|
To start off, the common mode choke (CMC) I'm addressing is also referred
to as a transmission line transformer. Physically it consists of a single
bifilar set of windings on an appropriate ferrite toroid. The windings
form a short (for HF) transmission line on the toroid core with unknown
impedance. Here are methods on measuring that impedance using a vector
Network Analyzer. The NanoVNAs work well for the purpose.
All the following are reflection measurements, so no need to complete the
through and isolation calibration options on the VNA. I'm assuming the VNA
is properly calibrated over the required frequency range including any
fixtures or adaptors which might be used. It's best to attempt all of the
following at a relatively low frequency unless you are specifically
interested in the higher frequencies. A wide sweep of frequencies is not
recommended. I usually use something around 2 to 3 MHz. This avoids the
effects of introducing unknown parasitics into the measurement.
All the following is without renormalizing which can introduce unknown
additional errors to the measurements.
From what I've read and utilized, the three methods are:
METHOD 1:
1) Connect one side of the CMC choke to the s11 port or source port.
2) With the "other" side of the CMC open terminated, measure and note the
capacitance.
3) With the "other" side of the CMC short circuit terminated, measure and
note the inductance.
4) Calculate the Zo using the following formula:
Zo = SQRT [L / C]
Be sure to use basic units, Farads and Henries.
1 pF = 1E-12 F 1 ?H = 1E-6 H
METHOD 2:
1) Connect one side of the CMC to the s11 port or source port.
2) Terminate the "other" port with a known (measured) potentiometer of
nominally 100 to 150-ohms over the frequency range of interest. Use
minimal leads from the pot. to the CMC.
3) While observing the Smith Chart on the VNA, adjust the potentiometer
such that both the capacitance (-j) and the inductance (+j) are
simultaneously minimized along the real, horizontal, axis on the Smith
Chart. The real axis is the only place on the Smith Chart that is purely
resistive.
4) Now remove the potentiometer and read its DC resistance on a DMM. That
is Zo.
METHOD 3 (my favorite):
1) Connect one side of the CMC to the s11 port or source port.
2) Terminate the "other" port of the CMC with a known potentiometer
measured to be non-reactive over the frequency range of interest. A value
of 100 to 150-ohms is useful since most of the CMC chokes have a Zo between
70 to 125 ohms. Keep leads to an absolute minimum.
3) While observing the Smith Chart on the VNA, adjust the potentiometer
such that both the capacitance (-j) and the inductance (+j) are minimized
simultaneously along the central (real) axis of the Smith Chart.
4) Observe the numerals at the top of the VNA. You can read all the
"interesting" parameters of your CMC from that data, including the Zo of
the choke.
No calculations or renormalizing is required for this third and last
method, minimizing potential sources of error.
Dave - W?LEV
--
Dave - W?LEV
|
Dave, I have been waiting for this post! Shall try out the first method as soon as possible. For the second and third methods, I have seen only potentiometers of 10K and above here. Shall check again. Can you kindly add a note on how to measure inductance and capacitance with NanoVNA? Usually I measure that with my LCR meter. Never tried measuring inductance and capacitance of the CMC which I homebrewed recently on an FT240-43 toroid using RG316 (). 73 Jon, VU2JO On Wed, May 14, 2025 at 5:01?AM W0LEV via groups.io <davearea51a= [email protected]> wrote: To start off, the common mode choke (CMC) I'm addressing is also referred
to as a transmission line transformer. Physically it consists of a single
bifilar set of windings on an appropriate ferrite toroid. The windings
form a short (for HF) transmission line on the toroid core with unknown
impedance. Here are methods on measuring that impedance using a vector
Network Analyzer. The NanoVNAs work well for the purpose.
All the following are reflection measurements, so no need to complete the
through and isolation calibration options on the VNA. I'm assuming the VNA
is properly calibrated over the required frequency range including any
fixtures or adaptors which might be used. It's best to attempt all of the
following at a relatively low frequency unless you are specifically
interested in the higher frequencies. A wide sweep of frequencies is not
recommended. I usually use something around 2 to 3 MHz. This avoids the
effects of introducing unknown parasitics into the measurement.
