Common Mode Current Chokes Comparison - NanoVNA Measurements

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A short common mode current chokes comparison, trying to decide which one is better. I picked the coaxial cable version and the insulated copper wire version for this test as they are more common and easy to make. I only had the FT 140-43 toroid available. I would recommend the 31 material instead ( FT 140-31 or even better FT 240-31 ).

73, YO6DXE.

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[multilecful]

Good video Chip. I use the coax version straight through to the radio, no joins. About 200mm from the radio. No problems with it.

[Niels.OZNO]

finally someone who both explains what he does and shows how he does it. I myself have a Nano VNA and now I know how to make a setup that shows how to measure a CMCC.

I have a question, why do you make a connection between shields on the 2 connectors.
with an external wire?
and not the shield which is in coax

[acestudioscouk-Ace-GACE]

Thanks for this useful video, it was interesting to see how similar they are. I wonder if less turns made a difference.

[gordslater]

I suspect that the change in SWR is becuse your VSWR is being adversely affected by common-mode currents. That is to say: it is reading wrong, until you fit the choke and get a more correct reading.

I have seen this many times in my old shack where all of my antennas were very imbalanced due to antenna layout and size problems. All of the antennas could be matched to a very good (1:1) VSWR, yet I always got bad RF burns because the rig and ATU case were hot with RF. I could not install a balanced antenna due to the shape of the garden.

Once I fitted a good coax choke before the antenna switch (which was right at the input of the ATU) the best matched VSWR always read higher - 1.5:1 or 2:1 was the best match on some bands but there was no problem with unwanted RF voltages in th shack. As a bonus, rx QRM was lower too, and much lower TVI (this was 1990's so analogue TVs were difficult in my area)


When I move to a new location in an apartment, I saw similar on a 21MHz 1/4wave vertical - the SWR would change at each position depending on the length of the coax - over 6 months I moved my rig further away from the antenna so added extra coax length. The VSWR reading dramatically changed depending on the coax length. This was a bad sign of common-mode currents on the coax.

Again, I thought of the coax choke and once I fitted it, I had the a higher VSWR at but identical readings at every distance/coax length no matter if I operated from the living room, the kitchen bench or the far bedroom. Overall performance was much better despite higher VSWR readings. So my conclusion is that the VSWR meter does not read properly unless it carries no common-mode current

The way my brain envisages it: freezing time for a split second, imaging the centre conductor showing 100 volts of RF at the meter diodes, but the coax shield being at 50 volts of RF - ie: the coax shield is "hot" with common mode. The difference between the two is 50 volts RF potential.
Now, with no common-mode flow, the shield will be at 0V but the centre conductor will be at 100V, so there will be a much higher difference. So the meter diodes read a higher voltage, but gives a more true (higher) reading of VSWR with a common-mode choke.

The meter is lying to you if it has common-mode flowing because it displays the ratios (the ratio difference of fwd:reflected) between the currents/voltages.

If you measure the VSWR at the antenna feedpoint and then again at the back of the radio, with common-mode problems you get very different readings. But with a good common-mode choke, or a perfectly balanced antenna (or both) comparing your VSWR reading at both positions will ONLY be affected by coax attenuation (and even then, at HF and using good quality coax cables, this will not be a lot unless you have hundreds of metres of coax run)

This is why some people think that coax length affects VSWR of the antenna. (they say "use exactly multiple of half wavelength coax " or "use only X metres of coax") The length shouldn't matter but it will appear to be so if there are common-mode problems.

[romanescu_misha]

I did similar choke using enameled copper wire, and the parameters depended on the winding method. In my case, keeping black&white close to each others caused worse results, probably by adding capacitance. When I separated them to equal distance the difference was huge on plus.

[LLAT]

Very interesting. I wound choke with coax. As i remember it was 4 wound from one side and 4 from another side. I made it when my antenna wasn't matching. i had manual antenna tuner and it was arking and choke removed those problems. Back then i didn't had nano vna to check it, but it worked great for me

[zsradioham]

Enjoyed your explaination and demonstration. 73, GL GD DL ZS1XB

[gordslater]

5:30 to stop reflections on the screen, use a grey or dark coloured bed sheet over your head, camera and the VNA. Like a tent over you. The screen will be easily visible because it has it's own source of light. Even a white sheet may be better than nothing - try it if you have nothing else.

But really, it's clear enough, so not a big problem.

PS: great vid

[rolnas]

If swr changes with chock it means you have common mode current on coax screen. For symetric dipole chock should be installed at antenna, for EFHW it could be installed just after 49:1 or after 5% lambda from it.

[АлександрКулеш-эж]

What type of coax did you use? RG-178? Are These thoroids both FT 140-43? And the final question: what diameter (or AWG number) of insulated copper wire?

[IKXOO--Paolo]

5:45 The test is correct in the procedure but the resulting attenuating level in decibel doesn't make sense because it is performed on 50 ohm VNA impedance but it is not the same impedance that common mode impedance will "see" on the external surface of the radianting coax braid.
Vurthermore the choking is broadband and that impedance will vary enormously as the frequency varies.
So the data in decibels can only be comparative but it is better in that case to refer to the reactance

[johnwest]

Connector loss at HF is minuscule, but fewer connectors means much higher reliability.

[rudiwiedemann]

DUDE! By measuring only ONE HALF of the toroid winding, you're essentially measuring the RF rejection of a simple inductor used as a simple RF choke and NOT a "Common-Mode Choke". You need to connect the center taps of the NanoVNA to BOTH HALVES of the toroid winding (at input AND output to each NanoVNA center tap, respectively) and you should find that they deliver bettern than -40dB of "common-mode suppression or rejection". That's because with this test you're essentially trying to SEND current of the SAME polarity down BOTH SIDES of the CMC which is the definition of "common-mode current".

[FarleyHillBilly]

Why put a 1000:1 attenuator between TX and antenna ?

[piterstein]

так оплетка же тоже является частью контура дросселя, а вы ее исключили замкнув вход с выходом