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Core for EFHW 1:49 transformer

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WB2UAQ, Oct 20, 2019.

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  1. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Check out the Fair-Rite Products EMI cable suppressor for use in the EFHW transformers. Part number is 2643251002. Mouser has them in the catalog for about same as FT-240-43 ($5). Has larger Ae (equiv cross section of about 50 % larger than the FT-240) for lower peak flux density. Should run cooler and is a more compact device. I tested the loss for a 1:49 to be about 0.5 dB at 7 MHz into 2400 ohms vs about 1 dB for the FT-240-43, I use 20 AWG, good enough, but should handle 16 AWG if you feel the need for a heavier gauge. 73, WB2UAQ
    BTW, I measured the impedance looking in and a 100 pF shunt cap across the input looked good. Used the NanoVna for the first time for this stuff:)
    Another EDIT....the picture in Mouser is is much more like a toroid than a cylinder:)
    Last edited: Oct 20, 2019
    KU3X likes this.
  2. KU3X

    KU3X Ham Member QRZ Page

    Mouser does not do well with using the correct pix for 240 cores no matter what material you look at.

    How much power are you planning of running? Just a curiosity question. I only use my HWEF40 for portable QRP and use a 64 to 1 UnUn with an FT-140 43 core. Works great with an almost perfect match on 10, 15, 20 and 40 meters. If I remember correctly, the UnUn loss on 40 meters is around .3db. I checked mine with a spectrum analyzer using the tracking generator with two UnUn's back to back.
    I also shunted a 100 pf cap across the 50 ohm side of the UnUn.

    KC9UR and KX4O like this.
  3. KU3X

    KU3X Ham Member QRZ Page

    That was a good find. The FT 140-43 (2643802702) core gets hot in my 64 to 1 UnUn at 100 watts on 20 meters. The above will fit in the same size enclosure and I already know it will easily handle 100 watts.

    I just ordered 12 of them.

    I have the Fair Rite catalog but missed that one.

    Barry, KU3X
    Last edited: Oct 20, 2019
  4. PA1ZP

    PA1ZP Ham Member QRZ Page

    I have build a number of EHFW antenna transformers and antennas.
    For 100 watts SSB or CW I would say go for an FT240-43.
    If you have troubles finding them go for a 2 or 3 stack of FT140-43 .

    look at Steves ellingtons facebook group for many files and details in how to build and place EFHW mono and multiband antennas and transformers (all the 1 :49 1 : 56 or 1 : 64 Ununs with a capacitor or even without for monobanders you can build these ununs without capacitors but you might need to take of a secundary winding or so to get SWR OK.

    73 Jos
  5. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Hi Barry,

    I only run 100 W PEP. SSB and CW.

    Yes the FT-140-43 has a pretty small cross section and the primary reactance must be pretty low as well. The beauty of this core is, as mentioned already, the large cross section.

    The reason you might have missed this core is because it is not listed under the toroids (26 prefix vs 59) and it is considered a Cable EMI Suppressor.
    I have been doing a bit of transformer work for some other application and was looking thru a ancient sample kit from Fair-Rite. This kit had a pair of them. First thing, I checked to see if it is still a current product and then checked Mouser.

    I think Fair Rite misses the point with us hams and other users not looking for EMI suppression. Also, the charts for frequency range are misleading for other than EMI suppression.

    Fair Rite has an amateur radio forum but I have not figured out how to join.

    If you bought 12 pieces, you bought out almost 10% of Mouser's stock:)

    If you think if it, please let us know how things worked out.

    Thanks and 73!
  6. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Thanks Jos.
    You have to measure the impedance looking in and come up with your own shunt C. It is easy to calculate the C and I'll work out an example if you'd like.

    I say go with this 2643251002 for 100W instead of the FT-240-43 (5943003801, I think). It is smaller and you can use lighter AWG no matter what core you use. I don't waste the time twisting the primary over the secondary.
    73, Pete
  7. KU3X

    KU3X Ham Member QRZ Page

    Yup.....I looked again and found it in the catalog.
    Everybody falls for that. I'd like to know who came up with that idea ?

    The other one is the cross over idea. It's only so the wire starts at the bottom and ends at the top, no other reason !

    The last 64 to 1 UnUn I built to handle 100 plus watts, I used a 2643102002 with two turns on the primary and 16 turns on the secondary. This core, or bead has more mass than the FT 140 43 core and can easily fit in a small enclosure. All Electronics # MB-133. I use a BNC for the coax and #22 teflon insulated wire for the antenna. I only use mine for portable operating. The entire antenna fits in my pants pocket. My smaller 25 watt version fits in my shirt pocket. FT 114-43 core for the QRP version.
    Barry, KU3X
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    Actually, there is another reason. The SelfResonantFrequency of any CM choke is a function of the tiny capacitance that exists between the two pieces of wire/coax at the start and end of the winding. If you cross over, the wires/coax winding ends are further apart reducing the capacitance which raises the SRF and makes the Choke usable over a wider frequency range. The last thing you want to do is have the winding ends right next to each other....

    I just wound two chokes consisting of 8 turns of RG400 TFE coax on a Type 31 core and measured mag(S21) on a nanoVNA, with and without the crossover. With the crossover, the usable bandwidth of the choke is about 25% wider...
    G5TM likes this.
  9. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Here's a shot of the 2643251002 core wound with 20 AWG. 2 turns on the prim and 15 on the sec. 100 pF shunt across the input. The BNC connected to the 2.4 k resistor has a shorting cap. I place a 50 ohm power detector here to indirectly measure the insertion loss.

    Attached Files:

    CT2FZI likes this.
  10. VK2TIL

    VK2TIL Ham Member QRZ Page

    This may explain why 'UX's #43 core gets warm.

    I have re-drawn the Fair-Rite data for #31, #43 and #61 materials on a semi-log scale that makes the differences in u' (real permeability) and u" (imaginary permeability) rather clearer.

    It is u' that determines inductance/reactance whilst u" determines resistance/loss (ie heat).



    Note that, at 20m/14 MHz, the resistance/loss of #61 is negligible whilst the resistance/loss of #43 is substantial, not too far from its peak value.

    Ferrite material choice is often a trade-off.
    K0UO, WA7ARK, WB2UAQ and 1 other person like this.

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