All the following is without renormalizing which can introduce unknown
additional errors to the measurements.
From what I've read and utilized, the three methods are:
METHOD 1:
1) Connect one side of the CMC choke to the s11 port or source port.
2) With the "other" side of the CMC open terminated, measure and note the
capacitance.
3) With the "other" side of the CMC short circuit terminated, measure and
note the inductance.
4) Calculate the Zo using the following formula:
Zo = SQRT [L / C]
Be sure to use basic units, Farads and Henries.
1 pF = 1E-12 F 1 ?H = 1E-6 H
METHOD 2:
1) Connect one side of the CMC to the s11 port or source port.
2) Terminate the "other" port with a known (measured) potentiometer of
nominally 100 to 150-ohms over the frequency range of interest. Use
minimal leads from the pot. to the CMC.
3) While observing the Smith Chart on the VNA, adjust the potentiometer
such that both the capacitance (-j) and the inductance (+j) are
simultaneously minimized along the real, horizontal, axis on the Smith
Chart. The real axis is the only place on the Smith Chart that is purely
resistive.
4) Now remove the potentiometer and read its DC resistance on a DMM. That
is Zo.
METHOD 3 (my favorite):
1) Connect one side of the CMC to the s11 port or source port.
2) Terminate the "other" port of the CMC with a known potentiometer
measured to be non-reactive over the frequency range of interest. A value
of 100 to 150-ohms is useful since most of the CMC chokes have a Zo between
70 to 125 ohms. Keep leads to an absolute minimum.
3) While observing the Smith Chart on the VNA, adjust the potentiometer
such that both the capacitance (-j) and the inductance (+j) are minimized
simultaneously along the central (real) axis of the Smith Chart.
4) Observe the numerals at the top of the VNA. You can read all the
"interesting" parameters of your CMC from that data, including the Zo of
the choke.
No calculations or renormalizing is required for this third and last
method, minimizing potential sources of error.
Dave - W?LEV
--
Dave - W?LEV
|
Jon, please wait for tomorrow as it's supper time here in N. Colorado. I'll do another procedure using the Smith Chart and the VNA on measuring the C and L. Dave - W?LEV On Wed, May 14, 2025 at 12:26?AM Jon via groups.io <vu2jo0= [email protected]> wrote: Dave,
I have been waiting for this post! Shall try out the first method as soon
as possible.
For the second and third methods, I have seen only potentiometers of 10K
and above here. Shall check again.
Can you kindly add a note on how to measure inductance and capacitance with
NanoVNA? Usually I measure that with my LCR meter. Never tried measuring
inductance and capacitance of the CMC which I homebrewed recently on an
FT240-43 toroid using RG316 ().
73
Jon, VU2JO
On Wed, May 14, 2025 at 5:01?AM W0LEV via groups.io <davearea51a=
[email protected]> wrote:
To start off, the common mode choke (CMC) I'm addressing is also referred
to as a transmission line transformer. Physically it consists of a single
bifilar set of windings on an appropriate ferrite toroid. The windings
form a short (for HF) transmission line on the toroid core with unknown
impedance. Here are methods on measuring that impedance using a vector
Network Analyzer. The NanoVNAs work well for the purpose.
All the following are reflection measurements, so no need to complete the
through and isolation calibration options on the VNA. I'm assuming the VNA
is properly calibrated over the required frequency range including any
fixtures or adaptors which might be used. It's best to attempt all of the
following at a relatively low frequency unless you are specifically
interested in the higher frequencies. A wide sweep of frequencies is not
recommended. I usually use something around 2 to 3 MHz. This avoids the
effects of introducing unknown parasitics into the measurement.
All the following is without renormalizing which can introduce unknown
additional errors to the measurements.
