HP 772D Dual Directional Coupler
Coaxial, N (F), 50 Ohms, 2 - 18 GHz
Manufactured from the late 1960s (the picture shows a model from this period) to the early 2000s (then under the Agilent and Keysight brands).
Still widely used today, prices on the second-hand market are around $1,000.
I've even seen it used as a paperweight on a boss's desk – for large, thick envelopes as a practical, albeit non-technical application ...
Directional couplers separate the forward and reflected waves - whose superposition forms the standing wave - by inductive and capacitive coupling via parallel line paths between the feed (input) and the “output” (test port) - the "main line" - and the paths to the connectors for the decoupled waves, sometimes called "auxilary arms".
At the test port, the DUT (device under test) is connected. A standing wave can also be deliberately provoked: by mismatches and total reflections OPEN and SHORT.
The incident wave can be tapped at the incident port and the reflected wave at the reflected port, each with a typical attenuation of 20 dB.
As usual in the world of RF, unused connections must be terminated with the correct wave resistance also known as characteristic impedance.
I use directional couplers for demonstration purposes and measurement exercises in my lectures on “Selected Topics in RF Technology - Standing Waves on Transmission Lines.”
For example, this is an inexpensive way to estimate the SWR (standing wave ratio) of the input of a measuring device.