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Hi Victor,

Thanks for your feedback, you ask a good question concerning its validity.

I’m not aware of any specific articles on the technique I mentioned, I derived that configuration from a simple circuit analysis of the differential mode wiring of a CMC = a series connected circuit with voltage drop measured across a load resistor.

I’d be interested to hear what concerns you have with my test arrangement.

Kind regards

Ed, G8FAX

Hello Ed,

I have not much now to look into this, but this article:

Gives your method (Fig 3b and Fig 7a.
So one can measure some aspects of the choke.

I need to check how close this is to the DM mode.

All the best,

Victor

Op ma 11 jul. 2022 om 08:20 schreef Ed G8FAX <ed@...>:

On Sun, Jul 10, 2022 at 08:09 PM, Jerry Stuckle wrote:

Actually, that's a common, but incorrect statement. Current - RF or otherwise
- always flows along ALL available paths, split according to the inverse of
the resistance.
//
Jerry, AI0K

Hi Jerry,

You're absolutely right. It's not all or nothing. A semantically better way to say it is: RF current always favors the path of least impedance. The degree of "favor" is divided between paths according to impedance. That's consistent with your point.

In the case of Miro's original question about problems with RF current flowing in the internal ground connection between the ports, the answer is still no. The impedance in PATH1 of his drawing (attached) is too high for there to be a problem even though the DC resistance is probably very low. The RF return current is confined almost entirely in the transmission line.

The attached drawing shows a double-sided PCB. The bottom has a solid copper ground plane. The top has a single trace between the ports. An RF input signal is applied between the trace and ground plane. The load is connected between the trace and ground plane.

At 10 MHz the RF return current will be confined to a narrow path in the ground plane directly below the top trace. Openings in the PCB provide isolated current paths A and B for easy measurement. RF current in these paths will be negligible.

At RF frequencies, the impedance through paths A and B is high primarily due to the geometry. The further the distance from the trace, the higher the series inductance and the lower the parallel capacitance. These both mean higher reactance. The area in the ground plane directly below the trace has the lowest impedance. That's where almost all the return current flows even though it's surrounded by an area with very low DC resistance.

Like so many things with RF, this behavior is counter-intuitive to our understanding of what happens with DC and low-frequency AC.

Dave NU8A

A clasic way to explain this was to connect 2 ports with coax,
connect the coax shields on 2 ports with a short piece of wire and
try to measure the current in the wire but I like this graphic, it is quite
intuitive.

Hi Victor,

Thank you for your update and information.

There are different ways to measure CMC performance as there are different 'lumped' component models of a CMC taking account of parasitic elements of a practical or real component.

There is merit in making a test configuration and method for how the component will be used in a particular application. If I were designing and making a CMC filter, I would use a different method of testing to my simple proposal, one very similar to your preferred configuration.

I do not have any rf 1:1 center-tapped baluns so am not able to compare results. If and when you have time to make comparisons, I would be very interested in your findings.

Kind regards

Ed, G8FAX

Hello Ed,

I postpone my experiments until I can build these transformers needed to do
a proper(?) DM measurement (as described in the earlier article, Fig. 16b).
A month ago I ordered some small toroids (also to make a three way RF
meter), still awaiting them.
I think your T1 measurement is only part of getting a DM model/measurement.

All the best,

Victor

P.S. The article is about CMC build of non-coax/twined cable, so the
model/results I somewhat different then for a 'transmission line' (RF) CMC:


Op di 12 jul. 2022 om 08:58 schreef Ed G8FAX <ed@...>:

In the sketch


from
bobine

It uses two transformers head to tail, which seems logical.
--
F1AMM Fran?ois

Hi Victor,

Thank you, I await your results on DM measurement after your parts arrive, if you are kind enough to post them. I hope you have better experience than I on delivery, I ordered some items from China just after Christmas, they arrived last week!

What else do you see as needed with my DM method to obtain a complete model for DM measurement?

Our discussion started out as, is there a difference, significant or otherwise between your DM measurement technique and my simpler approach to practical measurement. It has widened now.

Re models and your footnote link -

This is part of a very long and detailed webpage of an unknown authors investigation and experimentations “to determine which Hybrid Choke/Transformer to use for a 40m OCFD”. As of today, no conclusion is yet written.

The section you bring to my attention today appears to relate to the ‘OC’ measurement and model rather than DM/CM representations. I do not see any comparisons between OC and DM/CM i.e. how well does OC compare to DM/CM, do you have any information on this?

