Re: Measuring swr on fan dipole or windom antenna
Hello
The nanoVNA sees all antennas as a localized dipole. The word DUT is used during measurements.
The NanoVNA measures, for you, the impedance shown by this dipole as a *function of the frequency*. In the particular case of a fan-dipole, it is the resultant of all the effects and reactions of the wires between themselves and with the ground which is displayed, for you. The nanoVNA does not know that it is a fan-dipole.
73
--
F1AMM
Fran?ois
De la part de Steve Johnson
Envoy¨¦ : vendredi 9 juin 2023 06:29
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Measuring swr on fan dipole or windom antenna
When using a NanoVNA to measure SWR on a multi-element antenna, with different length elements but all fed from the same feedpoint, which element¡¯s swr is the Nanvna displaying data for?
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Re: Estimating coaxial cable length - using TDR
Kindly allow me to restate my objective
Find out if the unknown length of coaxial cable is in multiples of 1/4 wavelength of selected frequency.
My tool of choice is nanoVNASaver and its TDR option.
I can estimate the cable VF or use preassigned cable selections which includes VF.
At this point I am making some progress using nanoVNASaver and do not want to dilute my efforts by using other tools.
Since my task is to determine the suitability of the existing unknown length of the cable to act as "in line transformer" in 14 MHz band
I like to know why other authors use "sweep " starting at 50 KHz ?
If I set nanoVNASaver to sweep from 50KHZ to 14,5MHz my cable length comes out over 22 meters,
with sweep range 13 to 14,5 MHz I get 13 meters ,
I cannot physically verify either - my cable *(dual RG58 )runs thru walls and attic...
But I can run known length "in the open" for verification purposes.
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Re: Estimating coaxial cable length - using TDR
At 50 kHz for paired telephone cable, the the velocity of propagation is somewhat lower than at higher frequencies. For 22 gauge plastic insulated pairs, AIEE Paper 59-778, "Transmission Characteristics of Polyethylene Insulated Telephone Cables at Voice and Carrier Frequencies," lists velocities that correspond to a VF of about 57 percent at 40 kHz and about 64 percent at 1 MHz.
Coax is, of course, not identical to paired cable, but using a low end of 50 kHz makes me wonder whether the velocity might be lower than the published high frequency value sufficiently to skew the results a bit. Maybe using a higher frequency low end would be better. I don't know enough about the characteristics of coax at low frequencies to be sure, just raising a caution here.
73,
Maynard
W6PAP
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On 6/8/23 14:09, Roger Need via groups.io wrote: On Thu, Jun 8, 2023 at 11:41 AM, Stan Dye wrote:
A far easier and more accurate way to measure cable length is by using the
"Measure -> Cable" function in the nanovna firmware (if you have a firmware
with this function).
Just connect to an open-ended cable, and read the length and cable loss on
the left side of the screen. You still have to set the correct VF, of
course. But this method does not use TDR and transforms, with the
associated complications. Instead, it finds the quarter-wavelength
frequency of the cable by finding the first S11 phase reversal, and does
the simple calculation.
Stan,
I have to disagree with you on this one. The method uses the 1/4 wavelength measurement method but it is prone to more error that the TDR method. Here are some actual measurements to illustrate my point...
I used the same test cable as in my previous post. This was a 9.38 meter long RG58 A/u cable with a measured 64% VF using a pulse/scope TDR technique. Using NanoVNA Saver the calculated value was 9.34 meters. Using the NanoVNA transform yielded 9.33 meters. These are both 5 cm (about 2 inches).
Using the Cable Measure feature the length depended on the calibrated frequency range. Here are the results with various frequency ranges and an open or short BNC cal terminations on the end. VF set to 64%. Screenshots attached.
50 kHz to 900 MHz. 9.240 meters (open) 9.331 meters (short)
50 kHz to 100 MHz. 9.586 meters (open) 9.608 meters (short)
50 kHz to 20 MHz. 9.588 meters (open) 9.610 meters (short)
One can clearly see that the results are much worse than the TDR method in Saver and on the NanoVNA
Looking at the screenshots you can see the following. The 1/4 wavelength frequency is about 5 MHz.. VF varies with frequency (see attached plot) and does not level off to the "nominal VF" published by the manufacturer until you are much higher in frequency. So this 1/4 wavelength method is subject to the user inputting a VF which is not correct for the 1/4 wavelength measuring frequency. The problem gets worse for longer cables because the 1/4 wavelength frequency is even lower! The second issue is that the user has to calibrate for a frequency range that does not result in too much interpolation which is what happened for the 50 kHz. to 900 MHz. measurement posted above.
