Hello
I'm struggling to understand parts of this YouTube Video
https://youtu.be/aWvPB299U60, which explains how to measure the velocity factor and (physical) cable length by using a NanoVNA.
Specifically the fact, that the video determines a specific frequency (those 61 MHz in the video) by calculating the difference of two frequencies (those 171 MHz minus those 110 MHz in the video) which are determined using a 360° phase shift during a frequency sweep? After that, 300 (speed of light) are devided by this 61 MHz - the same formula is used to calculate a wave lenght.
I'm simply not able to recognize the context between a number (61 MHz) which is treated like a "normal" radio frequency to calculate its wave length
and the manner where this number comes from; a calculation of the difference of two frequencies (171 MHz - 110 MHz), which are determined by a 360° phase shift during a frequency sweep ?
![Shocked :o](https://www.eevblog.com/forum/Smileys/default/shocked.gif)
If I express myself unclear; sorry, english is not my native language.
Regards
Roman