Kind regards

Ed

Hello Ed,

I mostly do CM0 measurements on Chokes, but I checked the OC one to see
what it did. I think G3TXQ used OC, but not 100% sure (he does not fully
describe his setup in: .
OC and CM measurements are close (except for power-supply chokes the OC
shows an additional resonance freq. which does not really emerge when I do
the OC on RF chokes)
<by the way, the webpage of 'unknown author' is me>

The OC is T4 (Fig 3e) in this article:
It also gives the
equivalent circuit for it: ig. 9a

Just to be sure, my main point is looking at Miro's question: how to do DM
measurements. I have not really done Dm measurements (as I was interested
in CM values of my Chokes), I think most people do DM measurements without
the two additional transformers (to go from unbalanced VNA post to balanced
ports to connect the DUT, Fig 16b). Which is related to Miro's question. So
I want to see what the difference is between these two DM measurements with
and without the Transformers.
I ordered these cores for these transformers a month ago, I hope to receive
them soon. I also don't have any winding advice. If someone has this, I am
interested.

All the best,

Vicctor

Hi Francois,

Thank you for your contribution.

The image you refer to gives a method for testing aspects of a 4;1 voltage balun, see link in the other ref you provided

Here we are discussing measuring certain parameters of a CMC, so not directly applicable.

If the CMC was placed between the two baluns' then the arrangement would allow some tests to be made on its performance. The effects of the two baluns would need to be compensated for.

Kind regards

Ed, G8FAX

Hi Victor,

Oh wow, what a fantastic webpage you have, full of useful information. Your attention to detail, investigations and experiments is exemplary and I congratulate you for all your efforts.

I must confess, I have only read parts of it. What springs to my mind, is the choice of ferrite material and power handling, this will alter any model if capabilities are exceeded. I’m sure you are aware of that and if not addressed this already, will do so.

Also, have you considered an open wire feed rather than coaxial? Again forgive me if you have already looked at this alternative.

When we are discussing models, the OC model comprises of a CM part and DM part. The CM parts has parallel resistances and reactances. I’m sure you are aware that these can be also be represented by equivalent series circuits. Thus by translating the shunt inductors with the resistors, the circuit could be re-drawn with a form similar to that of a transmission line (TL). By the way I would call that configuration a “balanced Pi” arrangement. Anyone following this discussion, not familiar with converting parallel to series circuits, I refer in the first instance to

I am relatively new to the nanoVNA (I have H4.2) and still exploring the very many menu options of firmware 1.2.00. It appears that the instrument now has capability of presenting results in either series or shunt form, I have yet to investigate this, and many more things. This might help when producing values for a model.

In respect of the G3TXQ link, I agree, it is not clear. What I do get from reading it is that his chokes are coils made from coax cable wound on a toroid and wonder as it is not a transformer arrangement if it is applicable/relevant to our current discussion? I also think there is a terminology issue with calling this ‘common mode’, a point you raise on your lovely webpage, to me I see it more as a ground loop between inner and out of coax line. Thoughts? Considering his test arrangement and firmly bonding grounds of generator and measuring ports, I wonder/think he connects croc clips to each end of the braid, as that is where the ‘choking’ is required, else what what be measured on the inner route and centre?

I am not familiar with the ‘open’ measurement technique, to be honest, never heard of it until you raised it in a previous post, so can not give any helpful comments on how useful it is for determining CM & DM performance of a CMC. I will study the ‘HAL’ link information and get back to you.

Like you, I have also been wondering about suitable baluns (transformers), if there are any reasonably priced commercial items available or how to make one, so very interested in what core you use, number of turns and wire gauge. I think some experimentation is required and happy to give it a go, but my schedule means in August or later. I was also wondering if a suitable CMC wired or used in voltage mode would be a suitable item as a transformer here, thoughts?


Kind regards

Ed, G8FAX

Hello Ed,

Op vr 15 jul. 2022 om 11:23 schreef Ed G8FAX <ed@...>:

Hi Victor,

Oh wow, what a fantastic webpage you have, full of useful information.
Your attention to detail, investigations and experiments is exemplary and I
congratulate you for all your efforts.

Thanks, but when I get a better grip on the OCFD (aka being ableto model
and include in that moe the Choke) I plan to revisit teh webpage(as it is
now somewhat cluttered with all kidn of experiments I did. I think all are
valid but it coudl be better structured with the present knowledge I have;-)

I must confess, I have only read parts of it. What springs to my mind, is
the choice of ferrite material and power handling, this will alter any
model if capabilities are exceeded. I’m sure you are aware of that and if
not addressed this already, will do so.

I did not look at power issues (I think I will using my equipmentin the QRP
range), but I agree it could be extended with that. Who knows, when I have
the need.

Also, have you considered an open wire feed rather than coaxial? Again
forgive me if you have already looked at this alternative.

