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Hi Folks,

A hats off to everyone who has contributed to the NanoVNA. I got mine last week and what a fantastic tool for the cost.

I'm trying to understand the TDR measurements. I've attached a plot from my setup. You can easily see the following parts:
1) This is the SMA->UHF cable, about 1M long.
2) impedance bump from the connector
3) 10ft of cable in the shack
4) female uhf barrel connector through a door
5) 30 ft of lmr-400
6) female to female connector
7) 10 more feet of lmr 400
8) Comet duplexer and antenna

My return loss measurement is just fine, -35dB at 2m, -55dB at 70cm.

So...do I care?

thanks!
--chris
ki7dln

Hi Chris,

Somehow it looks like your 30ft length(5) is not lmr-400. On both ends the jump in impedance as well as the reflection do not match.
Could you try to measure just the 30ft peace?

The return loss might be good depending on the position. Just 1/4 wavelength more and it might be very poor.

Eric

On Fri, Jan 17, 2020 at 07:43 PM, Chris Prosser wrote:

" My return loss measurement is just fine, -35dB at 2m, -55dB at 70cm. So...do I care? "
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If you your concern is the impedance measurements; then no, I wouldn't. care. The NanoVNA-saver impedance plot tends to stack impedances and both Rune and Sali (the originator of the impedance algorithm) are working towards future solutions. So far the only program that accurately returns NanoVNA TDR impedance measurements is written in MATLAB, but the functions used are not available in the language (Python) that NanoVNA-saver is written in.

- Herb

Hello Chris,

the NanoVNA can do the TDR measurement without external software (Firmware updated last year November) :

- CH0 Real (Display-> Format -> more-> Real), Smith
- Display -> Transform -> Low Pass
- Transform on
- Velocity factor to be defined in % e.g. 0.6 = 60% (is needed to determine the right distance)

In the attachment a twisted pair cable with 50Ohms Termination was measured.
The 103 Ohms is compared to the ~150 Ohms determined by L (shorted) and C (open) a bit low and the 50Ohm return not perfect, but it can be used as indicator for line impedance changes.

One comment to your mesurement:
"8) Comet duplexer and antenna"

The VNA TDR measurement consists of calculating results of a frequency sweep. For this the cable should have the same impedance for the hole frequency range used in the measurement. This is not the case with having an antenna at the End of the cable. Best would be to replace the Antenna with 50 Ohms Termination to measure the cable!

This is the difference to TDR measurements with the reflection method, where steep short pulses are used for the measurement.

vy73 de Karsten, DD1KT

NanoVNA can do the TDR measurement without external software

FWIW, the Wiki main page /g/nanovna-users/wiki/home#Documentation
now clarifies that measurement techniques and tips are collected here:
/g/nanovna-users/wiki/Application-Notes

Thanks for the tips everyone. After sending I realize what I'm particularly interested in is measuring cable and insertion loss ala .

Given that I'm looking at seven days straight of rain (the joy of Seattle in the winter) disconnecting parts of the transmission line and testing them independently is not a near-term project :-).

I had previously seen the collective wisdom part of the wiki, some great gems in there.

Thanks,
Chris

measuring cable and insertion loss ala

Added to "External Links" wiki page
/g/nanovna-users/wiki/External-links#related-28RF29-links