In summary the Cable measure function in the nanoVNA requires fewer setup steps but is prone to greater error because the VF that is entered is different than the nominal published VF or one measured using a pulse/scope or TDR instrument. Users need to be wary of using a feature without know how it works or its limitations.
Roger
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Thank you everyone for your input. I will give the nanoVNA a shot. I also came across this little book which explained much that concerned me. ¨C "A guide to the NanoVNA" by Christoph Schw?rzler, Maximilian Schw?rzler.
I think this link gives a sample.
Edward
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Re: Estimating coaxial cable length - using TDR
On Thu, Jun 8, 2023 at 07:34 AM, Bryan Curl wrote:
Still its just an estimate.
...which totally depends upon the correct velocity factor; any error in that will have a proportionate effect on your (physical) length measurements. 73, Don N2VGU
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Re: Estimating coaxial cable length - using TDR
On Thu, Jun 8, 2023 at 11:41 AM, Stan Dye wrote:
A far easier and more accurate way to measure cable length is by using the
"Measure -> Cable" function in the nanovna firmware (if you have a firmware
with this function).
Just connect to an open-ended cable, and read the length and cable loss on
the left side of the screen. You still have to set the correct VF, of
course. But this method does not use TDR and transforms, with the
associated complications. Instead, it finds the quarter-wavelength
frequency of the cable by finding the first S11 phase reversal, and does
the simple calculation.
Stan, I have to disagree with you on this one. The method uses the 1/4 wavelength measurement method but it is prone to more error that the TDR method. Here are some actual measurements to illustrate my point... I used the same test cable as in my previous post. This was a 9.38 meter long RG58 A/u cable with a measured 64% VF using a pulse/scope TDR technique. Using NanoVNA Saver the calculated value was 9.34 meters. Using the NanoVNA transform yielded 9.33 meters. These are both 5 cm (about 2 inches). Using the Cable Measure feature the length depended on the calibrated frequency range. Here are the results with various frequency ranges and an open or short BNC cal terminations on the end. VF set to 64%. Screenshots attached. 50 kHz to 900 MHz. 9.240 meters (open) 9.331 meters (short) 50 kHz to 100 MHz. 9.586 meters (open) 9.608 meters (short) 50 kHz to 20 MHz. 9.588 meters (open) 9.610 meters (short) One can clearly see that the results are much worse than the TDR method in Saver and on the NanoVNA Looking at the screenshots you can see the following. The 1/4 wavelength frequency is about 5 MHz.. VF varies with frequency (see attached plot) and does not level off to the "nominal VF" published by the manufacturer until you are much higher in frequency. So this 1/4 wavelength method is subject to the user inputting a VF which is not correct for the 1/4 wavelength measuring frequency. The problem gets worse for longer cables because the 1/4 wavelength frequency is even lower! The second issue is that the user has to calibrate for a frequency range that does not result in too much interpolation which is what happened for the 50 kHz. to 900 MHz. measurement posted above. In summary the Cable measure function in the nanoVNA requires fewer setup steps but is prone to greater error because the VF that is entered is different than the nominal published VF or one measured using a pulse/scope or TDR instrument. Users need to be wary of using a feature without know how it works or its limitations. Roger
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Re: Estimating coaxial cable length - using TDR
A far easier and more accurate way to measure cable length is by using the "Measure -> Cable" function in the nanovna firmware (if you have a firmware with this function). Just connect to an open-ended cable, and read the length and cable loss on the left side of the screen. You still have to set the correct VF, of course. But this method does not use TDR and transforms, with the associated complications. Instead, it finds the quarter-wavelength frequency of the cable by finding the first S11 phase reversal, and does the simple calculation. It uses a good interpolation function to calculate the exact phase-crossing point for very good accuracy. You only need to set the frequency range such that the smith chart trace does at least a half-circle (i.e. the range needs to contain the frequency corresponding to a quarter-wavelength resonance). I usually just use 50kHz - 30MHz, although you will need to go much higher for very short cable lengths. If the smith chart trace does many circles, you may want to decrease the upper frequency to improve accuracy. If your firmware does not have this feature, you can use the same technique manually, by simply displaying the S11 phase trace. When connected to an open-end cable, it will give a "saw-blade" trace. Just move the cursor to the first phase crossing, read the frequency, and calculate the wavelength using the VF for correction. Stan KC7XE On Thu, Jun 8, 2023 at 11:22?AM Roger Need via groups.io <sailtamarack= [email protected]> wrote: Follow up to my previous post...
