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On Wed, Oct 28, 2020 at 10:20 AM, Erik Kaashoek wrote:

On Wed, Oct 28, 2020 at 02:45 AM, OneOfEleven wrote:

Can you post the S1P file from that measurement for me to test with ?

Here it is. Tell me if you need more

For the calculation maybe take 3 points some/many MHz apart and calculate the
center of the circle that goes through these three points. This enables
plotting the change of impedance over frequency (if any)

Yes I was wondering if I could just maybe show the mean point on the smith chart as the impedance, but that would be accurate if you have a complete single circle, otherwise the mean will be offset.

Anyway, I'll have a think and a play with your data to see what I can do.

It maybe that I just take the nearest to 50R point and the farthest from 50R point and show the average of the two as the line impedance.

On Wed, Oct 28, 2020 at 02:45 AM, OneOfEleven wrote:

Can you post the S1P file from that measurement for me to test with ?

Here it is. Tell me if you need more

For the calculation maybe take 3 points some/many MHz apart and calculate the center of the circle that goes through these three points. This enables plotting the change of impedance over frequency (if any)

--
NanoVNA Wiki: /g/nanovna-users/wiki/home
NanoVNA Files: /g/nanovna-users/files
Erik, PD0EK

On Tue, Oct 27, 2020 at 03:24 PM, Erik Kaashoek wrote:

I'm trying to measure the (frequency dependent) characteristic impedance of a
3cm stripline using a nanoVNA-H
Attached the measurement using nanoVNA-App.

Can you post the S1P file from that measurement for me to test with ?

On Wed, Oct 28, 2020 at 08:44 AM, RayC wrote:

Erik - you were right on the money in your original post. Look at your S11
plot. It draws a semi-circle from 50 ohms, around some center. That center is
your transmission line's characteristic impedance. I'd have guessed around 65
ohms. Check out 1/4 wavelength microstrip transformers on the web to see this
effect.

ah so, it's circulating around the lines impedance then :)

Like this then ? ..

65 = sqrt(50 * 84.6)

Erik - you were right on the money in your original post. Look at your S11 plot. It draws a semi-circle from 50 ohms, around some center. That center is your transmission line's characteristic impedance. I'd have guessed around 65 ohms. Check out 1/4 wavelength microstrip transformers on the web to see this effect.

The frequency at which your S11 plot hits the real axis again at around 80 ohms, is your 1/8 wavelength. If you had a good measurement higher in frequency it would continue to rotate around the chart and pass through 50 ohms again at 1/4 wavelength. It would keep going around that circle over and over every 1/4 wavelength, forever.

As to what the Vp is, you can take the physical length of the line and back calculate the relative permitivity of the dielectric and/or the phase velocity from that 1/8 wavelength frequency. Strictly speaking that's not a perfect fit but it's good enough for what most of us will ever do.

On the variation vs frequency, I don't think you'll be able to measure that. I don't know FR4 material but I make t-lines all day long and they look so close to constant impedance from 50MHz to 50GHz that I have to have a careful setup to measure the dispersion. There's some loss due to Dk vs frequency that will push your impedance up, and maybe you'll see that in the few GHz range, but it looks like you're hitting the noise floor of the equipment enough at over 1GHz that the Zo variation will be lost in the measurement accuracy.

Roger's link to Bogatin's work is a good one. I forget that at very low frequency, lower than I've ever looked at a t-line, skin effect really kicks impedance up a lot.

And here is the 0-2GHz scan of a 3cm coplanar stripline on FR4 that seems to confirm the impedance is changing with frequency
The marker is at 1GHz



--
NanoVNA Wiki: /g/nanovna-users/wiki/home
NanoVNA Files: /g/nanovna-users/files
Erik, PD0EK

REAL and IMAG parameters are quite different from RESISTANCE and REACTANCE parameters. REAL and IMAG apply to reflection coefficient Γ, in its complex form (a+j.b). That's why values are always in the [-1,1] interval, without any associated unit. When REAL=-1 and IMAG=0, it is the Short circuit situation. When REAL=1 and IMAG=0, it is the Open circuit situation. When REAL=0 and IMAG=0, it is the normal Loaded (50 ohms) situation.

LINEAR is the Γ modulus form of combined REAL and IMAG values, and finally POLAR is the geometric representation of REAL, IMAG and PHASE values. When POLAR is displayed by the NanoVNA, and even if data values are exactly displayed as for Smith Chart, results must not be read in the same way. Have a try by displaying two CH0 traces, POLAR and SMITH.

