Re: Measuring coax characteristic impedance
Use Smith chart display.? Sweep the frequency to find the place where the curve passes the first quarter of the circle, which is one eighth wave.? Move the marker to that spot and read the reactance,
At least that's how I remember doing it.
Another method is to terminate the line in a variable resistor and adjust for SWR = 1.00.? Then measure the resistor or read the impedance.
There are other ways also.
Bob
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On Friday, January 1, 2021, 12:02:26 PM PST, Noji Ratzlaff <nojiratz@...> wrote:
How can I use a NanoVNA to measure the characteristic impedance of an unmarked length of coaxial cable? I have a NanoVNA-H4.
I saw another thread here titled "characteristic impedance," but the conversation quickly got off-topic, and remained off-topic, so I bring it up again.
Thanks,
Noji
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Measuring coax characteristic impedance
How can I use a NanoVNA to measure the characteristic impedance of an unmarked length of coaxial cable? I have a NanoVNA-H4.
I saw another thread here titled "characteristic impedance," but the conversation quickly got off-topic, and remained off-topic, so I bring it up again.
Thanks,
Noji
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On Fri, Jan 1, 2021 at 08:51 AM, David Eckhardt wrote:
Once again, he demonstrates that transformers do not make baluns as he
measures only -3 to -4 dB of CMR of the typical current balun.
And......the function of the balun is not necessarily to transform
impedances. The sole function of a balun or CMC choke is to map a common
mode topology into a differential mode topology - nothing more.....nothing
less. That's all in its name: Bal = balanced (differential mode), un =
unbalanced (common mode). Get it?
Dave - W?LEV
Yes. Thanks Fred KB4QZH
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Re: Measure CMC - bit confused
Well, I've sent this out twice yesterday, but I'll attach the tabulated
data again. I probably need to take pictures and write this up to make it
more useful to those who don't have all the test equipment.
A month or so ago, I ordered 2.3" OD type 31 cores to do this
'experiment'. In the past, 43 material was the preferred core material,
but 31 material came along with considerably greater ?r that 43 material at
the lower HF bands. My measured data indicated that 31 material is,
indeed, better on 160-meters and marginally better on 75-meters than
equivalent bifilar windings on 43 material. However, 43 material offered
considerably better performance on 40-meters on upward. The measured data
on the 43 material cores formed a smooth convex curve from 1 through 50 MHz
with no resonances indicated over that range. Remember, these are
configured as CMCs with simple bifilar (no twists) windings on the cores,
no contorted or crossed windings in some form or another on the cores.
Yea, I need to write this up so it's a bit more useful to others.
Dave - W?LEV
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On Fri, Jan 1, 2021 at 4:35 PM AG6CX <edwmccann@...> wrote: Happy New Year Dave!
Do you have a link to one your attachments or presentations that can guide
those of us (who can¡¯t find the HP 8753C in the shack under all the other
stuff!) using our nanovnas and conversion formulae as required?
Thanks!
Ed McCann
AG6CX
--
*Dave - W?LEV*
*Just Let Darwin Work*
|
Once again, he demonstrates that transformers do not make baluns as he
measures only -3 to -4 dB of CMR of the typical current balun.
And......the function of the balun is not necessarily to transform
impedances. The sole function of a balun or CMC choke is to map a common
mode topology into a differential mode topology - nothing more.....nothing
less. That's all in its name: Bal = balanced (differential mode), un =
unbalanced (common mode). Get it?
Dave - W?LEV
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On Fri, Jan 1, 2021 at 5:31 AM Fred <fmoeves@...> wrote: Kevin,
I make Tx chokes out of Rg-400 as per K9YC cookbook.
And for Rx antennas 75 ohm RG-179.
I would post photos but no way to upload.
Also have a look at these links he knows his stuff.
Because of the language ...He pronounces Balun as Balloon and coax is corx.
73
Fred KB4QZH
--
*Dave - W?LEV*
*Just Let Darwin Work*
|
Re: Measure CMC - bit confused
Happy New Year Dave!
