VNA measurements on Fair-Rite 2643167851 ferrite material

After reading the nice publication by GM3SEK (http://www.ifwtech.co.uk/g3sek/in-prac/inpr1005_ext_v2.pdf) about the Fair Rite ferrite matrial that gives a high resistive component when used for RF choking applications I wanted to do some measurements myself vith my VNA on that material used as a RF choke.

DSC_5105

Above is a three choke setup

3chokes

The blue trace above is the real Z ( R ) plotted from 0-30 MHz. The scale is 480 ohms/div. As you can see the material gives a resistance over a fairly large BW well over 1000 ohms peaking around 5K ohms resistive at the low frequency range. However over approx 18 Mhz, the resistive component is not that great. For QRO operation on 15 meter and 10 meter some further measures should be taken if RF currents are high.

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Here is a three stage RF-choke setup. First the three element RF-choke, then a one element RF-choke and a two element RF-choke with the material stacked on top.

3-2-1chokes

Note that the scale here is 620 ohms/div. The measurements show that the three in line setup didnt change the resistive part of the choke a lot. This may indicate that for QRGs over 18-20 MHz, a material with more loss in that frequency range could be found. I am later going to experiment with different winding diameters and cable diameters to see if that will change the resistive elements in the upper parts of the HF spectrum.

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6 Comments

  1. I built a combination choke of the the SEK HF and LF variety laid concentrically in a 160 x 160 plastic box. I used just one length of RG58 from an N type on the box with 5 turns through the first 3 cores on average diameter 130mm then 3 turns to a pair of cores taped together. The RG58 exits through a hole in the base of the box for an in-line connection. I think this should be an efficient layout but I have not measured its performance. I can send you a photo if you wish. Do you think there will be any interaction between the 2 chokes?

    • Hi David. Thanks for info.

      I don’t think there will be much interaction between separate chokes as long as the turns of coax doesn’t cover both cores. In fact, what we are trying to achieve with using a choke is to concentrate the magnetic field inside the core. That is why a core can give you more inductance than an air wound core of similar dimensions and with the same number of turns.

      You can send some photos if you like. You will find my e-mail adress over at QRZ.

  2. Hi Marius,
    Last weekend I´ve been doing some test with those ferrites, as well.
    You can see some of the results in next link:
    http://foro.ea1ddo.es/smf/index.php?topic=3.msg31
    Sorry, it is spanish, but you can see the results.
    By the way, I am using S21 mode, instead S11. I am not sure if there is any difference in the results.
    What I do, is to export the data from my VNA (Antirsu MS2024A), then I do the maths and plot in Excel.
    Whe can try to replicate some test to compare them, or any other test interesting for you.
    Thank you
    73, Maximo – EA1DDO

    • Hi Maximo. Cool!! Basically either I measure reflection or I measure the transmission thru the device under test (for the latter I need to use two ports on the VNA)

  3. Hi Marius,
    I’ve asked Jim K9YC, a very well know choke expert, and this is what he said;

    “> Jim, I´ve seen someone else making a S11 measurement for CM choke?
    >
    > Is that correct?

    That’s a reflection-based measurement, and has VERY poor accuracy when Z
    being measured is much greater than or much less than the system
    impedance (50 ohms).

    > http://www.lb3hc.net/archives/1358

    This appears to be an S11 measurement. The common measurement error is
    that the measured resonant frequency is much lower than the actual
    resonant frequency.

    > I always use S21.

    This has the greatest potential for success, but it requires a load
    resistance to form a voltage divider with the choke. This is the method
    shown in that set of slides, and it correlated well with a set of round
    robin measurements done by others using classical methods (Q-meter,
    etc.). The choke being measured was wound on a material that seemed to
    be #61, thus a fairly high Q, and thus quite sensitive to measurement
    error.

    73, Jim K9YC”

    I would like you to replicate same measurements but using S21, two ports mode, and see any difference.

    Thank you

    73, Maximo – EA1DDO

    • Hi Maximo, what K9YC mentions may be correct but doesn’t apply to the objectives I had when I did these measurements. I was not looking for extremely high accuracy.
      What I was looking for was to quickly determine IF the choke X and R was high enough to present proper suppressive action of the potential RF current that could occur on a transmission line as common mode RF current.

      I wanted to develop a method to do such measurements quickly so if one comes over some surplus RF chokes, one can do measurements by simply threading one wire thru the choke a couple of turns and doing a S11 measurement.

      S21 measurements (port 2 response from port 1, thru measurement) is too complicated for such a quick measurement was the reasoning at that stage if IIRC. I was also looking for some indication of where X and R curves cross each other to determine where Q passed 1.

      If I see that the S11 measurement peaks out of the instrument’s measuring range at a certain QRG and I know that the instrument measures within a reasonable accuracy up to approx 2 kohm, then I don’t need more information than that. I know the choke is likely good for RF choking applications on that frequency.

      However one must always watch for self resonances and take those into account.

      Unfortunately I don’t have time for doing S21 measurements at this stage and the DUT is long put into service (and works well). However, I don’t expect the S21 measurements to show any different results (when taking the objectives into account).

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