Measuring velocity factor and (physical) cable lenght with NanoVNA? - Page 1
Forum section: electronics / rf-microwave

author: RK_aus_S
author_rank: Contributor
timestamp: July 21, 2020, 02:50:07 pm
content: HelloI'm struggling to understand parts of this YouTube Videohttps://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 lengthandthe 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.RegardsRoman

author: FenTiger
author_rank: Regular Contributor
timestamp: July 21, 2020, 03:47:08 pm
content: 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.

author: iMo
author_rank: Super Contributor
timestamp: July 21, 2020, 03:50:41 pm
content: 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..

author: RK_aus_S
author_rank: Contributor
timestamp: July 21, 2020, 04:06:38 pm
content: @FenTiger, @imoThanks! Your explanations help me a lot further to understand it...Regards

author: aristarchus
author_rank: Regular Contributor
timestamp: July 22, 2020, 06:34:34 am
content: Just have a look at these videos, they are a treasurehttps://www.youtube.com/user/w2aew/videos

author: cdev
author_rank: Super Contributor
timestamp: August 04, 2020, 12:54:10 pm
content: 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 thetransform modeits 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.