SWR and LOGMAG (Return Loss) are derivated from Γ modulus (LINEAR). For educational purposes I have created an ods file (see below), showing and calculating NanoVNA parameters. You can play with it by entering values in the blue fields, and also checking what are the arithmetic relations behind the different results. Here Group Delay is not relevant as calculations are done for a discrete (CW) frequency.

A last word about the use of REAL and IMAG parameters. The following case (see attachment) is an opened coaxial cable (length 2 meters), creating a quarter wave stub (at red marker). An opened coaxial cable remains a good use case for education and increase of knowledge. On the NanoVNAsaver snapshot we see clearly that displayed values between RESISTANCE/REACTANCE and REAL/IMAG do not allow an immediate comparison. For example at red marker, R+jX or Smith Chart highlight a value of 0+j0 ohms (short circuit situation), and checking this with REAL/IMAG chart you get -1 (REAL) and 0 (IMAG) which is the same thing. Be careful with REAL and IMAG curves which follow sinus and cosinus rules, it reflects simply a monotonous variation of PHASE.

REAL is also interesting if you want to measure a coaxial cable length, thanks to advanced TDR function.

73 from Jean-Roger / F6EGK

On 10/27/20 1:09 PM, Jose Mihotek via groups.io wrote:

HI Erik,
The characteristic Impedance of a transmission line is not frequency dependent.A coax is 50 Ohm at 1MHz or at 1000MHz.A TDR will measure Zo (Impedance) as well as transitions in the time domain which can be translated to distance.A VNA will measure Zo vs frequency, but it can measure like a TDR at CW.I don't know if the software in NanoVNA supports TDR.When you measure a length of coax it is doing exactly that.Look into that function.It should display the Zo for a reasonable lenght of transmission line.3 cm is very short, I don't know if it can measure that.

Well... A transmission line with frequency independent construction is not frequency dependent. But there are plenty of examples of transmission lines with non-constant impedance, even at a single frequency For example, there are tapered lines that have different impedances at each end. And actual coax is also frequency dependent.

Z0 = sqrt ((R+j*omega*L)/(G + j*omega*C))


So, for things like a air dielectric coaxial line at low frequency with constant sized outer and inner conductors, L per unit length and C per unit length are fixed by the geometry .

But real life coax has non-zero R and G, and those tend to vary with frequency, the former mostly from skin effect, the latter from dielectric losses.

Real life low loss coax has very low R and G, so you can probably ignore them for most practical purposes.

However, get up to microwave frequencies and this starts to make a difference, particularly for small diameters. And, of course, there's funky delay line coax, with a helical center conductor that's ferrite loaded and has a Z of around 1000 ohms.

Best regards,
JoseN0GU
On Tuesday, October 27, 2020, 03:50:11 PM EDT, John Gord via groups.io <johngord@...> wrote:
Eric,
Take a look at message 17063.? It discusses how to set up a NanoVNA to do TDR in a manner that allows direct reading of the impedance along a connected line.? Using the reflection times, you can also get the velocity.
Last time I looked, NanoVNA-Saver did not do impedance readout in TDR correctly, but the firmware does.
--John Gord
On Tue, Oct 27, 2020 at 08:24 AM, Erik Kaashoek wrote:

I'm trying to measure the (frequency dependent) characteristic impedance of a
3cm stripline using a nanoVNA-H
Attached the measurement using nanoVNA-App.

Using RFsim99 I calculated that a 3cm stripline with impedance of 65ohm and a
phase velocity of 1.25e+8m/s gave the same S11

How can I calculate from the data shown in nanoVNA-App to the characteristic
impedance of 65ohm? Would that be of 1GHz? (50 + 85)/2 (the center of the
impedance circle?) is 67ohm?

So using some points (one below, one at and one above the frequency of
interest?) on the impedance circle,? should it be possible to calculate the
stripline impedance for any frequency?
--
NanoVNA Wiki: /g/nanovna-users/wiki/home
NanoVNA Files: /g/nanovna-users/files
Erik, PD0EK

David:
Funny!
Too bad it happens far more often than not, and most are unaware of it.
As for YOUR social security, I will not be contributing much longer!
I am going solo, and getting back to real radio, not this broken volume
control trash I have been stuck with.
A Duplexer here, a repeater there...covid hoax is everywhere!
60+ and my immune system must be top notch!
Waiting for my fiancé to get stateside, and finally away from military
minutia.
Moving out to heaven immediately after.