From what I've read and utilized, the three methods are:
METHOD 1:
1) Connect one side of the CMC choke to the s11 port or source port.
2) With the "other" side of the CMC open terminated, measure and note the
capacitance.
3) With the "other" side of the CMC short circuit terminated, measure and
note the inductance.
4) Calculate the Zo using the following formula:
Zo = SQRT [L / C]
Be sure to use basic units, Farads and Henries.
1 pF = 1E-12 F 1 ?H = 1E-6 H
METHOD 2:
1) Connect one side of the CMC to the s11 port or source port.
2) Terminate the "other" port with a known (measured) potentiometer of
nominally 100 to 150-ohms over the frequency range of interest. Use
minimal leads from the pot. to the CMC.
3) While observing the Smith Chart on the VNA, adjust the potentiometer
such that both the capacitance (-j) and the inductance (+j) are
simultaneously minimized along the real, horizontal, axis on the Smith
Chart. The real axis is the only place on the Smith Chart that is purely
resistive.
4) Now remove the potentiometer and read its DC resistance on a DMM. That
is Zo.
METHOD 3 (my favorite):
1) Connect one side of the CMC to the s11 port or source port.
2) Terminate the "other" port of the CMC with a known potentiometer
measured to be non-reactive over the frequency range of interest. A value
of 100 to 150-ohms is useful since most of the CMC chokes have a Zo between
70 to 125 ohms. Keep leads to an absolute minimum.
3) While observing the Smith Chart on the VNA, adjust the potentiometer
such that both the capacitance (-j) and the inductance (+j) are minimized
simultaneously along the central (real) axis of the Smith Chart.
4) Observe the numerals at the top of the VNA. You can read all the
"interesting" parameters of your CMC from that data, including the Zo of
the choke.
No calculations or renormalizing is required for this third and last
method, minimizing potential sources of error.
Dave - W?LEV
--
Dave - W?LEV
-- *Dave - W?LEV* -- Dave - W?LEV
|
Thanks a lot Dave, for the quick response. Here the sun is just rising up! Shall look out for your post by evening (that will be 'tomorrow' for you!). 73 Jon, VU2JO On Wed, May 14, 2025 at 6:04?AM W0LEV via groups.io <davearea51a= [email protected]> wrote: Jon, please wait for tomorrow as it's supper time here in N. Colorado.
I'll do another procedure using the Smith Chart and the VNA on measuring
the C and L.
Dave - W?LEV
On Wed, May 14, 2025 at 12:26?AM Jon via groups.io <vu2jo0=
[email protected]> wrote:
Dave,
I have been waiting for this post! Shall try out the first method as soon
as possible.
For the second and third methods, I have seen only potentiometers of 10K
and above here. Shall check again.
Can you kindly add a note on how to measure inductance and capacitance with
NanoVNA? Usually I measure that with my LCR meter. Never tried measuring
inductance and capacitance of the CMC which I homebrewed recently on an
FT240-43 toroid using RG316 ().
73
Jon, VU2JO
On Wed, May 14, 2025 at 5:01?AM W0LEV via groups.io <davearea51a=
[email protected]> wrote:
To start off, the common mode choke (CMC) I'm addressing is also referred
to as a transmission line transformer. Physically it consists of a single
bifilar set of windings on an appropriate ferrite toroid. The windings
form a short (for HF) transmission line on the toroid core with unknown
impedance. Here are methods on measuring that impedance using a vector
Network Analyzer. The NanoVNAs work well for the purpose.
All the following are reflection measurements, so no need to complete
the
through and isolation calibration options on the VNA. I'm assuming the VNA
is properly calibrated over the required frequency range including any
fixtures or adaptors which might be used. It's best to attempt all of the
following at a relatively low frequency unless you are specifically
interested in the higher frequencies. A wide sweep of frequencies is
not
recommended. I usually use something around 2 to 3 MHz. This avoids
the
effects of introducing unknown parasitics into the measurement.
