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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*

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.

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

Hello. Today i received my new NanoVNA v2 SAA-2 .. Is Hugen's AA Firmware compatible with this device? I search for a Firmware with a bit more large Letters.

Thanks for help!

Greetings Wolf

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

American Scientific Surplus has some good instrument cases.

Have you tried using the TDR method to find the VF of the cable?

Roger

Jim,

A great solution. I would use that, except that I now want to include my Anritzu SiteMaster. Which is as big as both the Nanos combined and then some.

Your balun should have much larger inductive reactance than your antenna, that is the best advise I have found.

On Tue, Dec 29, 2020 at 11:40 PM, Roger Need wrote:

Martin I posted your Guide on several NanoVNA Facebook Users Groups. The
response from dozens of people was to thank you very much! Many of them just
got a NanoVNA for Christmas and your guide was just what they needed.

Happy New Year - Roger

Thank you Roger. Very good idea.
Some members of this group sent me email pointing to typos in the Guide. I wish to thank them all.
It will be corrected in a future update.

If anyone notice error, typo or have suggestion regarding English language,
please do not hesitate to write here or PM.

Happy New Year
Martin 9A2JK

I can actually fir both the VNA and SA in the Harbor Freight box I use, as well as the short jumbers and USB cables and a plethora of adapters. It just depends on how you arrange the dividers. There's even a 10 Watt 30 dB 50 Ohm attenuator pad in there now.

73

-Jim
NU0C


On Tue, 29 Dec 2020 10:55:33 -0800
"GEO BADGER via groups.io" <w3ab@...> wrote:

What a bunch of great responses. While I was reading them I realized they were all for a single instrument, and I want to carry my nanosa in the same case. Then I got greedy, or project creep, and decided I want to also carry my Anritzu SiteMaster as well. Plus all the cables & BSAs that I want to schlep along. I believe a top loader case might work for me. The instruments slip in vertically with a large pocket to store the aux items. I have some camera cases I no longer use that may fit the bill with pluckable foam.

On Tue, Dec 29, 2020 at 04:13 AM, Martin J.K. wrote:

Thank you all for your kind words.
I’m glad if I helped someone start working with NanoVNA.

Martin I posted your Guide on several NanoVNA Facebook Users Groups. The response from dozens of people was to thank you very much! Many of them just got a NanoVNA for Christmas and your guide was just what they needed.

Happy New Year - Roger

I bought handgun cases for my instruments on ebay. They have foam,
different sizes and inexpensive.

On Tue, Dec 29, 2020, 1:55 PM GEO BADGER via groups.io <w3ab=
[email protected]> wrote:

What a bunch of great responses. While I was reading them I realized they
were all for a single instrument, and I want to carry my nanosa in the same
case. Then I got greedy, or project creep, and decided I want to also carry
my Anritzu SiteMaster as well. Plus all the cables & BSAs that I want to
schlep along. I believe a top loader case might work for me. The
instruments slip in vertically with a large pocket to store the aux items.
I have some camera cases I no longer use that may fit the bill with
pluckable foam.

Hello,

I'm trying to measure a 40 degree delay line on 1.83 MHz for a 75 Ohm cable.
Would the S21 Phase reading be reliable given that the in/out of the NanoVNA is 50 Ohm?

I cut the cable for S21 Phase of 40.3 degr which is close enough.
The issue is that if I measure the same cable using the L/4 method ( min(|Z|) I do see a different VF than what the S21 Phase would imply (about 5% difference in VF from S21Phase (VF = 0.82), min(|Z|) - L/4 (VF = 0.84) and cable datasheet (VF = 0.87)).
The cable length is 14.89 m. Frequency for min(|Z|) is 4.26 MHz. S21 Phase is -40.3 degree.

Thanks,
Dani

What a bunch of great responses. While I was reading them I realized they were all for a single instrument, and I want to carry my nanosa in the same case. Then I got greedy, or project creep, and decided I want to also carry my Anritzu SiteMaster as well. Plus all the cables & BSAs that I want to schlep along. I believe a top loader case might work for me. The instruments slip in vertically with a large pocket to store the aux items. I have some camera cases I no longer use that may fit the bill with pluckable foam.

Based on the picture, position 3 is for a series component between the in
and out connectors, positions 1 and 2 are for shunt components on the input
side, and positions 4 and 5 are for shunt components on the output side.
Positions 10-14 are the same layout for the bottom set of sma connectors.

The decision of where to place components and what values to use depends
completely on what you are trying to accomplish. As an example, you could
build a low pass filter between the top two connectors by placing
appropriate values of capacitors at positions 2 and/or 4, and an
appropriate inductor at 3.

So, what are you trying to accomplish with the board? You mentioned SOLT
calibration... to use the board for that purpose, you would place a 50 ohm
resistor in shunt on one of the connectors for load,and place a solder
bridge in shunt on another connector for short. There are far more
interesting uses for such a board than that, though...

I am sure you would not mind showing off the results?!?! Pics welcome :)

--
73 es God Bless de Ron KK1L <><
KK1L.COM ( )

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