You can also use the Transform option in the NanoVNA menu to do TDR
measurements without connecting to a PC. From the Display menu you select
a trace and set it to S11 mode and set it to Linear display. From the
Transform menu select Transform On and Low pass impulse. You set the VF
(in %) which is for the cable being tested. You need to look this up in
the manufacturers spec or measure it using a scope and pulse generator.
Be aware that the actual VF can be different by a percent or two from the
nominal published spec due to manufacturing tolerances. It also varies
between manufacturers for the same cable type. Below are 3 screenshots
taken on a NanoVNA-H4 using a VF of 63, 64 and 65 %. The measured VF using
the pulse generator/scope method was 64%. Physical cable length was 9.38
meters and using 64% VF NanoVNA calculated 9.327 meters
Roger
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Re: Estimating coaxial cable length - using TDR
Follow up to my previous post...
You can also use the Transform option in the NanoVNA menu to do TDR measurements without connecting to a PC. From the Display menu you select a trace and set it to S11 mode and set it to Linear display. From the Transform menu select Transform On and Low pass impulse. You set the VF (in %) which is for the cable being tested. You need to look this up in the manufacturers spec or measure it using a scope and pulse generator. Be aware that the actual VF can be different by a percent or two from the nominal published spec due to manufacturing tolerances. It also varies between manufacturers for the same cable type. Below are 3 screenshots taken on a NanoVNA-H4 using a VF of 63, 64 and 65 %. The measured VF using the pulse generator/scope method was 64%. Physical cable length was 9.38 meters and using 64% VF NanoVNA calculated 9.327 meters
Roger
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Re: Estimating coaxial cable length - using TDR
On Thu, Jun 8, 2023 at 04:34 AM, Bryan Curl wrote: I always had this question as well
Since you must enter a VF it must be calculating physical length.
Things that effect accuracy are stop freq. I got better accuracy when the
indication filled the graph. I adjust the stop to get best fit then
recalibrate. Also setting it up to do 401 pts for the sweep improved accuracy.
Still its just an estimate.
The NanoVNA does not calculate the physical cable length using traditional Time Domain Reflectometry (with pulses on the cable). It calculates using frequency domain S11 data and then performs an Inverse Fast Fourier Transfom (IFFT) and uses the velocity factor VF (input by the user) to calculate the physical length. It can be quite accurate if you know the VF and select an appropriate frequency range. Below are some tests I completed using NanoVNA Saver on a 9.38 meter (30.75 foot) piece of Beleden 8259 RG-58A/U cable. In the first test I selected a cable type that was similar from the drop down menu and Saver calculated the physical length as 9.628 meters (31.6 feet) using a VF of .66. I measured the actual VF using an analog TDR system and the VF for this cable was .64. I set this value in Saver using the custom option from the drop down menu and Saver calculated 9.34 meters which is very close the the actual cable length of 9.38 meters. The frequency range used in my tests was 50 kHz. to 900 MHz. and I used the Manage button in Saver to select 401 data points for the measurement. This works well for cables up to 30 meters in length. For longer cables you need to reduce the stimulus frequency range. For much shorter cables the stimulus range should be higher if you want a better estimate. Roger
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Re: Estimating coaxial cable length - using TDR
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Photo Screenshot from 2023-06-08 10-58-50.png uploaded
#photo-notice
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Re: Estimating coaxial cable length - using TDR
As expected - verifying by using known length of KNOWN coax it is easily said then done ....
I found "Belden YR 16664 " "factory cable" and cannot find any data on it...
