Using directional couplers to provide reference signals for hydrogen line radio astronomy

Andy,

A directional coupler is a device which allows RF to pass along one path in either direction with very low insertion loss while tapping off a reference signal at a reduced power level consisting of power flowing in only one direction. Commonly this reduced level is 6 to 20 dB down from the main path power level. Directional couplers can be used to inject reference signals with some level of attenuation into the RF path as well as tapping off power in one direction from reflection off of a mismatched load. Because of this, they are often used to measure reflected and forward power in order to calculate VSWR or S11 magnitude. I had good success using 2nd hand ones for low cost VSWR measurements of 5.8 GHz antennas before COTS SWR meters were affordable at that frequency. They are also critical to using a spectrum analyzer with noise source or tracking generator to make S11 measurements of microwave devices.

Here’s an example of use for VSWR measurements at 5.8 GHz: https://www.youtube.com/watch?v=F69ZKZZhd-k

Spectrum Analyzer use w/ Noise Source: https://www.youtube.com/watch?v=SP8u0ceGs0Q&list=PLynuGts93wlBOOu-Jmgf-fH-tb1pvp1lZ&index=15

And microwaves 101 is always a good resource for more info: https://www.microwaves101.com/encyclopedias/directional-couplers

-Robert