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Is there any way to tell if the dielectric is PE or foam?

73,
Joseph W6JHP

I did ask Mts Goggle and got no (real) answer

Here is the label on the cable

Eagle Aspen 39B2 RG-6 2.25 GHz Digital Coaxial Cable, 6FT, Black, High Performance 2.25 GHz CMX 18AWG 75°C UL E227171

Please share if you know the velocity factor - VF .

Thanks

Sigi:

Did you write anything up or have pictures of your Bluetooth add-on.

I’d like to give it a try.

Thanks

Ed McCann
AG6CX

On 6/10/23 11:22 AM, Anne Ranch wrote:

Jim Lux,
as always, your replies are clearing few of mine misconceptions, thanks.
With all this FFT (frequency to pulse and time ) it is still unclear how CHANGING the "sweep" changes the "estimated cable length".
...and how it still "calculates" the cable length AFTER it is hooked to the actual load / antenna (variable with frequency ) - assuming that TDR can be calculated from open , shorted and "real terminated" but unknown and variable with frequency termination.

The open/short/load just makes sure the reflection measurement is correct. It has nothing to do with the actual measurement of a feedline.

The reason you can "see" connectors and the antenna in the TDR is that they are not exactly the same impedance, so you see the discontinuity. If you had a single length of coax, and a load that was well matched to the coax, you probably wouldn't be able to measure the length.

Think of it as estimating the size of a room with your eyes closed by listening how long it takes for the echo to come back when you clap your hands. Easy in a boxlike room with hard walls. Impossible in an acoustically dead room, because the sound just goes out and is absorbed.

It is the rare antenna that is matched at all frequencies. So even it's a good match (and has no reflection) at a few frequencies, for most of the other frequencies, it reflects back significant energy. And that's enough to make the TDR work.

To kind of use a different conceptual view, imagine that you measure the phase shift (and ignore amplitude) for a series of frequencies. You plot those on a graph (unwrapping where needed), and then fit a straight line to the points. The slope of that line corresponds to the length of the cable. And even if there's some missing points (because the amplitude is small where the antenna is well matched) you can still fit the line. So you get a more accurate measurement - sort of like averaging.

The cool thing about the FFT approach or the "fit a straight line" (versus the "look for zero or 180 phase in the reflection and it's a multiple of 1/4 wavelength" ) is that it combines multiple measurements into what you want - the delta phase vs delta frequency. (FWIW, Delta phase/delta frequency is "group delay")

It also lets you know if you're measuring something that's dispersive (where the delay varies with frequency), because then, the plot of phase vs frequency won't be a straight line. That's important for things like waveguides, ionized media, traveling wave tube amplifiers, etc - because a group delay that's not flat means that pulsed waveforms will be distorted.

With real load connected the calculation must include "real load" somehow...

On 6/10/23 11:04 AM, Roger Need via groups.io wrote:

Those interested in how the TDR was implemented in NanoVNA Saver will find the details in this groups.io post by Herb Walker.
/g/nanovna-users/message/9651
For more details on how the S11/IFFT method of TDR works this tutorial by Agilent is quite informative. It includes details on why "windowing" the data is necessary, how to determine the upper stop frequency required based on estimated cable length, why the lower start frequency should be close to DC and other considerations.

Roger

The Keysight Ap Note is a good reference.

For what it's worth, in the older NanoVNA-Saver version I have, there are some problems in the implementation, specifically because it does not deal with the "non zero" starting frequency. If you set 10 MHz to 60 MHz sweep, I don't know that you'd get the right results, because it just assumes that you're feeding the IFFT evenly spaced points from 0-{highest freq}.

It is possible that when you select TDR mode it changes how it builds the measurement segments so that the lowest frequency is equal to the spacing between frequencies, and then puts in a zero for the DC term.

I do know that the Keysight boxes (e.g. the fieldfox) do handle both lowpass and bandpass correctly. When you pay $20k, that's something you get. For other applications, with uneven spacing, I've written code to deal with it.

Jim Lux,
as always, your replies are clearing few of mine misconceptions, thanks.
With all this FFT (frequency to pulse and time ) it is still unclear how CHANGING the "sweep" changes the "estimated cable length".

...and how it still "calculates" the cable length AFTER it is hooked to the actual load / antenna (variable with frequency ) - assuming that TDR can be calculated from open , shorted and "real terminated" but unknown and variable with frequency termination.

With real load connected the calculation must include "real load" somehow...

Those interested in how the TDR was implemented in NanoVNA Saver will find the details in this groups.io post by Herb Walker.

/g/nanovna-users/message/9651

For more details on how the S11/IFFT method of TDR works this tutorial by Agilent is quite informative. It includes details on why "windowing" the data is necessary, how to determine the upper stop frequency required based on estimated cable length, why the lower start frequency should be close to DC and other considerations.