All the following is without renormalizing which can introduce unknown
additional errors to the measurements.
From what I've read and utilized, the three methods are:
METHOD 1:
1) Connect one side of the CMC choke to the s11 port or source port.
2) With the "other" side of the CMC open terminated, measure and note the
capacitance.
3) With the "other" side of the CMC short circuit terminated, measure and
note the inductance.
4) Calculate the Zo using the following formula:
Zo = SQRT [L / C]
Be sure to use basic units, Farads and Henries.
1 pF = 1E-12 F 1 ?H = 1E-6 H
METHOD 2:
1) Connect one side of the CMC to the s11 port or source port.
2) Terminate the "other" port with a known (measured) potentiometer of
nominally 100 to 150-ohms over the frequency range of interest. Use
minimal leads from the pot. to the CMC.
3) While observing the Smith Chart on the VNA, adjust the
potentiometer
such that both the capacitance (-j) and the inductance (+j) are
simultaneously minimized along the real, horizontal, axis on the Smith
Chart. The real axis is the only place on the Smith Chart that is
purely
resistive.
4) Now remove the potentiometer and read its DC resistance on a DMM. That
is Zo.
METHOD 3 (my favorite):
1) Connect one side of the CMC to the s11 port or source port.
2) Terminate the "other" port of the CMC with a known potentiometer
measured to be non-reactive over the frequency range of interest. A value
of 100 to 150-ohms is useful since most of the CMC chokes have a Zo between
70 to 125 ohms. Keep leads to an absolute minimum.
3) While observing the Smith Chart on the VNA, adjust the
potentiometer
such that both the capacitance (-j) and the inductance (+j) are
minimized
simultaneously along the central (real) axis of the Smith Chart.
4) Observe the numerals at the top of the VNA. You can read all the
"interesting" parameters of your CMC from that data, including the Zo
of
the choke.
No calculations or renormalizing is required for this third and last
method, minimizing potential sources of error.
Dave - W?LEV
--
Dave - W?LEV
--
*Dave - W?LEV*
--
Dave - W?LEV
|
Hi Dave
This method of direct experimental measurement, without any computational artifice and without the risk of modeling errors or theoretical estimations, is simply my own used method, which I have already presented multiple times and illustrated in several recent topics in this IO group. Indeed, this experimental method is very elegant, accurate, and reliable, and involves no computational tricks.
I have specifically presented it as the method for measuring the characteristic impedance of coaxial or twin-lead lines, offering greater precision than other methods already shown in YouTube videos or even the classical recommended techniques that I have come across in this highly respectable IO group.
It has been suggested to me to use the recently introduced macro function by DiSlord called "Measure" + "Cable" in his latest firmware version 1.2.40. This function directly displays the cable¡¯s characteristic impedance, its length, its attenuation in dB, and its specific attenuation in dB/100m. It's already a very good and reliable function, and above all, it's easy to use¡ªthere¡¯s virtually nothing to do except connect the open-ended coax to port 1 for S11 measurements.
This function is indeed useful, but it provides a single Zc value valid across the entire frequency range. It is based on quasi-static C0 measurements, which are more relevant at relatively low frequencies, and it does not fully account for skin effect and other second-order behaviors that are difficult for a typical user¡ªsuch as a radio amateur seeking ohm-level accuracy¡ªto model precisely.
For example, I have already published the different characteristic impedance values of my RG213 cable depending on the frequency band, using the experimental method that involves centering the impedance circle at the center of the Smith chart with a fixed resistive load close to the nominal characteristic impedance of the coax, or by referring to the first-order value displayed by DiSlord¡¯s macro function.
The method is simple, robust, and original because it is based on the very solid foundational concept of the characteristic impedance of an RF transmission line. This is, by definition, the impedance that produces a purely traveling wave and absorbs all the energy without any reflection¡ªas if the line were infinitely long.
