 470 Ohm resistors in series with the capacitance to be measured (the soil moisture probe), as shown here:
View attachment 108535
Here, the voltage is being measured across the capacitance of the soil moisture probe.
To me, this feels to be a much more appropriate measurement method than the use of an RLB.
JimB