Roger

Jim Lux,

That was a well written explanation of how "TDR" is done in the NanoVNA. Thanks for taking the time to write it up.

Roger

i added a bluetooth module to my sark100 and can now measure "wireless" (use smartphone to display sweeps)

dg9bfc sigi

Am 10.06.2023 um 16:33 schrieb KK4ITX John via groups.io:

On 6/10/23 7:36 AM, Anne Ranch wrote:

The calculations are in ./Windows/TDR.py (not ./Charts/TDR.py, that's what actually does the plotting)
Little discouraging, but I found the same "source code ".....
I also realized that , also little to late nanoVNASaver is "just the GUI" , but I had to start somewhere...

Not at all - NanoVNA-Saver also does the calculations - for calibration, for instance, as well as TDR.

My next step is to find the actual TDR processing,,,

It's in ./Windows/TDR.py

Maybe after I muddle thru the nanoVNASaver TDR undocumented / uncommented source code

If you know how inverse FFT to generate the time domain works, there's really not a lot of commenting needed.

There's the following steps:

1) concatenate all the segment sweeps into one array
2) apply a suitable window (it uses the blackman window from numpy)

3) do the inverse FFT (also in numpy)


Now you've got the time domain impulse response
4) To convert that to a step response (because that's what you need for a impedance display) you convolve it with a step (array of all ones) (yeah, using numpy )

5) Then convert the step response to impedance using the usual Z0 * (1+rho)/(1-rho) formula.

( quite expected ) I will make (some) more progress.
BTW - for all those who keep telling me to "use S11 " phase reversal " - this TDR GUI does use S11 , so
it looks as TDR GUI is "just another layer" of software...
I am still in KISS mode - TDR in principle is
" start timer, then send a pulse and measure time when such pulse returns "
it cannot be much simpler,

That's one way to do TDR - but that's now how VNAs do it. What a VNA does is measure the reflection coefficient at a series of frequencies. That gives you a frequency domain response. Then, you use the Fourier transform to turn that into the time domain response.

As of today - I still do not know how nanoVNA generates such pulse and how "sweep" frequency (?) setup
is related to this simple principle of sending a pulse down the line.

It's not.

That pulse down the line is (idealized) an "impulse" which contains all frequencies. The impulse response (what you get with classic TDR) is the reflected voltage vs time from that pulse. The challenge is that you want a fair amount of energy in the pulse (so your reflection measurement has good SNR) AND you want it to be really narrow, which means a high voltage fast pulse, which is hard to generate. It also might "break" something in the system to put a HV pulse. The problem with a short pulse is that although it's "wideband" the energy is spread out, so the power at any one frequency is small.

What the VNA does is rather than send an infinitesimally short pulse, it sends single frequencies, for a long time, at lower power. It eventually sends "all frequencies" (or, at least enough different frequencies) so it's spanning the spectrum, just like the short impulse did. Even better, each of those individual measurements has more energy, hence better SNR. The Fourier Transform lets you convert all those individual measurements spanning the frequency into an equivalent time response.

For what it's worth, a similar technique is used in modern radars - A radar is just TDR through the air. The shorter the pulse, the higher the resolution, the more power in the pulse, the better the SNR. But the problem is that there's a maximum power (can't keep building bigger tubes or amplifiers - and high power breakdown limits how much power you can push) - so they do what's called pulse compression - they turn the short impulse into a little FM chirp - same average power, but lower peak power. Then on receipt of the signal, they dechirp it to generate the time domain response. In traditional pulse compression they use dispersive delay lines (or these days DSP) - and, as a practical matter, you don't have to use chirps, you can use PSK coding too. But overall, it's the same general idea you use the time domain/frequency domain swap.

PS
When this is all done I may break down and
open the RG6 I have
observe the dielectric - my guess it is poly-whatever
and actually use nanoVNA to measure the VF...
what a goal...
ah yes - my current cable is 9 (nine) times of 1/4 wavelength long @ 14MHz....
Purfect for Field Day...

The calculations are in ./Windows/TDR.py (not ./Charts/TDR.py, that's what actually does the plotting)

Little discouraging, but I found the same "source code ".....
I also realized that , also little to late nanoVNASaver is "just the GUI" , but I had to start somewhere...
My next step is to find the actual TDR processing,,,
Maybe after I muddle thru the nanoVNASaver TDR undocumented / uncommented source code
( quite expected ) I will make (some) more progress.
BTW - for all those who keep telling me to "use S11 " phase reversal " - this TDR GUI does use S11 , so
it looks as TDR GUI is "just another layer" of software...

I am still in KISS mode - TDR in principle is
" start timer, then send a pulse and measure time when such pulse returns "
it cannot be much simpler,

As of today - I still do not know how nanoVNA generates such pulse and how "sweep" frequency (?) setup
is related to this simple principle of sending a pulse down the line.