Do you have a link to one your attachments or presentations that can guide those of us (who can¡¯t find the HP 8753C in the shack under all the other stuff!) using our nanovnas and conversion formulae as required?
Thanks!
Ed McCann
AG6CX
|
Re: Measure CMC - bit confused
As a wide hand wave, yes. Looking at my measured data for those CMC I
measured both attenuation and resistance, there is a correlation between
loss and resistance. However, it is not linear. Bottom line is that
greater attenuation does indicate greater resistance, but not a simple
linear mapping. I believe there was a simple formula given in a previous
post within this thread which relates resistance based on s21.
Dave - W?LEV
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On Fri, Jan 1, 2021 at 3:02 PM Don - KM4UDX <dontay155@...> wrote: I too wanted to measure the resistance of my CMCs with my nano. And came
to exactly the same point:
LOGMAG via CH0 and CH1 giving me plots of -db vs freq.
So to get resistance we need some extra math, which is why there are
spreadsheet to do the drill. I follow that. I think.
So if the funky math is just a linear transform of log mag into
resistance, then can I take the -db readings a a perfectly good enough
proxy for resistance? Is it enough to say that (for example) -30db
rejection of CM current is really good, and -35db even better. Is there a
minimum value of negative db values like -25db? Or -xyz, that we simple
types could use as a "good enough" value for CMC suppression on our
antenna feed lines?
If the transform is linear, then using logmag as a proxy for resistance
seems reasonable? Oh please....
--
*Dave - W?LEV*
*Just Let Darwin Work*
|
Re: Measure CMC - bit confused
Download ZPLOTS from Dan, AC6LA. You run an S21, save the touchstone files. Then open the zplots spreadsheet, import the touchstone file and it will tell you all you want to know.
--Shane
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Re: Measure CMC - bit confused
I too wanted to measure the resistance of my CMCs with my nano. And came to exactly the same point:
LOGMAG via CH0 and CH1 giving me plots of -db vs freq.
So to get resistance we need some extra math, which is why there are spreadsheet to do the drill. I follow that. I think.
So if the funky math is just a linear transform of log mag into resistance, then can I take the -db readings a a perfectly good enough proxy for resistance? Is it enough to say that (for example) -30db rejection of CM current is really good, and -35db even better. Is there a minimum value of negative db values like -25db? Or -xyz, that we simple types could use as a "good enough" value for CMC suppression on our antenna feed lines?
If the transform is linear, then using logmag as a proxy for resistance seems reasonable? Oh please....
|
Kevin,
I make Tx chokes out of Rg-400 as per K9YC cookbook.
And for Rx antennas 75 ohm RG-179.
I would post photos but no way to upload.
Also have a look at these links he knows his stuff.
Because of the language ...He pronounces Balun as Balloon and coax is corx.
73
Fred KB4QZH
|
I did some Velocity Factor (VF) measurements today on 9.4M of Belden RG-58A/U. The Belden specs give the VF as .66 or 66% of the speed of light in free space for this cable type.
The first step was to measure the round trip delay using the Transform Function in the NanoVNA-H4. It was 97.6 nanoseconds or 48.8 nanoseconds one-way. In free space it takes 31.34801 nanoseconds to go 9.4M. So the VF is 31.34801/48.8 = .6424 or 64.24%
I connected the NanoVNA-H4 to NanoVNA Saver and the VF was measured at 64.37%.
The next step was to use a signal generator and a 2 channel scope to measure the differential arrival delay in nanoseconds due to the cable over a range of frequencies - 1 to 20 MHz. The results are attached as a graph and show VF increasing with frequency.
The second attachment is a plot for the same cable type using the excellent TLDetails program by AC6LA
You can clearly see that my results have the same overall shape as the the TLDetails plot but are slightly lower. This can be due to manufacturing differences or that my cable which did not have the same Belden cable code was slightly different.
