quick measuring cable characteristic wave resistance

DJ7BA

Hi,

probably the easiest way to obtain the characteristic wave resistance of a coax cable is

the Lambda/8 method:

(After an OSL CAL for the roughly expected frequency range:)

Connect a piece of open ended cable to CH0 (no need to know length or Vf).
Find the lowest (=Lambda/4) resonance frequency. (Impedance there is about 0 Ohm.)
Set stimulus CW frequency to half of the above (to make the cable Lambda/8).

The magnitude of the Lambda/8 reactance value is the cable's wave impedance.

Example: At Lambda/8, I read reactance = -51.4
It is a 51.4 Ohm characteristic wave resistance cable.

73, Hans
DJ7BA

-----Ursprüngliche Nachricht-----
Von: nanovna-users@groups.io <nanovna-users@groups.io> Im Auftrag von Andy UA3RAW
Gesendet: Mittwoch, 25. Dezember 2019 20:23
An: nanovna-users@groups.io
Betreff: Re: [nanovna-users] New version of NanoVNA-Saver: 0.2.1

Hi, Rune!
I will continue here.
There was a task to find out an impedance of unknown teflon coax cable, which looks like 75 Ohm.
I did it with the help of NanoVNA TDR measurements. A 500 mm piece of 50 Ohm teflon coax cable with SMA connector was plugged into the device and a piece of unknown cable was soldered to its other end and left open. The results are on the device screen photo.
Then I decided to do the same with the help of the built in TDR function of NanoVNA-Saver, as you suggested.
What I saw in TDR window, looks correctly, but when I put marker on the curve, only distance was displayed, but not an impedance value. See the next screenshort. The impedance value can only be read approximately from the impedance axis. I think that it would be very useful to display an impedance under marker.
After that I put device in TDR mode again, menu item - TRANSFORM/LOW PASS STEP and run Sweep in NanoVNA-Saver. The results were very interesting. TDR window displays something wrong. But “S11R+jX” window seems to be alike device screen. See the next screenshort. The markers shows true values of impedance of two pieces of cables. Only horizontal axis is in MHz. :) I think, that it is one of the ways to measure transmission line impedance with NanoVNA-Saver.
Thank you very much!
Merry Christmas!
73! Andy, UA3RAW

Helmut Wabnig

On Thu, 26 Dec 2019 09:33:20 +0100, you wrote:

Hi,

probably the easiest way to obtain the characteristic wave resistance of a coax cable is

the Lambda/8 method:

(After an OSL CAL for the roughly expected frequency range:)

Connect a piece of open ended cable to CH0 (no need to know length or Vf).
Find the lowest (=Lambda/4) resonance frequency. (Impedance there is about 0 Ohm.)
Set stimulus CW frequency to half of the above (to make the cable Lambda/8).

The magnitude of the Lambda/8 reactance value is the cable's wave impedance.

Example: At Lambda/8, I read reactance = -51.4
It is a 51.4 Ohm characteristic wave resistance cable.

73, Hans
DJ7BA
Nice trick.

w.

Nick

On Thu, Dec 26, 2019 at 08:33 AM, DJ7BA wrote:

the Lambda/8 method:
Do you have any citations for this method?

Corneliu

Hi Hans,
Thank you vm for for sharing your method with us.
I do not know which is the explanation behind this method but I can confirm it is working very well.
I have 2 pieces of professional made high quality 50 ohm cables about 38 cm long and they both measured 50 ohms +/- 0.1 ohms.
The cables supplied with nanoVna measured 53.7 and 55.2 Ohms respectively.
Thank you very much!
Merry Christmas!
73! de YO4AUL

Martin Rickes

Hi,

wave-theory of open transmission-line (Z_l = inf) is Z =
-jZ_line*cot(beta*len_line).
If the line has a length of the line is labda/8 this gives cot(pi/4), which
is one.

Thus we have Z = -j*ZLine | lengh_line = labda/8

And we have just the wave-impdenace of the line left in the equation.

Correct me, if I'm mistaken here please, but sounds like a clever idea :)

Best Regards,
Martin

Am Fr., 27. Dez. 2019 um 16:49 Uhr schrieb Corneliu via Groups.Io
<coralenka=yahoo.com@groups.io>:

Hi Hans,
Thank you vm for for sharing your method with us.
I do not know which is the explanation behind this method but I can
confirm it is working very well.
I have 2 pieces of professional made high quality 50 ohm cables about 38
cm long and they both measured 50 ohms +/- 0.1 ohms.
The cables supplied with nanoVna measured 53.7 and 55.2 Ohms respectively.
Thank you very much!
Merry Christmas!
73! de YO4AUL

Roger Need

I experimented with this Lambda/8 method using the program TLD (transmission Line Details) and got within .4 ohms of the cable impedance after doing a number of different tests. TLD contains the characteristics of many kinds of cables and calculates input impedance at various frequencies/line lengths based on a user input load impedance.

TLD is a free program and is available here >>> https://ac6la.com/tldetails1.html. Highly recommended

DJ7BA

Hi

Helmut, Nick, Corneliu and Martin,

thanks for the kind replies - indicating that the Lambda/8 method is not commonly known.
The value measured that way is good for just that one Lambda/8 frequency.
The Method is fast and easy. It saves time and quite often is all you need.
That is the strength of this method, that I first learned from DC1MKR in our local club.

If maximum precision is your need, not just fast practical results, you will prefer the frequency
dependent cable properties described by AC6LA. Find it, for example, in:

Zplots.xls https://ac6la.com/zplots1.html and applied in
SimSmith http://www.ae6ty.com/Smith_Charts.html

for many popular cable types and makes.

