240-31 vs 240-43 ferrites - VNA chart

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W0KNI, Jul 16, 2020.

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  1. W0KNI

    W0KNI XML Subscriber QRZ Page

    Working on a 1:1 transformer and thought I would share two charts. Let me say right up front that this is not a scientific comparison. Things to note:

    50 ohm (2 x 100 ohm resisters in parallel) used for dummy load
    Calibrated NANO-VNA-H4 used with a range of 1Mhz-50Mhz.

    GREEN = Impedance
    PURPLE = Reflection
    RED = SWR

    Transformer #1 - 240-31
    Copper PVC coated #14 primary wire / 7 turns ea.

    7 turn 240-31 ferrite graph-20200716-103134.png

    Transformer #2 - 240-43
    Steele Silicone coated #14 wire / 7 turns ea.

    7 turn 240-43 ferrite graph-20200716-103326.png

    As expected, the 31 mix performed the best. Charts attached.

    Some thoughts to improve the higher frequencies would be to use a smaller gauge wire like #18 where I could get 10-12 turns instead of 7. I'll be posting those once I get them done.

    73
     
  2. AB6RF

    AB6RF Premium Subscriber QRZ Page

    Wait, these are the S21 plots of the transmission line through the choke?
    As such, they basically tell how good 50 Ohm line you've made, not so much about how good the choke is.

    To compare the chokes, you need to measure the common mode impedance of the choke.
     
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  3. W0KNI

    W0KNI XML Subscriber QRZ Page

    @AB6RF - This was of S11. There was a 50Ohm dummy load at the end since this transformer will be placed just downstream of the radio but before the 9:1 transformer for the long wire antenna.

    I'm hooking this up in the field this weekend and will post what I found. Just getting started with all this while working on my Extra.

    73.
     
  4. AB6RF

    AB6RF Premium Subscriber QRZ Page

    Yes, you're correct, I got my S-parameters mixed up.
    But anyhow, so far you're measuring the impedance of the transmission line used to make the choke.
    The interesting part will be the choking effectiveness, in other words common-mode impedance through the choke.
     
    W0KNI, KA0HCP and KU3X like this.
  5. W0KNI

    W0KNI XML Subscriber QRZ Page

    Absolutely! The charts are nice and pretty since it's a dummy load. I'm anxious to see how well it does in real-world tests! Stay tuned, I will post the result for sure.

    I'm going to run a random wire of #18 Poly-Stealth CCS line, out 260 ft with a max hight of about 100'. It will then be slopping down about the center, so I suppose it will look like an inverted "V" EFHW. From there I'll see what bands the antenna is resonant on and then start cutting for an exact 1/2 wave on the lowest band I can get to. I don't have a tuner so I'll just have to be happy with narrow options but should be fun!

    Using a quad to fly the line up to the top of a very tall Red Cedar. 73.
     
  6. EA1DDO

    EA1DDO Ham Member QRZ Page

    How you suggest to measure common mode here?

    VNAs only measure magnitude and phase. Rest is only maths.

    I've made a tool to calculate and show Zcm (Common Mode) among others, from S21 measure. You can download the Excel tool calles ZPlots S21 Xtra here

    73, Maximo
     
  7. AI3V

    AI3V Ham Member QRZ Page

    So your transmitter is going to be battery operated, with no connections whatsoever except the single antenna wire?

    This also means the stray capacitance between the rig and universe needs to be negligible, probably wamt to build a 6 or 10 foot tall insulating support to keep the rig clear of everything.

    ...

    What you really are building is a antenna where the connection to the rest of the world comes from 2 places, all the wires connected to the rig, including the house wires , and the capacitive coupling from the rig to the universe.

    How do you propose to measure this?

    Rege
     
  8. W0KNI

    W0KNI XML Subscriber QRZ Page

    You are correct, I don't plan on running a from the end of the EFHW back to the S21 of the VNA, so yes it would be only math. I'm very new to this so seems like the only way I could determine power coming back on the coax is via a hand held antenna analyzer/watt meter?

    I'm open to suggestions!
    Gracias SeƱor!
     
  9. W0KNI

    W0KNI XML Subscriber QRZ Page

    I don't plan on running a from the end of the EFHW back to the S21 of the VNA, so yes it would be only math. I'm very new to this so seems like the only way I could determine power coming back on the coax is via a hand held antenna analyzer/watt meter?

    I'm open to suggestions!
     
  10. EA1DDO

    EA1DDO Ham Member QRZ Page

    Not sure what you meant when saying "power coming back".
    It could be VSWR, then you need a VSWR meter.
    If you need to measure common mode, then it is not "coming back", just taking the "third wire", outside coax braid to ground. then you need a common mode meter like MFJ-854.
    Remember common mode currents are in the coax, not the antenna.

    Not sure if you can use a VNA to measure common mode currents.

    73, Maximo
     

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