The measurements done by the NanoVNA or traditional fast rise time-short pulse methods both use very high frequency stimulus so they yield VF numbers that are higher than those at low frequencies of only a few Megahertz.
Roger
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Dave,
Thanks for the chart. Were all of the measurements taken with 1 turn except the BRN which you indicated 11 turns?
Mike N2MS
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On 12/31/2020 6:46 PM David Eckhardt <davearea51a@...> wrote:
The 'big boys' who design the materials use one turn through the core to
characterize the ferrite material. They measure in a 50-Ohm system.
Remember, one turn equates to passing a conductor through the core, NOT one
turn *around* the core!
I have recently characterized most of my larger cores using this method.
It works nicely to compare the measured impedance curves (linear magnitude)
against the published data curves for each core material. The measured data
matched the published data well enough to determine the core material. I
built a fixture to accept the one-turn (based on the single pass through
the cores) to accomodate serial testing of my cores. The measured data was
made in the transmission mode so a complete cal. is required. I even did
isolation, just because I could. The measured data matched the published
data curves up to 200 MHzrelatively well. I didn't calibrate or measure
above that frequency as I was interested only in HF through 50 MHz. Of
course, the fixture was calibrated into the measurement setup.
This is also a good method for detecting resonances (to be avoided!) in
home brew CMCs and current 'baluns'. Getting to the data I put out earlier
today, many designs showed resonances which nixed the design of that
specific CMC. That which won consisted of 11 bifilar turns (no twists) of
AWG #10 stranded and insulated copper conductor wound on two stacked 3" OD
43 material cores. It measured a nice smooth convex curve from 1 MHz
through 50 MHz with no resonances in that range. That is the choke noted
as (BRN) in my data compilation as that was the color of the wire (I'll
attach the data again just for reference). However, the 31 material still
won out on 160-meters showing no resonances in the frequency range of
concern but was not as good at mid-HF frequencies.
Dave - W?LEV
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Line length on a "mixed" transmission line system
I'm trying to get the length of the ladder line portion of a feedline that includes both co-ax and ladder-line. The radio connects directly to 6 feet of coax, then a balun, then 50-some feet of ladder line to a horizontal dipole antenna. I've used the TDR measurement function of the NanoVNA, and love it, but in this case I'm not sure if there is a way to measure from the radiio end of the coax. I'd like to avoid climbing up to the balan:ladder-line connection if this is possible to do down at the radio.
I know the Velocity Factor of the coax (.80) and I think the ladder line is about 0.90 VF. With the known length of coax and its VF, can I "factor out" the time delay in that section to give me just the ladder line length?
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The 'big boys' who design the materials use one turn through the core to characterize the ferrite material. They measure in a 50-Ohm system. Remember, one turn equates to passing a conductor through the core, NOT one turn *around* the core! I have recently characterized most of my larger cores using this method. It works nicely to compare the measured impedance curves (linear magnitude) against the published data curves for each core material. The measured data matched the published data well enough to determine the core material. I built a fixture to accept the one-turn (based on the single pass through the cores) to accomodate serial testing of my cores. The measured data was made in the transmission mode so a complete cal. is required. I even did isolation, just because I could. The measured data matched the published data curves up to 200 MHzrelatively well. I didn't calibrate or measure above that frequency as I was interested only in HF through 50 MHz. Of course, the fixture was calibrated into the measurement setup. This is also a good method for detecting resonances (to be avoided!) in home brew CMCs and current 'baluns'. Getting to the data I put out earlier today, many designs showed resonances which nixed the design of that specific CMC. That which won consisted of 11 bifilar turns (no twists) of AWG #10 stranded and insulated copper conductor wound on two stacked 3" OD 43 material cores. It measured a nice smooth convex curve from 1 MHz through 50 MHz with no resonances in that range. That is the choke noted as (BRN) in my data compilation as that was the color of the wire (I'll attach the data again just for reference). However, the 31 material still won out on 160-meters showing no resonances in the frequency range of concern but was not as good at mid-HF frequencies. Dave - W?LEV On Thu, Dec 31, 2020 at 3:26 PM Manfred Mornhinweg <manfred@...> wrote: But if the choke balun is highly inductive and not very resistive, it will
form a resonant circuit with the antenna's total capacitance to ground, and
other reactances in the antenna/feedline/ground circuit. If this resonance
happens to fall near your operating frequency, the balun will make matters
worse then they are without a balun!