PS
When this is all done I may break down and
open the RG6 I have
observe the dielectric - my guess it is poly-whatever
and actually use nanoVNA to measure the VF...

what a goal...

ah yes - my current cable is 9 (nine) times of 1/4 wavelength long @ 14MHz....
Purfect for Field Day...

We all have our favorites, mine is the SARK 100, got it from Amazon for around $50.

It has a bright backlit screen, apply 12v and you’re ready to go, in 30 seconds you have your SWR. Absolutely great in the field and since I use the same battery for my QCX-Mini not much extra to carry.

Yes, the VNA can be used quite well for this purpose but it’s no well suited for field work IMHO.

John
KK4ITX

Hi Fran?ois,

As an example, I routinely use a nanoVNA or antenna analyser to adjust my 136 and 475 kHz antennas at my home and remote QTH's, via a switch or PC-selected port which routes the antenna path to either the Class D/E amplifier or the analyser. To avoid the sort of trouble you correctly point out, activating the switch simultaneously disables the amplifier drive.

For my remote station, VK6MJM, I use a RigExpert AA-30.Zero "naked"analyser under station PC control, largely because I've not found any nanoVNA application reliable enough to connect faultlessly over long periods. The RigExpert Antscope software, although relatively basic, performs better in this regard. (The station is 300 km away).

I like your proverb: there's a near equivalent in English which talks about "casting pearls before swine" .

73, Peter.

On Thu, Jun 8, 2023 at 01:53 PM, Jim VE7RF wrote:


<Before the VNA was available, I would just measure the C between center conductor + braid. Knowing how many pf per foot the cable was, it's a simple math calculation for the physical length. Knowing the VF, the electrical length is also calculated.

This works very good on a known length of coax...dead on actually. On coax that you don't know how many pf per foot, cut exactly 1'-2'-3' of coax off the end of the run / roll...and measure it.

MFJ also has about 8 different antenna analyzers available, some analog and some digital.
Comet has very nice ones, too, for around $500.

NanoVNA is by far the cheapest, but comes with no instructions or documentation.
The NanoVNA-F has a different type of screen than the NanoVNA-H models, and it's screen is more visible when outdoors - but still not too good in the sun. I _really_ need a sunshade for it in Florida.

To get a 'feel' for using a nano, watch some youtube videos - there are many.
That might help for other analyzers too.
--
Doug, K8RFT

When you adjust a fan dipole trim the high frequency one first. Then the lower one, and
lower and so on. Look up G3BDQ's book on Practical Wire Antennas.

That's good, but anyway, even if the resonance has been found for each of the two frequencies (zero reactant), the radiation resistance will not be identical and neither will be 50 ?.

That's why I match the impedance with a double 'L' in the center of the antenna. Thus, at the two frequencies, the impedance will be 50 + j0 ?. As the double 'L' adapter can also compensate, if necessary, the reagent on each frequency it is not necessary to take the cabbage to cut the strands at the small onions (but it does not harm either) [se prendre le chou pour tailler les brins aux petits oignons] . With this system the antennas are not necessarily resonant but it is better to be close to them so as not to degrade performance. The reagent induces more important caurants which are useless but which produce losses; such as cos(phi) in electrical distribution.

We measure the S11 without the adapter with a nanoVNA, we calculate the adapter, we put it in place and, normally, that's 50+j0... if we're not mistaken.

73
--
F1AMM :)
Fran?ois

-----Message d'origine-----

De la part de F4WCV
Envoyé : samedi 10 juin 2023 09:12

When you adjust a fan dipole trim the high frequency one first. Then the lower one, and lower and so on. Look up G3BDQ's book on Practical Wire Antennas.

I mainly just want to use it to find the best position on my coil! Lol

Hello
Some OMs, in fact, temporarily replace their Tx with a nanoVNA to adjust their tuning box. You have to be careful that the TX does not end up in front of the NanoVNA.

In France we call that: giving jam to the pigs ("donner de la confiture aux cochons") :)
--
F1AMM
Fran?ois

De la part de Mike Anderson/KF?AWL
Envoyé : samedi 10 juin 2023 06:50

Hopefully, if we ignore you, you will stop your BS as you have done in other groups and just fade away.
We are willing to help those who are really wanting and needing it. Not those who spread crap like you have and do.
Also quit hiding behind, the "ANNE RANCH" BS. I have personally informed her of you using her name and causing trouble.
I am sure, she is going to be contacting you. You can not hide from her.
Clyde

I mainly just want to use it to find the best position on my coil! Lol
Its small a lot more portable then a mfj and affordable if it breaks in the field.
The stock owner manual goes way overboard with things for what I want lol