Measuring velocity factor and (physical) cable lenght with NanoVNA?

[RK_aus_S]

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 ? 

If I express myself unclear; sorry, english is not my native language.

Regards
Roman

[FenTiger]

At 110 MHz, the cable is N wavelengths long.

At 171 MHz, the cable is N+1 wavelengths long.

So, at 61 MHz, the cable is one wavelength long.

[iMo]

My understanding:

1. at frequency F1 the phase shift will be 0degree (because we decided so, it is the start)
2. at frequency F2 the phase shift will be 360degree (because we count the phase mod 360)
3. that means that in F2-F1 freq difference the signal made 360-0 degree phase shift difference (one period of phase shift change through the cable length)
4. that is "equivalent" to say "at F1 the signal start was at the beginning of the cable, with rising frequency the phase shift of the start increased slowly, at F2 the start moved up to the end of the cable"
5. the 360deg turn of something is 1 period, at Fd=F2-F1 difference the 1 period is "lambda" long (a wavelength in meters), and therefore 300/Fd=4.91m..

[RK_aus_S]

@FenTiger, @imo

Thanks! Your explanations help me a lot further to understand it... 

Regards

[aristarchus]

Just have a look at these videos, they are a treasure
https://www.youtube.com/user/w2aew/videos

[cdev]

You need to know one OR the other first to get the other.  (If its handy, measure it if your coax type is unknown)

I've been using the method Alan describes in his video on it. But there are other methods.

When you are in the transform mode its giving you your info in time, nanoseconds and length (using the VF if you provided one) . For longer measurements you need to use lower frequencies and vice versa.

The point you calibrate it at (you just need SOL) is your beginning.

Coax VF varies by dielectric material. RFcafe has a table containing the VFs for most coax but there are slight variations from it which it would be useful to know if you are using one coax a lot. .66 is a good starting figure. On the NanoVNA just enter in the integers dont enter in the decimal point. Once you have entered it in it will use that until you change something.