For that reason many authors suggest to use lossy core materials,
including authors already mentioned in this thread. Ferrite has a complex
permeability. At low frequency it's almost purely inductive, while at
higher frequencies it becomes increasingly resistive. For that reason, at a
sufficiently high frequency for the material chosen, a choke balun will
oppose little inductance, but a lot of resistance to any common-mode
current. This makes it work very well as a balun, without any risk for the
mentioned resonances in that frequency range. But its resistance must be
high enough to keep the loss negligible, or at least acceptable.
This comes down to selecting a core material that has a mostly resistive
permeability all over the intended operating frequency range, and winding
enough turns on it to keep the flux density at a level at which the loss in
the core is low. At the same time there shouldn't be so many turns that
inter-turn capacitance becomes a problem.
I have found the NanoVNA to be a great tool for measuring the
characteristics of ferrite cores! You can take an unknown core, wind one or
two turns of wire on it, or several if it's a small core, hook it up to the
NanoVNA, and see the resulting series RLC graph over the desired frequency
range. This gives you an instant idea of the core material's inductive and
resistive permeability curves! By comparing to various
manufacturer-provided data, you can make a good guess which material your
core is made from.
-- *Dave - W?LEV* *Just Let Darwin Work*
|
Re: Measure CMC - bit confused
Nice work Dave.
Mel, K6KBE
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On Thursday, December 31, 2020, 10:55:20 AM PST, David Eckhardt <davearea51a@...> wrote: What you have constructed is not quite a common mode choke (CMC).? It is a current choke or current "balun".? Technically, it isn't even a true current "balun".? It works by dening current flow on the outside of the coaxial braid.? A true balun would accomplish this function in addition to assuring the currents on the two conductors - the inside of the coax braid and the outside of the inner conductor - are of equal amplitude and opposite phase.? But the second requirement is pretty much assured using coax cable as what occurs on the inside of the coax is not (greatly - depending on the integrity of the braid) influenced by what goes on outside the cable. I, as well, have constructed and measured several (maybe as many as 15 or so) true CMCs on 31, 43, and 75 material.? These consist of, for the most part, 10 to 18 turns of bifilar wound heavy stranded and insulated copper conductor (#14 and the last on 43 material, #10).? The last one on 43 material uses two stacked 3" OD cores.? The others are wound on two stacked 2.3" OD cores.? The single 31 material is wound on 5 stacked cores of 2.3" OD.? Since I can run the full legal limit to my parallel conductor feeders (no coax), I don't want to sense any or absolutely minimal heating in either the cores or the conductors.? I'll attach the results of my measurements.? I cheated and used the HP 8753C for the measurements as it has most of the required conversions built in. Note the 31 material shines on 160 and good on 75.? However, the last one, (BRN) which is wound of #10 stranded (11-turns, bifilar) and insulated wire on two stacked 3" OD cores of 43 material is pretty much the winner for general use on 75 through 10-meters. With a house filled with new appliances the chokes not only transform CM to DM for my parallel conductor feeders, but keeps the SMPS noises from the appliances (with love, fromChina) out of the feedline and antenna. Dave -W?LEV On Thu, Dec 31, 2020 at 3:19 PM Torbj?rn Toreson <torbjorn.toreson@...> wrote: Hello,
I have made (several) CMC-filters by wrapping RG58 on a FT240 mix 31
toroid, the result is very good. I have a special instrument to check the
current on the coax outside shield and that goes to zero when I connect the
choke on the transmitter side.