Therefore, in practice, we should observe a constant, purely real impedance across the entire measurement band as a first approximation (theoretically, a line¡¯s characteristic impedance may not be purely real, but for low-loss lines it is often very close to real and commonly approximated by the well-known square root of L/C¡ªthis remains a very good first-order approximation).
Thus, for a ferrite balun, it is experimentally straightforward to connect an adjustable resistor in place of the antenna and connect the other side to port 1 for S11 measurements, in order to center the impedance plot across an entire frequency band on real axe dot.
This is entirely feasible for a small balun or a coiled coaxial cable whose second end is accessible in the shack, allowing the connection of an adjustable resistor (non-inductive, of course) in order to find the value that yields the smallest possible Smith chart impedance plot ¡ª ideally, a single point located on the real axis over the entire desired frequency band. This value can then be directly read using the NanoVNA cursor. It's one of the best methods to measure the characteristic impedance (Zc), while also enjoying the clear and safe visual illustration of its fundamental definition on a simple and tangible Smith chart using a compact NanoVNA.
So far, so good ¡ª but unfortunately, this method cannot always be applied so easily, especially when the cable is already deployed in space, fixed in place, and its far end is no longer accessible from the shack near the NanoVNA.
Fortunately, the new firmware by DiSlord (version 1.2.40) offers a solution by allowing the renormalization of the equivalent impedance for Smith chart display purposes. It¡¯s important to understand that the actual impedances values remain exactly the same ¡ª only the graphical representation changes. So, there¡¯s no risk of error introduced by the renormalization of Zc in S11 measurement mode.
The goal is no longer to focus the impedances on a single real point (since we can¡¯t adjust accurately the resistor remotely ¡ª unless two people coordinate via phone), but rather to connect a fixed resistive load that is approximately correct, and then use the Zc renormalization feature of DiSlord to center the impedance circle on the middle of the Smith chart, indeed the impedances are no longer all reals but remain on a small cicle around the renormalized Zc end value.
The value of Zc that centers the impedance circle is the one to retain in the end. that why i appreciate a lot this renormalize graphical option offered By DiSlord firmware 1.2.40 and ask him many times if it"s possible to add a graphical X4 or X6 zoom option around Smith shart center , it will be very usefull , indeed here all is graphical with Smith Shart , zero external computing .
73's Nizar .
|
Hi Dave
Here My message about RG213 coax measurements posted here on Avril 5 2025:
for My RG213 cable (25m length) loaded by a 50.3 Ohm resistor ,
I used the centered impedances circle methode on smith graph with the renormalized Z0 impedance ( option added by DiSlord) for different ferquency's band (span always fixed to 4 Mhz) :
2Mhz ---> Zc = 52.6 Ohm
3Mhz ---> Zc = 52.5 Ohm
7Mhz ---> Zc = 52.0 Ohm
14Mhz ---> Zc = 53.0 Ohm
18Mhz ---> Zc = 53.0 Ohm
21Mhz ---> Zc = 54.0 Ohm
24Mhz ---> Zc = 54.0 Ohm
29Mhz ---> Zc = 52.0 Ohm
50Mhz ---> Zc = 49.0 Ohm
100Mhz ---> Zc = 43.5 Ohm
Same coaxial Zc varie from 43.5 Ohm to 54.0 Ohm depend on the frequency band .
Direct measurement with Dislord "Cable" function gives Zc = 51.77 Ohm with same cable over all the band wich is correct enought but not too accurate depend on the frequency band .
73's Nizar
|
Assuming that "wires" (transmission line) used to make CMC are much shorter then "lambda" across useful frequencies, how much impact do you expect from "mismatched" characteristic impedance of CMC?