Both freeware programs are highly recommended to all NanoVNA users.

73, Hans
DJ7BA

-----Ursprüngliche Nachricht-----
Von: nanovna-users@groups.io <nanovna-users@groups.io> Im Auftrag von Roger Need via Groups.Io
Gesendet: Freitag, 27. Dezember 2019 19:19
An: nanovna-users@groups.io
Betreff: Re: [nanovna-users] quick measuring cable characteristic wave resistance

I experimented with this Lambda/8 method using the program TLD (transmission Line Details) and got within .4 ohms of the cable impedance after doing a number of different tests. TLD contains the characteristics of many kinds of cables and calculates input impedance at various frequencies/line lengths based on a user input load impedance.

TLD is a free program and is available here >>> https://ac6la.com/tldetails1.html. Highly recommended

hwalker

On Sat, Dec 28, 2019 at 06:31 AM, DJ7BA wrote:

" ,,, the Lambda/8 method is not commonly known... "
=================================================

The method is more useful when you have an instrument like the NanoVNA where you can visually determine the first zero ohm Lambda/8 resonance point and then quickly set the Lambda/4 CW frequency and read the impedance.

The method would be even more useful if the algorithm could be automated to program the NanoVNA to perform all the above and display the cable impedance.

- Herb

larry36@...

TDR

Andy UA3RAW

Hans, many thanks!
Very fine method!
Merry Christmas and Happy New Year!
73! Andy, UA3RAW

On Thu, Dec 26, 2019 at 11:33 AM, DJ7BA wrote:

Hi,

probably the easiest way to obtain the characteristic wave resistance of a
coax cable is

the Lambda/8 method:

(After an OSL CAL for the roughly expected frequency range:)

Connect a piece of open ended cable to CH0 (no need to know length or Vf).
Find the lowest (=Lambda/4) resonance frequency. (Impedance there is about 0
Ohm.)
Set stimulus CW frequency to half of the above (to make the cable Lambda/8).

The magnitude of the Lambda/8 reactance value is the cable's wave impedance.

Example: At Lambda/8, I read reactance = -51.4
It is a 51.4 Ohm characteristic wave resistance cable.

73, Hans
DJ7BA

Bob Albert

I set this up and am convinced it's the best way to measure characteristic impedance.

You set up the Smith chart to find the first eighth wave resonance which for an open end line will be at the bottom center of the chart. Note the frequency on the top of screen. Change the frequency to a little below and to a little above that frequency. Note the capacitance displayed.

Then select phase and carefully adjust the marker until the phase reads 90 degrees, or as close as possible.

Then comes the hard part. You use the reactance formula, where X = 1/(2 * pi * frequency * capacitance). My calculator does this in a flash. Or use a reactance calculator or chart.

That gives X at the marker frequency. I get results that are within less than 1 Ohm of 50 Ohms on various cables.

A line that is short has a high frequency for 90 degrees. A long line does it at a lower frequency. The capacitive reading adjusts itself to satisfy the equation.

Better than my earlier method using a variable resistor.

Andy UA3RAW

The screenshort shows how to make Lambda/8 measurements with the help of NanoVNA-Saver step by step.
DUT- is the cable supplied with NanoVNA.

Rudi

@Andy
Could you please tell the length of the coax cable, and if the end was open or shorted.
I know of possible length of 30, 50, 70 cm.
73, Rudi DL5FA

Andy UA3RAW

Hi, Rudi!
The length of cable does not matter in this measurement. My cable length was 33cm and it was open.

Nick

@Andy
What calibrations did you do before making your measurements?

W5DXP <w5dxp@...>

For what it's worth, here's an article I wrote on coil and transmission line measurements using an AIM-4170D.

http://www.w5dxp.com/aircore/aircore.htm

Rudi

Hello Andy,

Thank you very much for your good description in message #8836.
I could reproduce that measurement with a 30 cm coax cable.
I measured 48.6 Ohm wave resistance.
See the attached screen shot: Coax_Lambda-div-8_30cm_48R6.png

73, Rudi DL5FA

Andy UA3RAW

Nick, it was common OSL calibration on CH0 50kHZ-900MHz.

On Mon, Dec 30, 2019 at 05:18 PM, Nick wrote:

@Andy
What calibrations did you do before making your measurements?

Andy UA3RAW

W5DXP Cecil, thank you very much for your weblink. There are many interesting information on your site.
In your article, in Transmission Line Stub Measurements I read a note: "A calibration procedure was performed to calibrate out the effects of the balun between the AIM4170 coax connector and the balanced feedline." Can you explain, how to make such calibration? If, for example, I want to measure an impedance of some kind of ladder-line with VNA.
Thanks!
73! Andy, UA3RAW

On Mon, Dec 30, 2019 at 06:12 PM, W5DXP wrote:

For what it's worth, here's an article I wrote on coil and transmission line
measurements using an AIM-4170D.

http://www.w5dxp.com/aircore/aircore.htm

Mario Vano

Thanks so much for the picture - it is indeed "worth a thousand words"!

I was unable to make much sense of some of the earlier explanations because I had no idea of the actual measuring contexts being described. Being able to look at the charting settings made everything clear! Please, everyone use this as an example!

I was able to confirm my long time suspicion that I had a surplus BNC jumper cable in use where it should not have been used. It was actually one built for 75 ohm video! It's a very nice cable, its connector dimensions and impedance have been off enough to cause me some confusion for years!

thanks again...

M

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