When I started to read K9YC "A hams guide to RFI, Ferrites, Baluns .. etc"
I wanted to measure the impedance, both resistance and reactance and not
only the attenuation as "LOGMAG". I have used to measure LOGMAG via CH0 and
CH1 and I get very probable results such as -35 dB at 5 MHz and -30 dB at
20 MHz. The connections is coax-shield on one side to CH0 and coax-shield
on the other side to CH1 (to center conductors).
I thought that I would see the impedance of the filter by using another
trace (CH1) and asign it the format Resistance to start with. The result
however is around 50 ohms when I expected some kohms. Measuring a 560 ohm
resistor also gives around 50 ohm. SOLT calibration for 2-30 MHz is
performed. What is wrong with my thinking?
If I measure the choke (of course always only the shield) only by CH0 I
get a credible result of e.g. 1,3 kohm at 5 MHz, but the resistance is
getting lower by increasing frequency, so this is probably not the right
way to measure the filters resistance.
Why can I not measure the impedance correct when the CMC-filter is
connected between CH0 and CH1?
I have tried with both a NanoVNA-H4 and a SAA2-N with similar results.
73/Torbjorn/SM6AYM
-- *Dave - W?LEV* *Just Let Darwin Work*
|
Re: Measure CMC - bit confused
What you have constructed is not quite a common mode choke (CMC). It is a current choke or current "balun". Technically, it isn't even a true current "balun". It works by dening current flow on the outside of the coaxial braid. A true balun would accomplish this function in addition to assuring the currents on the two conductors - the inside of the coax braid and the outside of the inner conductor - are of equal amplitude and opposite phase. But the second requirement is pretty much assured using coax cable as what occurs on the inside of the coax is not (greatly - depending on the integrity of the braid) influenced by what goes on outside the cable. I, as well, have constructed and measured several (maybe as many as 15 or so) true CMCs on 31, 43, and 75 material. These consist of, for the most part, 10 to 18 turns of bifilar wound heavy stranded and insulated copper conductor (#14 and the last on 43 material, #10). The last one on 43 material uses two stacked 3" OD cores. The others are wound on two stacked 2.3" OD cores. The single 31 material is wound on 5 stacked cores of 2.3" OD. Since I can run the full legal limit to my parallel conductor feeders (no coax), I don't want to sense any or absolutely minimal heating in either the cores or the conductors. I'll attach the results of my measurements. I cheated and used the HP 8753C for the measurements as it has most of the required conversions built in. Note the 31 material shines on 160 and good on 75. However, the last one, (BRN) which is wound of #10 stranded (11-turns, bifilar) and insulated wire on two stacked 3" OD cores of 43 material is pretty much the winner for general use on 75 through 10-meters. With a house filled with new appliances the chokes not only transform CM to DM for my parallel conductor feeders, but keeps the SMPS noises from the appliances (with love, fromChina) out of the feedline and antenna. Dave -W?LEV On Thu, Dec 31, 2020 at 3:19 PM Torbj?rn Toreson <torbjorn.toreson@...> wrote: Hello,
I have made (several) CMC-filters by wrapping RG58 on a FT240 mix 31
toroid, the result is very good. I have a special instrument to check the
current on the coax outside shield and that goes to zero when I connect the
choke on the transmitter side.
When I started to read K9YC "A hams guide to RFI, Ferrites, Baluns .. etc"
I wanted to measure the impedance, both resistance and reactance and not
only the attenuation as "LOGMAG". I have used to measure LOGMAG via CH0 and
CH1 and I get very probable results such as -35 dB at 5 MHz and -30 dB at
20 MHz. The connections is coax-shield on one side to CH0 and coax-shield
on the other side to CH1 (to center conductors).
I thought that I would see the impedance of the filter by using another
trace (CH1) and asign it the format Resistance to start with. The result
however is around 50 ohms when I expected some kohms. Measuring a 560 ohm
resistor also gives around 50 ohm. SOLT calibration for 2-30 MHz is
performed. What is wrong with my thinking?
