NE6F’s common mode current tester – Part 1 ended with the following:

Common mode current adjacent to a small choke

Consider a straight section of coaxial feedline not close to other materials, and with a small common mode choke inserted in the feedline. A “small” choke means one that is a very tiny fraction of a wavelength, say λ/100, from connector to connector.

Q1: Ask yourself that if say 1A of common mode current flows into one connector, what is the common mode current at the other connector?

Q2:What is your answer if you were told the balun was specified to have a CMRR of 20dB?

The answer to Q2 is relatively easy, CMRR is not a meaningful statistic for a common mode choke deployed in a typical antenna system, it would not change the answer to Q1.

The answer to Q1 needs a longer explanation… let’s do it!

Common mode current distribution is almost always a standing wave.

Above is a plot of current distribution of an example dipole antenna system with coax feed. The dipole is slightly off centre fed to drive a significant common mode current on the vertical coax feed  which is grounded at the lower end.

Note the standing wave on the vertical section, it has maxima and minima (if it is long enough), and these are unrelated to possible standing waves inside the coax, ie in differential mode.

Note that if we took spot measurements of the magnitude of common mode current |Icm| at points A, B and C we would get different results.

Broadly, Icm changes relatively slowly over a small electrical distance (say λ/100), except near deep minima if they exist.

Above is the same model with a common mode choke inserted near ground. Note that it has changed the magnitude and distribution of Icm, and more importantly, note that |Icm| at both ends of the choke are very similar, and in fact slightly lower on the antenna side of the choke.

If you make spot measurements either side of an electrically short (say length<λ/100), you will not usually find a large difference… except in the region of a current minimum.

The common notion that |Icm| is much lower on the tx / ground side of the choke is deeply flawed, and even worse is to think that CMRR can be used to calculate the reduction.

NE6F’s video of measurement of his balun’s effectiveness.

N6EF demonstrates measurement with his dual probe meter asserting that “this choke is doing a pretty good job,” but is the measurement a valid basis for that assertion?

Above is a capture from the video with my added notations of four currents, I1-4.

Having set his instrument to read 100% of scale on the Port A probe, he switches to display the Port B probe.

Above. a stunning drop, it is reading perhaps <1% of scale on Port B. Some might infer that proves CMRR>40dB.

Let’s look at that test setup again.

Above is a capture from the video with my added notations of four currents, I1-4.

NE6F demonstrates that I4 is less than 1% of I1 (though the meter response may not be linear at low readings).

Now look at the conductive bulkhead and the bulkhead adapter which connects the coax shield to the bulkhead. At this point a circuit node with three conductors is formed and currents annotated:

• I1: shield from the antenna side;
• I2: bulkhead conductor; and
• I3: shield to the choke.

The measurement made ignores the presence of current path I2, and so all conclusions are invalid.

With the information given earlier, you might well think that it is likely that I3 is approximately the same as I4 (measured at <1% of I1), and that I2 is probably almost the same as I1, ie that this shunts, diverts common mode current on the antenna feed line to ground. That might be an effective measure in preventing ingress to the equipment cluster, but it does not reduce Icm on the main feedline or the undesirable effects of that.

Of course that is just supposition… but it should drive measurement of |I3| to better infer |I2|.

Homework

Make or buy an effective common mode current meter, and make some measurements adjacent to a common mode choke to test your understanding of what it does or does not do.

A correspondent asked my thoughts on a Youtube video featuring…

NE6F’s common mode current tester

Above is the schematic of NE6F’s common mode current tester.

The concept is that current probes A and B are placed either side of a current mode choke, and by calibrating and switching between them, a relative reading of current on one side compared to the other may be found.

Note that the two transformers are intended to be current transformers, they have a ratio of 1:10 turns so the \(Is=\frac{I_p}{n^2}=\frac{I_p}{100}\) … provided there is a low impedance load (called a burden) connected to the secondary.

If the burden was say 100Ω, then the current transformer inserts an impedance of approximately 100/10^2=1Ω in the primary circuit. If a current sensing element does not have a very low impedance, it is likely to disturb the thing being measured.

A general rule about current transformers is that if there is no burden on a current transformer:

• excessive / dangerous voltage may be developed by the secondary winding;
• a high impedance may be inserted in series with the primary line.

So, this is a current transformer with a burden of megohms, a deeply flawed design, but that problem is easily fixed by connecting a resistance of say 100Ω across each secondary winding.

Common mode current adjacent to a small choke

Consider a straight section of coaxial feedline not close to other materials, and with a small common mode choke inserted in the feedline. A “small” choke means one that is a very tiny fraction of a wavelength, say λ/100, from connector to connector.