It's cool to know how to measure it, but should we ever be concerned that CMC might have fc different from the rest of the system?
|
12, 10, and 6 meters might be a bit of concern. But usually these are at or less than an electrical 0.1 wavelengths long. The characteristic impedance of all that I've built and measured (well over 20) come in between 70 and 110 ohms. The Vp measures around 0.55. Any length of transmission line or reactive circuit element inserted that has a Zo other than the system impedance will alter the system impedance coming through that component. Generally, any transmission of less than an electrical 0.1 wavelength and a Zo within reason is quite acceptable. No, I'm not concerned. If I were, I have the proper instruments to measure the results. Dave - W?LEV On Wed, May 14, 2025 at 9:16?PM Miro, N9LR via groups.io <m_kisacanin= [email protected]> wrote: Assuming that "wires" (transmission line) used to make CMC are much
shorter then "lambda" across useful frequencies, how much impact do you
expect from "mismatched" characteristic impedance of CMC?
It's cool to know how to measure it, but should we ever be concerned that
CMC might have fc different from the rest of the system?
-- *Dave - W?LEV* -- Dave - W?LEV
|
Hi Miro
I believe you asked the right question here.
In my opinion, not having the correct characteristic impedance (Zc) for the CRC that matches the Coax/Antenna system slightly alters the impedance seen by the coax from the antenna and also the propor resonnance frequency , CRC become a part of resonnance frequency . It also affects the impedance measurements taken at that point, especially if the NanoVNA is calibrated right at the point just below the antenna. In such a case, the measurements will be distorted by the transformation introduced by the CRC's twin-lead line.
For example, consider a CRC with a Zc of 140 Ohms used with a 50 Ohm coaxial system: the impedance measured with or without the CRC will be different, and correcting the antenna accordingly becomes much more difficult. On the other hand, if the CRC has a characteristic impedance equal to that of the Coax/Antenna system, the measured impedance will be the same before or after the CRC. This would greatly simplify both the calculations and the practical adjustments needed to optimize the antenna system.
73's Nizar
|
QUOTE: ........ but should we ever be concerned that CMC might have fc different from the rest of the system? All my CMCs are home brewed and meticulously measured with professional instruments. My usual frequency sweep runs from 1 MHz through 30 MHz. They are broadbanded. Dave - W?LEV On Wed, May 14, 2025 at 9:16?PM Miro, N9LR via groups.io <m_kisacanin= [email protected]> wrote: Assuming that "wires" (transmission line) used to make CMC are much
shorter then "lambda" across useful frequencies, how much impact do you
expect from "mismatched" characteristic impedance of CMC?
It's cool to know how to measure it, but should we ever be concerned that
CMC might have fc different from the rest of the system?
-- *Dave - W?LEV* -- Dave - W?LEV
|
QUOTE: In such a case, the measurements will be distorted by the transformation introduced by the CRC's twin-lead line. These CMCs are not made of just "twin lead line". They are intelligently chosen conductors, ideally insulated with Teflon, and properly wound in bifilar manner on an appropriate toroidal core. Opinions don't count in this game of antennas and transmission lines! Real properly measured data using the correct instruments and techniques are what's important. In other words, "show me the data" !!! Measure Z at the antenna. Any, and I do emphasize "any" reactance at that point when coupled to a good 50-ohm XMSN line will alter the impedance at the shack end of that line!!! The effect of a short length of transmission line embodied in the CMC will do typically less than any reactive component at the antenna. So, again, show me the data before offering "opinions". Please..... I'm only attempting to keep this thread on a technical basis and not based on opinion. Science and engineering rely on hard theory and data to back up the theories. Antenna and transmission lines rely on hard science and data, not opinion. Dave - W ?LEV On Wed, May 14, 2025 at 9:36?PM Team-SIM SIM-Mode via groups.io <sim31_team= [email protected]> wrote: Hi Miro
I believe you asked the right question here.
In my opinion, not having the correct characteristic impedance (Zc) for
the CRC that matches the Coax/Antenna system slightly alters the impedance
seen by the coax from the antenna and also the propor resonnance frequency
, CRC become a part of resonnance frequency . It also affects the impedance
measurements taken at that point, especially if the NanoVNA is calibrated
right at the point just below the antenna. In such a case, the measurements
will be distorted by the transformation introduced by the CRC's twin-lead
line.