If I measure the choke (of course always only the shield) only by CH0 I
get a credible result of e.g. 1,3 kohm at 5 MHz, but the resistance is
getting lower by increasing frequency, so this is probably not the right
way to measure the filters resistance.
Why can I not measure the impedance correct when the CMC-filter is
connected between CH0 and CH1?
I have tried with both a NanoVNA-H4 and a SAA2-N with similar results.
73/Torbjorn/SM6AYM
-- *Dave - W?LEV* *Just Let Darwin Work*
|
Re: Measure CMC - bit confused
Obtain the transmission Zt from a s21 measure requires a calculation.
Zt = Zo * 2(1-s21)/(s21)
Zo at 50 ohm.
Then find the real and imaginary parts.
|
But if the choke balun is highly inductive and not very resistive, it will form a resonant circuit with the antenna's total capacitance to ground, and other reactances in the antenna/feedline/ground circuit. If this resonance happens to fall near your operating frequency, the balun will make matters worse then they are without a balun!
For that reason many authors suggest to use lossy core materials, including authors already mentioned in this thread. Ferrite has a complex permeability. At low frequency it's almost purely inductive, while at higher frequencies it becomes increasingly resistive. For that reason, at a sufficiently high frequency for the material chosen, a choke balun will oppose little inductance, but a lot of resistance to any common-mode current. This makes it work very well as a balun, without any risk for the mentioned resonances in that frequency range. But its resistance must be high enough to keep the loss negligible, or at least acceptable.
This comes down to selecting a core material that has a mostly resistive permeability all over the intended operating frequency range, and winding enough turns on it to keep the flux density at a level at which the loss in the core is low. At the same time there shouldn't be so many turns that inter-turn capacitance becomes a problem.
I have found the NanoVNA to be a great tool for measuring the characteristics of ferrite cores! You can take an unknown core, wind one or two turns of wire on it, or several if it's a small core, hook it up to the NanoVNA, and see the resulting series RLC graph over the desired frequency range. This gives you an instant idea of the core material's inductive and resistive permeability curves! By comparing to various manufacturer-provided data, you can make a good guess which material your core is made from.
|
Measure CMC - bit confused
Hello,
I have made (several) CMC-filters by wrapping RG58 on a FT240 mix 31 toroid, the result is very good. I have a special instrument to check the current on the coax outside shield and that goes to zero when I connect the choke on the transmitter side.
When I started to read K9YC "A hams guide to RFI, Ferrites, Baluns .. etc" I wanted to measure the impedance, both resistance and reactance and not only the attenuation as "LOGMAG". I have used to measure LOGMAG via CH0 and CH1 and I get very probable results such as -35 dB at 5 MHz and -30 dB at 20 MHz. The connections is coax-shield on one side to CH0 and coax-shield on the other side to CH1 (to center conductors).
I thought that I would see the impedance of the filter by using another trace (CH1) and asign it the format Resistance to start with. The result however is around 50 ohms when I expected some kohms. Measuring a 560 ohm resistor also gives around 50 ohm. SOLT calibration for 2-30 MHz is performed. What is wrong with my thinking?
If I measure the choke (of course always only the shield) only by CH0 I get a credible result of e.g. 1,3 kohm at 5 MHz, but the resistance is getting lower by increasing frequency, so this is probably not the right way to measure the filters resistance.
Why can I not measure the impedance correct when the CMC-filter is connected between CH0 and CH1?
I have tried with both a NanoVNA-H4 and a SAA2-N with similar results.
73/Torbjorn/SM6AYM
|
On Wed, Dec 30, 2020 at 12:54 PM, Dani YO5LD wrote:
Hey Roger, thanks for your answer.
Running a TDR on the cable between 5 and 100 MHz (505 points) shows the
correct length of cable (14.89m) at 0.859 VF. (which is a 4th VF value..)
Dani
Velocity factor does change with frequency. Here is a plot of one type of Belden 75 ohm coax Roger
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Noji Ratzlaff
Don - KM4UDX
Roger Need
