Over on YouTube channel Bootstrap Workbench has been running a series on using an RTL-SDR and noise source to create a poor man’s spectrum analyzer. So far he has three videos available. The first shows how to install and setup Spektrum, his preferred Windows based wideband scanner for the RTL-SDR.
The second video shows how the RTL-SDR, noise source and Spektrum can be used to tune a cavity duplexer. A cavity duplexer is an adjustable set of filters that allows you to use a single antenna for TX and RX at different frequencies. It can be tuned by adjusting screws on the unit body.
In the third video he shows how to examine the response of a dual ferrite isolator/circulator which is a device that can be used to ensure RF only travels in one direction. This could be use for example to prevent damage to a TX power amplifier from reflected signals due to high VSWR or other nearby powerful signals.
If you’re interested in this we also have our own tutorial available about setting something like this up, but using alternative software.