For example, consider a CRC with a Zc of 140 Ohms used with a 50 Ohm
coaxial system: the impedance measured with or without the CRC will be
different, and correcting the antenna accordingly becomes much more
difficult. On the other hand, if the CRC has a characteristic impedance
equal to that of the Coax/Antenna system, the measured impedance will be
the same before or after the CRC. This would greatly simplify both the
calculations and the practical adjustments needed to optimize the antenna
system.
73's Nizar
-- *Dave - W?LEV* -- Dave - W?LEV
|
Hi Dave
I believe that everything I have presented here has already been demonstrated both experimentally and graphically, without relying on formulas or theoretical estimations. Nevertheless, approximation methods are always possible, depending on the required level of accuracy."
73s Nizar
|
Only one of the methods I presented involved a formula. What the dickens is wrong with a little very simple algebra?? The other two methods are simple and only need a non-reactive potentiometer, a VNA, or a DMM. Why have you taken offense to my offering? Dave - W?LEV On Wed, May 14, 2025 at 10:58?PM Team-SIM SIM-Mode via groups.io <sim31_team@...> wrote: Hi Dave
I believe that everything I have presented here has already been
demonstrated both experimentally and graphically, without relying on
formulas or theoretical estimations. Nevertheless, approximation methods
are always possible, depending on the required level of accuracy."
73s Nizar
-- *Dave - W?LEV* -- Dave - W?LEV
|
On Wed, May 14, 2025 at 06:50 AM, Team-SIM SIM-Mode wrote:
Here My message about RG213 coax measurements posted here on Avril 5 2025:
for My RG213 cable (25m length) loaded by a 50.3 Ohm resistor ,
I used the centered impedances circle methode on smith graph with the
renormalized Z0 impedance ( option added by DiSlord) for different ferquency's
band (span always fixed to 4 Mhz) :
2Mhz ---> Zc = 52.6 Ohm
3Mhz ---> Zc = 52.5 Ohm
7Mhz ---> Zc = 52.0 Ohm
14Mhz ---> Zc = 53.0 Ohm
18Mhz ---> Zc = 53.0 Ohm
21Mhz ---> Zc = 54.0 Ohm
24Mhz ---> Zc = 54.0 Ohm
29Mhz ---> Zc = 52.0 Ohm
50Mhz ---> Zc = 49.0 Ohm
100Mhz ---> Zc = 43.5 Ohm
Your measurements do not agree with the calculated values for typical RG213. The characteristic impedance of Belden coax is around 50.6 ohms at 1 MHz and slowly decreases to about 50 ohms at 100 MHz. You can see that in the attached graph which is based on the parameters for this type of cable. It decreases because the inductance is decreasing with frequency. The problem with your test method is that you will not have a pure resistance at the end of the cable as the frequency is increased. Any resistor (including SMD) will have some series inductance and there will also be capacitance across the resistance. The reactance associated with these components will be small at low frequencies but will be considerable at your highest measured frequency of 100 MHz. This is particularly true if you try to use any kind of small adjustable potentiometer for your test. The end result is that your measurements will not be correct because you don't have a pure resistance load to base your measurements on.
|
Hi Roger
Thanks, its not assumed that test terminaison resistor should be a pure real resistive value nor an accurate Zc muched impedance but just to be approximatly reduced to a relatively focused and small circle of impedances , what we measure in final is the circle center graphically value with help of graphical renormalisation option added by DiSlord without using Marker or cursur numerical displayed values over the desired freq band , circle impedances has obviosly reactive impedances values but turning graphically around the good center value to take on the end.
Hi Dave , Thanks , i have no offensive to mathematical approches, just i prefer that mathematical threads can be taken by firmware as possible ( as Dislord try to do) to facilitate nanovna use simply graphically especially on using magical circles of Smith plots .
73s Nizar
|
Roger Need