Ask yourself that if say 1A of common mode current flows into one connector, what is the common mode current at the other connector?

What is your answer if you were told the balun was specified to have a CMRR of 20dB?

Take your time… a follow up will be posted in a day or three.

This article presents a simple way to make measurements of a prototype Guanella 1:1 current balun, measurements that can guide refinement of a design.

The usual purpose of these transmitting Guanella 1:1 current baluns is to reduce common mode feed line current. Not surprisingly, the best measure of a device’s effectiveness is direct measurement of common mode current (it is not all that difficult), but surprisingly, it is rarely measured.

Above is the prototype transformer being a Fair-rite 5943003801 (FT240-43) wound with 11t of solid core twisted pair stripped from a CAT5 solid core LAN cable and wound in Reisert cross over style. Note that Amidon #43 (National Magnetics Groups H material) is significantly different to Fair-rite #43.

There is no need to wind a prototype using expensive materials, the measurements made with the above prototype are very useful in guiding development without wasting expensive materials. Further, the measured results are very close to the behavior of the ‘raw’ balun.

s21, s11 measurement of through transmission

Above is the configuration for the through test, the clothes pegs are used to clamp one conductor to the outside of the SMA jacks, the inner wire is poked into the inner pin, the VNA is sitting on a cardboard box, the whole thing is on a wooden table. An alternative to the clothes pegs are small zip ties.

Above is a screenshot from the NanoVNA of the through measurement. The relevant traces are s11 Logmag, S11 Smith, and S21 Logmag.

Above is an expanded graph of |s21|. At first glance, you might be horrified by the value of -1.8dB @ 30MHz. Many would assert that this shows a loss of 1.8dB… but more correctly it is InsertionLoss of 1.8dB and conversion of RF energy to heat is likely to be less.

See Measurement of various loss quantities with a VNA for discussion of Loss terms.

Above is a graph disaggregating InsertionLoss into (Transmission) Loss and MismatchLoss. Thicker conductors will have less Loss, and would be used in practical baluns.

Above is a graph showing InsertionVSWR wrt 50Ω and ReturnLoss wrt 50Ω.

The significant mismatch is due to the fact that the balun introduces about 800mm of transmission line with Zo around 100Ω.

If you want low InsertionVSWR wrt some reference Zo, then the transmission line (the twisted pair or coax) needs to be that same Zo.

Is the InsertionVSWR and associated impedance transformation a problem? Well if you are using an ATU, it should not matter, and imperfection is dealt with by the ATU.

s11 reflection measurement of Zcm

Above, the DUT was reconfigured for Zcm measurement by twisting the white wire around the green wire at each end to bond them, leaving a short straight section of the green wire for connection. One end is inserted into Port 1 jack inner, the other was clamped to the male threads of Port 2, ie grounded.

Note: my NanoVNA-H4 has been modified by addition of a short direct wire between the ground side of Port 1 and Port 2 jacks inside the case (see above).

(If wound with coax, Zcm measurements are made between shield ends, ignore the inner conductor.)

Above is a screenshot of the Zcm measurement, |Zcm| peaks at about 14.6MHz.

Above is a plot of |Zcm| taken from the .s1p file saved during measurement. |Zcm| > 2000Ω from 3MHz to more than 40 MHz.

Above is a plot of the R and X components of Zcm taken from the .s1p file saved during measurement.

Above is a plot of the ReturnLoss taken from the .s1p file saved during measurement.

Above is a plot of the InsertionVSWR taken from the .s1p file saved during measurement.

Conclusions

Good / valid measurements of a prototype Guanella 1:1 balun can easily be made with a NanoVNA using low cost materials in capable hands.

Keep in mind that you are measuring a sample of one or a small number and that ferrite has a wide tolerance specification, and is temperature sensitive.

Experimenting with measurement setups and prototypes is the beginning of understanding of these things, more so that soaking up the utterings of armchair experts.

Go build and measure some prototypes, try different cores, different turns, different winding layouts, twisted / untwisted / coax etc.

Downloads

Proto-G11-FT240-43-11t.7z

The Airspy YouLoop is a no tune, broadband, small footprint, affordable LF to HF receive antenna that works indoors. Its modular construction facilitates experiments to understand where the Möbius inspired electrical design provides benefit.

The post Airspy YouLoop LF/MF/HF Möbius Receive Antenna appeared first on Ham Radio . Magnum Experimentum.

 Well,

Lots of discussions about my baluns and the test setup, a common question seems to be how to measure the output impedance.

Here's my way:

Fun, dont you think?

 How about this for a bit of bonkersness?

Fun, dont you think?