I did not do that (yet?). Also here I am planning to use coax and thus I
restricted intially to that.

When we are discussing models, the OC model comprises of a CM part and DM
part. The CM parts has parallel resistances and reactances. I’m sure you
are aware that these can be also be represented by equivalent series
circuits. Thus by translating the shunt inductors with the resistors, the
circuit could be re-drawn with a form similar to that of a transmission
line (TL). By the way I would call that configuration a “balanced Pi”
arrangement. Anyone following this discussion, not familiar with converting
parallel to series circuits, I refer in the first instance to

The 'equivalent' circuit maps the best my measurmenets is the model of
Guanella (to ideal transformers; a transmission line to represent the DM
and a ractance+resistance to represent the CM); see also Owen Duffy's page:


I am relatively new to the nanoVNA (I have H4.2) and still exploring the

very many menu options of firmware 1.2.00. It appears that the instrument
now has capability of presenting results in either series or shunt form, I
have yet to investigate this, and many more things. This might help when
producing values for a model.

I use NanoVNA Saver for that (or the s1p/s2p files and my excel
spreadsheet).

In respect of the G3TXQ link, I agree, it is not clear. What I do get from

reading it is that his chokes are coils made from coax cable wound on a
toroid and wonder as it is not a transformer arrangement if it is
applicable/relevant to our current discussion?

I think this is part of the dsicussion of terminology;-) 1:1 Guanella
(which I use and also G2TXQ) is lso a 1:1 transformer, but I call it a
Choke as I use it mainly as a choking device.

I also think there is a terminology issue with calling this ‘common mode’,
a point you raise on your lovely webpage, to me I see it more as a ground
loop between inner and out of coax line. Thoughts? Considering his test
arrangement and firmly bonding grounds of generator and measuring ports, I
wonder/think he connects croc clips to each end of the braid, as that is
where the ‘choking’ is required, else what what be measured on the inner
route and centre?

Teh 1:1 Guanella Choke (whihc is also used by G2TXQ on that page (indeed
coax on a toroid), can be used in the CM0 (crocs on both end of braid plus
shield) or the CO (crocs on both end of braid or crocs on both end of
shield) onfiguration (he is certainly not used CM2 or CM3 configuration)

I am not familiar with the ‘open’ measurement technique, to be honest,
never heard of it until you raised it in a previous post, so can not give
any helpful comments on how useful it is for determining CM & DM
performance of a CMC. I will study the ‘HAL’ link information and get back
to you.

The HAL article is nice and also this one:

I think the OC, DM and CM are nicely explained and modelled.
But IMHO this article and HAL's are abouvt power-supply chokes (a lot of
windings), whcih are build a little different than RF chokes (more aligned
with transmission line).

All the best,

Victor

P.S. just received the toroids, need to test the complex mu so I can design
these 'ideal' Transfomers for the DM measurement.

Hi Victor,

Interesting reading about cmc models and ways of establishing by measurements and maths of obtaining values for the various elements of the models form the links you provided, thanks.

Dominguez-Palacios et al (DP) paper uses S21 Voltage measurements, whilst the Hal document works on S11 impedance measurements. Fig 2(a) of DP paper is equivalent to HAL Fig 3 (a) – T0 connection, or as you describe in your notes CM0 as you find no appreciable difference to CM3.

For DM, the DP paper makes use of two baluns. The equivalent in the HAL document would be fig 3 (b) = T1 connection, an arrangement close to my original suggestion for measuring CMC DM, the differences being what started our excellent discussions and something yet to be ascertained is if there is any appreciable difference of techniques. For sure if one considers a CMC filter (extra capacitance on the far side), yes I fully agree that the techniques are not comparable.

Thus, I see that the ‘open’ measurement of the DP paper (fig 2 (c) is part of a series of tests of a particular method for the purposes of modelling, not an alternative to more conventional means of establishing performance of a cmc. Just as are the additional configurations of the HAL document Fig 3 (c), (d) & (e) with T4 being equivalent to DP paper ‘open’.

Interesting to note that in the HAL document, the conclusion is that results not only depend on components (tolerances?) but also on respective positioning (lay-out).This reminds me of a quotation I heard many years ago (can’t remember where/who it came from) =”Theoretically, practice and theory are the same, in practice, they are not!”

I have also been refreshing and improving my knowledge on transmission line transformers (TLT’s) and found for me a very useful reference = Ruthroff transmission line transformers, Guanella baluns, lowpass and bandpass filters, and balanced transmission lines using multilayer technology, Riaz R Sobrany, M.Eng.