Robin:
I am connected to what appears to be the only 2 active antennas I am able to use, but I never have the time to access the roof, and perform
proper testing.
The ones responsinble for the utter disaster, are now long gone, and thankfully, I will be as well.
I am not going to unscramble this mess. I was going to look at the plots, and for sadistic humor, tey to make heads or tails of the data.
What I should do, is show the owner, and prove the mess he has, will require a lot of time and money to clean up.
I think I am going to simply decline future work on this unholy mess, as I am certain it will become my fault.
I hjave signal traces and other tools to track down cable problems, but after thinking about this for a while now, I am going to simply show the data
and walk away...I should not have begun this worthless journey to be honest.
73!
Ernest
KA9UCE

On Tue, Oct 27, 2020 at 08:24 AM, Erik Kaashoek wrote:

I'm trying to measure the (frequency dependent) characteristic impedance of a
3cm stripline using a nanoVNA-H

Eric,

The following article may be of interest to you. It measures at multiple of 1/8 wavelength and I know you want to look at frequencies other than this but it might give you several data points to compare with other methods. What is also important is how they physically attach the SMA connectors and terminated the microstrip.



Roger

On Tue, Oct 27, 2020 at 01:19 PM, Jose Mihotek wrote:

HI Erik,

The characteristic Impedance of a transmission line is not frequency
dependent.A coax is 50 Ohm at 1MHz or at 1000MHz.

The characteristic impedance of a transmission line does vary with frequency. As shown in the formula below the characteristic impedance is based on several cable parameters that can change with frequency. In particular R and L will change with frequency due to the skin effect. At low frequencies the change in R can be quite significant. At frequencies beyond 100 MHz. there is little change.

Attached is a plot from the program TLDetails showing calculated characteristic impedance of RG8X with frequency. another example for microstrips is discussed here:

>>

Roger

Ernest,

the periodic nature of the dips indicates a long cable, but you probably guessed that. If it was open circuit or short circuit one would expect a return loss getting close to 0dB, but that value will be reduced by the effect of two-way cable loss.

If you have access to the roof, or wherever the antennas are, then this looks like a job for a tone tracer or similar, preferably with a reliable assistant. If not, then some calculations may (or may not) be helpful.

1) estimate cable loss of 150' of whatever cable is in use at each frequency of interest. You should do this at the frequencies (if known) of each individual antenna. [Hopefully all the feeder cables are the same type.]

2) double that loss (for go & return) to calculate the aggregate cable loss

3) An open (or short, depending on how cleanly the cable was cut) circuit should have a return loss of 0dB, less the aggregate cable loss.

4) Whichever cable has roughly that loss is probably the one that was cut. There may be a ripple effect due to cable imperfections.

5) A classic dummy load is a long length of lossy cable (there's more to it, but that's the principle).

HTH, and you have my deepest sympathy,

Robin, G8DQX

On Tue, Oct 27, 2020 at 03:03 PM, Simen Tobiassen wrote:

What is the difference between the curves in "Real/Imaginary" vs "R+jX (Ω)"
Charts - in NanoVNA Saver and NanoVNA 2_2 ?

The first displays the complex scattering parameter (S11 or S21), the second display the complex impedance calculated from S11.

Regards
Christian

Imaginary = ±jX (just presented in different forms)

Capacitive reactance carries the negative sign (bottom half of the Smith
Chart).

Inductance reactance carries the positive sign (top half of the Smith
Chart).

Also remember, resonance is defined as zero imaginary or zero ±jX values -
purely resistive. It is the radiation (pure) resistance that radiates
energy from an antenna structure.

Also: Resistance is frequency independent.
Reactance is frequency dependent.

Resistance can dissipate power but cannot effect phase.
Reactance cannot dissipate power but does effect phase.

We'll not address parasitics of the real world implementations of the above
statements.

