A recent online video provides instruction on how to measure loss of a section of coax cable, loss to mean Matched Line Loss, \(MLL=\frac{P_{in}}{P_{out}}\) when the cable is terminated in its characteristic impedance Zo, and which can be expressed in db as \(MLL=10 log_{10} \frac{P_{in}}{P_{out}}\). Note that MLL in dB is ALWAYS a +ve value for a passive DUT such as this.
There is nothing new in the method, it appears in lots of analyser user manuals, and has a built in assist in many analysers.
The video deals with the case of an antenna analyser that has a ‘measure cable loss’ function and using a VNA. Lets use the VNA graphic as it shows more detail of what is happening.
![](https://owenduffy.net/blog/wp-content/uploads/2021/09/Screenshot-210905035948-300x196.jpg)
Above is the video’s graphic for the case. The narration says to use the dB magnitude of s11 or ReturnLoss as equivalents. They aren’t equivalent (a hammy Sammy thing), \(ReturnLoss=-20 log_{10}|s11|\) or \(ReturnLoss_{dB}=-s11mag_{dB}\) (both wrt the VNA reference impedance). Continue reading On testing coax cable loss with an analyser / VNA – part 1
Last update: 5th September, 2021, 3:03 PM