I found piece of worn-out what appears RG8 with barely readable description reading "microwave cable" .....useless
The u-tube is all about selecting menus on nanoVNA - I have given up on the menus long time ago ( my choice ).
I can get "TDR" on nanoVNASaver main display , but I cannot figure out how to "update" it....
...and coaxial cable is manual option AFTER "Time domain retroreflectormetry ... " is selected .... my error , but that does give option for VF ....
In theory - TDR can measure cable length - when the distant end is open - hence using reflection
OR it can measure "fault" AKA when there is known good coaxial cable and the distant end is shorted.
After all this is appears the measurement posted ( in nanoVNASaver) is physical length.
But still not verified, just a best guess for now.
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Re: Estimating coaxial cable length - using TDR
I always had this question as well
Since you must enter a VF it must be calculating physical length.
Incorrect
as I pointed out - the graph shows the coax cable type and its VF , it also gives the cable dielectric (how?) which in an essence effects the VF.
( again reading the source code should answer some of the concerns )
Things that effect accuracy are stop freq. I got better accuracy when the indication filled the graph. I adjust the stop to get best fit then recalibrate. Also setting it up to do 401 pts for the sweep improved accuracy.
Still its just an estimate. which was my goal in first place.
Yes, I will czech the u-tube....
BUT to answer my "length" question - if I use KNOW length of cable and then compare the nanoVNA TDR calculated "length" ... problem solved
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Re: Estimating coaxial cable length - using TDR
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.
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Re: Estimating coaxial cable length - using TDR
---
Since the TDR graph did identify the coax type it is not clear
if the "length" includes the VF -
hence is it
electrical or mechanical length ?
---
I always had this question as well
Since you must enter a VF it must be calculating physical length.
Things that effect accuracy are stop freq. I got better accuracy when the indication filled the graph. I adjust the stop to get best fit then recalibrate. Also setting it up to do 401 pts for the sweep improved accuracy. Still its just an estimate.
...for what its worth...
Bryan, n0luf.
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On Wed, Jun 7, 2023 at 10:17 AM, Jim Lux wrote:
I use 6-12" jumpers with SMA plug on one end and SO-239 (or PL-259) on the
other end. For HF, the 6" makes very little difference, and for VHF/UHF, you
can figure out how to calibrate it out.
Yes, that's a great idea! I happen to use one a bit longer - but I do use it often. Long and short USB C-to-C cables are also very helpful. -- Doug, K8RFT
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After you buy a sma to pl259 cable sttach i and the vna to plexiglass with velcro. That way you will not break. See attacheddei De K8HTB
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Re: Estimating coaxial cable length - using TDR
Take a look at this nanoVNA TDR tutorial by W2AEW:
VF must be entered numerically in order to obtain correct physical length.
The X axis scale is defined by the stimulus start/stop freqs.
W2AEW presents a formula to determine stop freq based on expected cable length and VF.
Did my fist TDR measurement yesterday and got reasonable results (length, attenuation).
73, Markus HB9BRJ
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It is great, just be aware of the connector used (UFL). These are tiny and aren't intended for many insertions.
I bought this same board (different vendor) and use it to train hams on the nanoVNA. After 10 or so 'careful' insertion the connector becomes very loose.
Thanks!
Allen Hill
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On June 7, 2023, at 3:35 PM, Roger Jollis <rjollis@...> wrote: I came across this NanoVNA/Analyzer test board on Amazon when I was shopping for some connectors. Anybody tried this? Seems to be a great way to validate operation. Garosa Frequency Test Board Demo Kit VNA Test Module Vector Network Analyzer Breadboard Test Protoboard Board Filter/Attenuator Module On Mon, Jun 5, 2023 at 4:42 PM RJOLLIS via groups.io <rjollis= [email protected]> wrote: Thanks for your reply, Jim. You said it and I was typing it. I'll do as
you described but I don't have a junk box to scavange. The wife made me get
rid of that stuff years ago on one or another relocation. Anyway, I just
wanted to know if somebody would look at my pictures and say, "NO WAY.
That's the latest piece of junk that the clone factories have been
shipping." But I haven't heard that so I guess I'll invest some time and
money into validating it before using it to cut wires!
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Donald S Brant Jr
Roger Need