Dave - W?LEV

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

HI Erik,

The characteristic Impedance of a transmission line is not frequency dependent.A coax is 50 Ohm at 1MHz or at 1000MHz.A TDR will measure Zo (Impedance) as well as transitions in the time domain which can be translated to distance.A VNA will measure Zo vs frequency, but it can measure like a TDR at CW.I don't know if the software in NanoVNA supports TDR.When you measure a length of coax it is doing exactly that.Look into that function.It should display the Zo for a reasonable lenght of transmission line.3 cm is very short, I don't know if it can measure that.
Best regards,
JoseN0GU

I once worked for a nickel-and-dime company in Albuquerque. We were
expected to make good engineering measurements in a lab with no AC in the
summer. Our lab ran 110 to 125 most of the time in afternoons. Without
permission from 'the boss' I called in an HVAC outfit to at least look at
the old and ratty swamp cooler system, if you could call it a system.....
No hope. I got fired for that. And, they were dumping extremely hazardous
and carcinogenic waste along the curbing as they didn't dare put it into
the sewer system. Oh, yes, should I mention the knife and bow-and-arrow
fights after hours..... Enough...... One in my career is more than
enough! I'm well familiar with those companies and thankful I'm retired.
NO MORE CORPORATE AMERICA for me!!!! Just keep contributing to our Social
Security.??

Dave - W?LEV

On Tue, Oct 27, 2020 at 7:52 PM ERNEST AEC-RADIO <aecradio1@...>
wrote:

David:
This shop has a history of people without any radio experience, and I am
being ignored as to my reasons why everything needs to be rebuilt.
I suspect the owner is treating a business, as a hobby, and is also very
cheap.
Nickel and dime things are okay, but big ticket items are not.
It has been more than a year since I was last on the roof, and I know the
person that has been up there, has no idea what he is doing, and has zero
interest in the work.
I offered to assist with his understanding of cables, loss and why they
need to be short, direct runs, but it goes in one ear and out the other.
The owner shows no concern or care as well.
Thankfully, I am changing my life soon, and getting out of Arizona. I hate
totalitarian police states, and the unconstitutional employees are ruining
a good life for everybody.
I just wish I could make the owner understand that the rat race of cables
is far excessive and, to properly tag each antenna with proper data, so I
know immediately what is up there, and what band/s they are tuned for.
Right now, it's a crap shoot!
Spock would go insane here....NO LOGIC!

On Tue, Oct 27, 2020 at 11:49 AM David Eckhardt <davearea51a@...>
wrote:

FIRST ATTACHMENT: With an 'average' RT of 15 dB, I doubt much RF is
getting to whatever is at the far end of the coax! The vast majority of
the data indicates lossy coax. The coax is pretty lossy!!! You might

have

a real antenna at the other end that wants to ring at roughly 450 to 455
MHz. But with that much loss, not much RF will reach this 'maybe'
antenna. Most of the data is due to lossy coax.

SECOND ATTACHMENT: Really can't make much sense of this. Everything
appears to be in milliohms. I don't know??

THIRD ATTACHMENT (Smith Chart): Again, lots of loss. Nearly all the

data

is within the 2:1 circle which indicates nothing but coax cable loss. It
shows multiple real-axis crossings near center (50-ohms), but I seriously
doubt much RF is reaching whatever is connected. Lots of loss!

FOURTH ATTACHMENT: Again, all resistance indications illustrate a lot

of

loss with all values between 20 and 100-ohms. This is all due to cable
loss.

FIFTH ATTACHMENT: I would presume the horizontal assembly of plates that
all the coaxial runs cross is copper and intended for lightning/discharge
remediation. I'm not addressing the sheet of plywood at the top. This

is

NOT a proper installation for that purpose!!!! What's more, the 'ground'
wire is too haphazardly routed and too small of a gauge to be of much

good

other than to serve as a return for small, low-frequency energy.

CONCLUSION: I don't know how long the coax runs are, but I'd suggest
replacing them all. All your plots show is loss in the cabling! The
mostly <2:1 SWR across your measurement bandwidth is all due to coax

loss.

There is far too much loss indicted by all your measurements. This time:
1) have someone familiar with RF install them and use good cable and 2)
Have someone familiat with proper return current (from close-by lightning
strikes) do the 'grounding' side of the installation.

You're right. It's a mess.........

Dave - W?LEV

On Tue, Oct 27, 2020 at 5:32 PM ERNEST AEC-RADIO <aecradio1@...>
wrote:

Okay, I saved each image, 1 through 4.
THese should now paint a better picture of what I am dealing with.
I have no clue which antenna these readings are for, and that is one

tiny

problem I doubt I will get an answer to.
The cable runs are excessive, well over 150 feet, and at least two
transitions in between.
The entire cable plant is a horrid nightmare!
The cables were hung/run/strung up by someone that had NO RF

experience!