Although aimed at microwaves and MICC type technology, chapter 3 covers basic theoretical operation of various Ruthroff TLT’s and chapter 4 Guanella types. Of interest to me in particular is his notes on testing baluns(on page 136 of pdf).

“Connecting two identical baluns with unknown performance back to back is a very bad test procedure to use.
All that can be determined is the degree of mismatch at the unbalanced port and the insertion loss of the balun. This test procedure is incapable of revealing values for the mode conversion parameters which means that the CMRR cannot be calculated, and even worse, provides no conclusive evidence that the balun actually functions as a balun at all!
A procedure to test a balun using a conventional 2-port VNA which determines the CMMR but does not measure the insertion loss or the port reflection coefficients is detailed in [113]. This test procedure requires three precision resistors and the accuracy of the measurement is dependent upon the accuracy of these resistors.”

With ref 113 being “O. M. Woodward, “Balance Quality Measurements on Baluns,” IEEE Transactions on Microwave Theoiy and Techniques, vol. 31, no. 10, pp. 821-824, Oct. 1983.” I have not located this, so no further comment on the technique at present.

This to me raises questions concerning the validity of using baluns in the measuring chain; what exactly is being measured and is making back-back compensation measurements valid, especially if one is using home made baluns. The same might well apply to commercial ones.

I did a little research in to commercial baluns, “mini-Circuits” produce a very wide range of rf transformers (500+), there may be something suitable there, I have yet to explore possibilities


Summarising so far on the way to measure a cmc depends rather on the objective, is it to find parameters for a model or to evaluate performance, different approaches are required. There is no universally applicable model for cmc’s it depends if low frequency or rf application. Your own experiments and studies support this when you say “The 'equivalent' circuit maps the best my remeasurements is the model of Guanella (to ideal transformers; a transmission line to represent the DM and a reactance+resistance to represent the CM)”.


Kind regards

Ed, G8FAX

On 7/17/22 11:26 PM, Ed G8FAX wrote:

Hi Victor,

“Connecting two identical baluns with unknown performance back to back is a very bad test procedure to use.
All that can be determined is the degree of mismatch at the unbalanced port and the insertion loss of the balun. This test procedure is incapable of revealing values for the mode conversion parameters which means that the CMRR cannot be calculated, and even worse, provides no conclusive evidence that the balun actually functions as a balun at all!

E.g. a "back to back" test of a pair of coax to banana jack adapter (which is not a balun) would show good S11 and S21.

Hello Ed,

Op ma 18 jul. 2022 om 08:26 schreef Ed G8FAX <ed@...>:

For DM, the DP paper makes use of two baluns.

Like the HAL document, Fig 16b.

Thus, I see that the ‘open’ measurement of the DP paper (fig 2 (c) is part

of a series of tests of a particular method for the purposes of modelling,
not an alternative to more conventional means of establishing performance
of a cmc.

But if you have the values you can also calculate the CM rejection (CMR).
But at the end I think that these two articles are more power-supply Choke
and not Transmission Line Choke. Which has a slightly different set of
parameters). The power supply choke have a fCO1 and fCO2, while I think
that the Transmission line Chokes have only fCO1:

Just as are the additional configurations of the HAL document Fig 3 (c),
(d) & (e) with T4 being equivalent to DP paper ‘open’.

DP Open is an OC S21 measurement. While HAL Fig 3 (c) T2, (d) T3 & (e) T4
are reflection measurements (to determine 'individual' choke parameters).

Interesting to note that in the HAL document, the conclusion is that
results not only depend on components (tolerances?) but also on respective
positioning (lay-out).This reminds me of a quotation I heard many years ago
(can’t remember where/who it came from) =”Theoretically, practice and
theory are the same, in practice, they are not!”

Certainly for a Choke it is important what is around it. That is why I test
on a styrofoam platform. So it is influenced by other objects (including
hands, etc.:


I have also been refreshing and improving my knowledge on transmission line

transformers (TLT’s) and found for me a very useful reference = Ruthroff
transmission line transformers, Guanella baluns, lowpass and bandpass
filters, and balanced transmission lines using multilayer technology, Riaz
R Sobrany, M.Eng.

Looks an interesting link as it handles 4:1 Ruthroff and Guanella (I think
he only discusses 4:1 and 16:1 Guanella) Thanks. Will study it

“Connecting two identical baluns with unknown performance back to back is a

very bad test procedure to use.
All that can be determined is the degree of mismatch at the unbalanced
port and the insertion loss of the balun. This test procedure is incapable
of revealing values for the mode conversion parameters which means that the
CMRR cannot be calculated, and even worse, provides no conclusive evidence
that the balun actually functions as a balun at all!
A procedure to test a balun using a conventional 2-port VNA which
determines the CMMR but does not measure the insertion loss or the port
reflection coefficients is detailed in [113]. This test procedure requires
three precision resistors and the accuracy of the measurement is dependent
upon the accuracy of these resistors.”

