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Hi all,how do i measure the loss on a long run of coax using the nanovna,ive about a 60ft run i want to check at hf,cheers .

I am in no way an expert, but if you connect your cable between CH0 and CH1, you will see the loss (or gain) at every frequency. You have to calibrate your VNA fully first .

Here is an alternative. It may not be precise but can give you an idea of
the loss. First you need to have access to the antenna end of the
transmission line.
1. Fully calibrate the NANOVNA using your best OS for the desired frequency
range.
2. Connect the short to the VNA and note the return loss using LOGMAG.
3. install the short at the antenna end.
4.Connect the VNA to the station end of the transmission line
5. Use the LOGMAG and perform a sweep
6. Note the return loss.
7. divide the return loss by 2.

This will get you close.

*Clyde K. Spencer*

W6LG just posted a video measuring coax loss using nanovna and -saver:

His measurements were pretty close to the coax loss calc on my site :-)
--kv5r

ive just taken some measurements of my coax,with my vna on cw at 3.775 Mhz,with the vna shorted i get logmag=0.75db,then with the short at the far end of my coax i get logmag=-0.42db,does that mean my coax is bad?

It depends on the length and type of the coax you are measuring.

hi ron,its about 60 feet of rg-213 type stuff.73

If you shorted CH0 to measure S11 (return loss), the nano should report back 0.00 dB. If not you need to do a cal (open, short, load). If the you get an RL of 0.5 dB for example, the loss of the cable is 0.25 dB (loss of signal going to the far and and loss of the signal returning).
However, if you short the far end and the length of the cable is a quarter wave long at your test freq, the impedance will be very high and the nano has trouble measuring this high Z. You might get a more accurate measurement of the cable's loss with the far end left open with a quarter wave length where the Z will be very low.
I think you are better off doing an S21 (Ch0 to Ch1 after a through cal). 73, Pete, WB2UAQ

Loss per 100 feet of RG-213:

1.0 MHz: 0.15 dB
10.0 MHz: 0.55 dB

Draw your own conclusions.

Dave - W?LEV

--

*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*

si in real terms is my coax bad at that frequency? 73,m3vuv.

You could look at the published loss figures (which I forwarded) and make
your own decision. 0.5 dB loss at the middle of the 75/80 meter band for
70 or so feet does not seem out of order to me.

A sure-fire method of determining the integrity of your coax is to measure
the power delivered into the coax at the transmitter end with the coax
terminated at the far end with a good dummy load. This needent be full
power, depending on your power meter. Then move the power meter to the far
end and measure again with the power meter terminated into the same dummy
load. The total loss in dB can then be easily calculated from:

Loss (dB) = Log [(measured power at FAR end) / (measured power at XMTR
end)]

The result will be negative indicating loss in dB. The log is base 10.

Dave - W?LEV

--

*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*

ERROR........OVERSITE.......

Loss (dB) = Log [(measured power at FAR end) / (measured power at XMTR
end)]

should be:

Loss (dB) = 10 X Log [(measured power at FAR end) / (measured power at XMTR
end)]

On Fri, Jan 31, 2020 at 8:28 PM David Eckhardt <davearea51a@...>
wrote:

You could look at the published loss figures (which I forwarded) and make
your own decision. 0.5 dB loss at the middle of the 75/80 meter band for
70 or so feet does not seem out of order to me.

A sure-fire method of determining the integrity of your coax is to measure
the power delivered into the coax at the transmitter end with the coax
terminated at the far end with a good dummy load. This needent be full
power, depending on your power meter. Then move the power meter to the far
end and measure again with the power meter terminated into the same dummy
load. The total loss in dB can then be easily calculated from:

Loss (dB) = Log [(measured power at FAR end) / (measured power at
XMTR end)]

The result will be negative indicating loss in dB. The log is base 10.

Dave - W?LEV


On Fri, Jan 31, 2020 at 8:21 PM <nanovnauser@...> wrote:

si in real terms is my coax bad at that frequency? 73,m3vuv.

--

*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*

--

*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*

well im still no nearer to an answer!!

Did you calibrate the NVNA?

--
Carey Fisher
careyfisher@...

yes it was calibrated with a sweep from 3.5 to 4 Mhz

Run the same test at 50, 100, and maybe even 200 MHz. Compare those loss
measurements against published loss data for your coax. I've experienced
coax cables that measure 'mostly' good at low frequencies, but become quite
lossy at higher frequencies. That should tell the whole story.

Dave - W?LEV

--

*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*

I ran a test case simulation as a point of reference to a vna measure. This is 100 feet of RG-213 coax. However, I deliberately made the coax much better and than much worse than typical to compare response differences. You will note the ripples in the return loss plot due to the periodic response of transmission line with a perfect reflection; a short in this case. I compare the S21 insertion loss of the coax with 2x higher loss than with Copper for the center conductor. As expected, the S21 response loss is 1/2 of the return loss due to the double travel of the signal. The two measurement techniques agree quite well. The advantage of course of the short at the end of the line, is no need to drag the coax end all the way back to the vna!

Alan

What about the fact that (Ifor+Iref)^2 does not equal (Ifor^2 + Iref^2)? Doesn't that mean that the matched line loss is not equal to 1/2 of the stub loss?

I've been confused about the ripples in the return loss measurements until I read this article:

Bottom line: Rather than taking RLoc/2 or RLsc/2, the true matched-line loss is very close to (RLoc+RLsc)/4

Again, there is no doubt that a more "laboratory" approach would yield more rigorous results. And my simple RL/2 is only a simple indication. But it is enough I feel to show the effect. I do though appreciate the feedback and will ponder Owen's "expert" comments.
John W6NBC