One walk through would incite laughter and hysteria...how any signal

can

reach any of those antennas, MUST be a miracle!
I will have to take one pic just to prove the mess I have to deal with.
In fact, I am going to try doing that now.

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

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

David:
This shop has a history of people without any radio experience, and I am
being ignored as to my reasons why everything needs to be rebuilt.
I suspect the owner is treating a business, as a hobby, and is also very
cheap.
Nickel and dime things are okay, but big ticket items are not.
It has been more than a year since I was last on the roof, and I know the
person that has been up there, has no idea what he is doing, and has zero
interest in the work.
I offered to assist with his understanding of cables, loss and why they
need to be short, direct runs, but it goes in one ear and out the other.
The owner shows no concern or care as well.
Thankfully, I am changing my life soon, and getting out of Arizona. I hate
totalitarian police states, and the unconstitutional employees are ruining
a good life for everybody.
I just wish I could make the owner understand that the rat race of cables
is far excessive and, to properly tag each antenna with proper data, so I
know immediately what is up there, and what band/s they are tuned for.
Right now, it's a crap shoot!
Spock would go insane here....NO LOGIC!

On Tue, Oct 27, 2020 at 11:49 AM David Eckhardt <davearea51a@...>
wrote:

FIRST ATTACHMENT: With an 'average' RT of 15 dB, I doubt much RF is
getting to whatever is at the far end of the coax! The vast majority of
the data indicates lossy coax. The coax is pretty lossy!!! You might have
a real antenna at the other end that wants to ring at roughly 450 to 455
MHz. But with that much loss, not much RF will reach this 'maybe'
antenna. Most of the data is due to lossy coax.

SECOND ATTACHMENT: Really can't make much sense of this. Everything
appears to be in milliohms. I don't know??

THIRD ATTACHMENT (Smith Chart): Again, lots of loss. Nearly all the data
is within the 2:1 circle which indicates nothing but coax cable loss. It
shows multiple real-axis crossings near center (50-ohms), but I seriously
doubt much RF is reaching whatever is connected. Lots of loss!

FOURTH ATTACHMENT: Again, all resistance indications illustrate a lot of
loss with all values between 20 and 100-ohms. This is all due to cable
loss.

FIFTH ATTACHMENT: I would presume the horizontal assembly of plates that
all the coaxial runs cross is copper and intended for lightning/discharge
remediation. I'm not addressing the sheet of plywood at the top. This is
NOT a proper installation for that purpose!!!! What's more, the 'ground'
wire is too haphazardly routed and too small of a gauge to be of much good
other than to serve as a return for small, low-frequency energy.

CONCLUSION: I don't know how long the coax runs are, but I'd suggest
replacing them all. All your plots show is loss in the cabling! The
mostly <2:1 SWR across your measurement bandwidth is all due to coax loss.
There is far too much loss indicted by all your measurements. This time:
1) have someone familiar with RF install them and use good cable and 2)
Have someone familiat with proper return current (from close-by lightning
strikes) do the 'grounding' side of the installation.

You're right. It's a mess.........

Dave - W?LEV

On Tue, Oct 27, 2020 at 5:32 PM ERNEST AEC-RADIO <aecradio1@...>
wrote:

Okay, I saved each image, 1 through 4.
THese should now paint a better picture of what I am dealing with.
I have no clue which antenna these readings are for, and that is one tiny
problem I doubt I will get an answer to.
The cable runs are excessive, well over 150 feet, and at least two
transitions in between.
The entire cable plant is a horrid nightmare!
The cables were hung/run/strung up by someone that had NO RF experience!
One walk through would incite laughter and hysteria...how any signal can
reach any of those antennas, MUST be a miracle!
I will have to take one pic just to prove the mess I have to deal with.
In fact, I am going to try doing that now.

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

Eric,
Take a look at message 17063. It discusses how to set up a NanoVNA to do TDR in a manner that allows direct reading of the impedance along a connected line. Using the reflection times, you can also get the velocity.
Last time I looked, NanoVNA-Saver did not do impedance readout in TDR correctly, but the firmware does.
--John Gord

PS: The equation might not be solvable analytically; among other things because tan() is a periodic function. When I did similar calculations, I already had an estimate for the parameters (impedance, phase constant) and I searched for the solution numerically. In my case, that turned out to be fairly stable.

Regards
Christian