Indeed one needs to know how good these Baluns are. But one needs something
to go from unbalanced the VNA port to balanced (the DUT).
I was wondering: if one includes the baluns in the calibration, would that
not calibrate that setup?

With ref 113 being “O. M. Woodward, “Balance Quality Measurements on
Baluns,” IEEE Transactions on Microwave Theoiy and Techniques, vol. 31, no.
10, pp. 821-824, Oct. 1983.” I have not located this, so no further comment
on the technique at present.

Indeed that is the reference I would like to read also. Does someone have a
copy?

I did a little research in to commercial baluns, “mini-Circuits” produce a

very wide range of rf transformers (500+), there may be something suitable
there, I have yet to explore possibilities

I think the 1:1:1 T-622-KK81 can do the function.

All the best,

Victor

On 7/18/22 12:23 PM, Victor Reijs wrote:

With ref 113 being “O. M. Woodward, “Balance Quality Measurements on
Baluns,” IEEE Transactions on Microwave Theoiy and Techniques, vol. 31, no.
10, pp. 821-824, Oct. 1983.” I have not located this, so no further comment
on the technique at present.

Indeed that is the reference I would like to read also. Does someone have a
copy?

google for "woodward balun balance quality 1983"

The paper works through the math of using a VNA to do the measurement as a 3 port device.

I did a little research in to commercial baluns, “mini-Circuits” produce a

very wide range of rf transformers (500+), there may be something suitable
there, I have yet to explore possibilities

Yes, and they publish N-port VNA measurements. 4 port for most of the baluns.

I think the 1:1:1 T-622-KK81 can do the function.

I use the T1-1T-65X - 6 pin DIP package 200 kHz to 80 MHz - it's 1:1 with a center tap. If you need 2:1 the T2-1T-65X has a secondary 2x the primary, with a center tap.


The 622-kk81 is a non catalog part. I'd find something that's in stock. If you need 3 isolated windings, I'm sure they've got something.

Jim Lux wrote: “google for "woodward balun balance quality 1983" “

Thanks, I no longer have academic access, so will need to buy or get site of the paper some other way. However, there might be alternatives/better solutions to pursue.

I found “Analysis and Performance of Antenna Baluns” by Lotter Kock

Victor Reijs might be interested in this. This models a balun as a delta-connected network of impedances, provides a good explanation of how the common mode current occurs (another question I think Victor has) as well as giving several means of testing, including back-back and the “Woodward” method, tabulating their usefulness.


Jim Lux wrote: “ I use the T1-1T-65X - 6 pin DIP package 200 kHz to 80 MHz - it's 1:1 with a center tap.? If you need 2:1 the T2-1T-65X has a secondary 2x the primary, with a center tap.
The 622-kk81 is a non catalog part. I'd find something that's in stock. If you need 3 isolated windings, I'm sure they've got something.”
It appears ordering direct from Mini-Circuits requires a minimum order value of $50. With shipping costs and import duties, the cost of 622-kk81 would be high, so thanks Jim for alternative part you use.

Neither part is available from Digikey or Mouser, so a look though what they have in stock to see if there is yet another alternative might be more fruitful and cost effective.
Kind regards

Ed G8FAX

On Mon, Jul 18, 2022 at 05:40 PM, Jim Lux wrote:

I use the T1-1T-65X - 6 pin DIP package 200 kHz to 80 MHz - it's 1:1 with a
center tap. If you need 2:1 the T2-1T-65X has a secondary 2x the primary, with
a center tap.

You have a typo in the component name - it's "X65", not "65X", but great find!

On 7/19/22 5:09 AM, Miro, N9LR via groups.io wrote:

On Mon, Jul 18, 2022 at 05:40 PM, Jim Lux wrote:

I use the T1-1T-65X - 6 pin DIP package 200 kHz to 80 MHz - it's 1:1 with a
center tap. If you need 2:1 the T2-1T-65X has a secondary 2x the primary, with
a center tap.

You have a typo in the component name - it's "X65", not "65X", but great find!

Oops, yes..

I used those because I had an application where I wanted to be able to change the turns ratio, so I have a 6 pin DIP socket, and I can plug in the T1, the T4 (2:1), or the T16 (4:1).

And it's easier to solder to DIP pins if you're dead bugging it, than to some tiny SMT pad.

As someone mentioned minicircuits does have a minimum order, so one might need to do some scrounging around for a source or alternate parts if you only want one or two.