Hi Team,

I need a confirmation from group member what I am doing is correct.

I am in the process of making RF chose for my HF antenna. Instead of making an ugly balun, I decided to make one from the Mix 31 ferrite beads. I made one last time for my vertical antenna but at that time I board MFJ-259B from friend of mine to measure it. This time I have my own nano vna. I think they way I have setup the nano vna to measure the choke is correct but need a verification. I have placed 4 beads and the impedance is around 550 ohm plus. I am posting some pictures and a video. Its not a best setup but just want to see if the impedance is 50 x 10 or more. I can add more but main thing that I would like to know is this showing correct that choke is 550 plus ohm.

Thank you,
Sohail VE3ITU.

That measurement setup is not so good for many things but is okay for
this application.
This isn't an impedance-critical application.

With a common mode choke we are just trying to make the feedline shield
path a much higher impedance than the desired counterpoise path, so the
vast bulk of current stays in the desired counterpoise and does not move
onto the shield outside layer. There is no fixed ratio of choke to
feedline differential impedance because the impedance necessary depends
on the specific system.

Isolating a single radial from the shield a choke would have to be
higher impedance for the same isolation as a choke in a system with 60
radials. As a matter of fact with 60 radials it doesn't take much of a
choke at all, just a few dozen ohms, to get common mode to near zero.

I don't even use chokes or baluns in some cases. My one 80M dipole
antenna has a 60 foot long feeder dangling down and connected to the
tower ground. The common mode on the feeder and dipole is essentially
zero and the dipole is balanced and there is no balun or choke at all.
As a matter of fact it will get worse, not better, with a balun or choke
at the feed point but so little worse it does not matter. It is mostly
academic that a choke makes it slightly worse.

My 80M dipole with a 140 ft feeder to ground has horrible common mode.
This is because of the ratio of shield impedance to dipole half
impedance. The shield at the dipole has a pretty low common mode Z, so
it unbalances the feed point and sucks current out of the shield dipole
half. It needs a choke balun, but since that half of the dipole is
around 30 ohms even less than 300 ohms would be 10 times more current
remaining in the dipole half connected to the shield. I use 1K ohms
which takes it to negligible values.

The common mode isolation we need has nothing to do with the
differential across the feedline. It is a shield CM impedance ratio
thing to the side we are isolating, not the impedance across the coax
from center to shield.

73 Tom

On 8/9/2025 7:14 PM, Sohail Anjum via groups.io wrote:
> Hi Team,
> I need a confirmation from group member what I am doing is correct.
> I am in the process of making RF chose for my HF antenna. Instead of
> making an ugly balun, I decided to make one from the Mix 31 ferrite
> beads. I made one last time for my vertical antenna but at that time I
> board MFJ-259B from friend of mine to measure it. This time I have my
> own nano vna. I think they way I have setup the nano vna to measure
> the choke is correct but need a verification. I have placed 4 beads
> and the impedance is around 550 ohm plus. I am posting some pictures
> and a video. Its not a best setup but just want to see if the
> impedance is 50 x 10 or more. I can add more but main thing that I
> would like to know is this showing correct that choke is 550 plus ohm.
> Thank you,
> Sohail VE3ITU.
>

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Hi Sohail,

I'm not sure that the way you are using the NanoVNA to make the common mode
measurements is correct. The way I have been shown to do and have done
myself such measurements is to apply the same signal on both of the bifilar
conductors. You seem to have a single wire with the beads on it. I hope
this is not what you are building as a common mode choke because it will
not work. To get proper common mode current suppression, you need a
transmission line choke, which uses the bifilar winding comprising two
wires in parallel, or can be in the form of a thin coaxial cable. The
bifilar winding is wound around the toroid, or you can try stringing the
beads on a short length of coax.. The way to measure the common mode
current suppression is to solder one end of the bifilar wires (or center
conductor and shield of a coax) each to a 50 ohm resistor. The free leads
of the 50 ohm resistors are shorted together and connected to the S11
terminal of the NanoVNA. At the other end, the two wires (or inner
conductor and shield of a coax) are again soldered each to a 50 ohm
resistor, which the free leads are shorted together and connected to the
S21 terminal. Then set up the NanoVNA to for through mode, your frequency
range, and to display the data on the LogMag display to see how much
suppression you are achieving. Ideally you should have at least 20 dB
suppression of the signal, which would be the least amount of attenuation.
Hopefully, the attenuation increases as you sweep to higher frequencies. In
your photos, you are showing the Smith chart display, which is only for
impedance or reflection measurements as you may know. The Smith chart does
not show the signal suppression. However, you can and should also measure
the SWR of the choke as well. This can be done by looking only at the
reflections from the S11 terminal with the NanoVNA . Those may be displayed
on a Smith chart. I will check on the exact procedure, but basically, you
want to make sure the SWR of the choke does not exceed 1.5 or maybe 2 at
most. If so, then either the impedance of the bifilar winding is not close
to 50 ohms and causing reflections, or it could be that the ferrite mix is
probably not the best, and you may want to switch to mix 43. But this may
depend on the frequency range of interest.

For more authoritative information, see TRX Lab YouTube videos #100 and
#101 on common mode choke baluns and the making/measuring thereof. This is
a very good series from Germany. He uses a benchtop VNA to do the
measurements, but it makes no difference if you use a NanoVNA. He shows the
proper way to make the connections and what to look for in the data.

Please let us know if this helps.

73,
Scott KF7GGN

On Sun, Aug 10, 2025 at 10:00 AM Sohail Anjum via groups.io <anjum_sohail=
hotmail.com@groups.io> wrote:

> Hi Team,
>
> I need a confirmation from group member what I am doing is correct.
>
> I am in the process of making RF chose for my HF antenna. Instead of
> making an ugly balun, I decided to make one from the Mix 31 ferrite beads.
> I made one last time for my vertical antenna but at that time I board
> MFJ-259B from friend of mine to measure it. This time I have my own nano
> vna. I think they way I have setup the nano vna to measure the choke is
> correct but need a verification. I have placed 4 beads and the impedance is
> around 550 ohm plus. I am posting some pictures and a video. Its not a best
> setup but just want to see if the impedance is 50 x 10 or more. I can add
> more but main thing that I would like to know is this showing correct that
> choke is 550 plus ohm.
>
> Thank you,
> Sohail VE3ITU.
>
>
>

--
Scott Gilbert
KF7GGN
[image: BCARES]

On 8/11/2025 8:08 PM, Scott KF7GGN via groups.io wrote:
> Hi Sohail,
>
> I'm not sure that the way you are using the NanoVNA to make the common
> mode measurements is correct. The way I have been shown to do and have
> done myself such measurements is to apply the same signal on both of
> the bifilar conductors. You seem to have a single wire with the beads
> on it.
>

He is okay if only trying to get a rough idea of the impedance offered
by the beads on his cables. I do something similar all the time.

It doesn't matter much anyway in his ultimate use because the common
mode rejection caused by inserting a choke is more dependent on the
system inserted in than it is on the choke bench measurements of impedance.

Some systems improve a great deal with just a few ohms added, some won't
change at all, and some might even become worse. We are sticking a known
CM impedance choke between an unknown common mode impedance source and
load. There is little reason to care about what the choke shows in
absolute numbers on a test bench, other than maybe someone wants it as
high a resistive impedance as possible at the operating frequency.

The entire exercise of high target impedance numbers is mostly
shoulder-shrug designing.

73 Tom


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Hi Scott,

Thank you for your input. Iwill check the videos. These ferrite beads are slip on sleeves. These cores have 1/4" hole and easily slip on the coax cable. Palomar engineers have these type of geed line choke and looks better then ugly balun.  One of my friend Ed Spingols VA3TPV introduced this kit some time back. I build it last time for my multi band vertical and if I recall I used 10 of theses beads.

https://palomar-engineers.com/antenna-products/Slip-On-Feed-Line-Choke-Isolator-1-4-6mm-coax-cable-RG-58-LMR240-1-8-300-MHz-p150201795

For 1MHz ~300 MHz, the Mix 31 is right option. The Mix 43 is good for 25 MHz~300 MHz. I am going to use it for my HF antenna , so Mix 31 is the right choice.

https://palomar-engineers.com/ferrite-application-experts-2/Ferrite-Core-Products-c21312657

I will check the videos you mentioned.

Sohail VE3ITU.