HWEF antenne for QRP using a BN43-2402 balun core

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by PD7MAA, Jan 20, 2017.

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  1. PD7MAA

    PD7MAA Ham Member QRZ Page

    This morning i made a test with 2 transformers back to back from 7-28MHz.

    TX : FT897 at 10 Watt FM.
    Output 40m : 5.5 Watt
    30m : 6.0 Watt
    20m : 6.5 Watt
    17m : 5.75 Watt
    15m : 4.8 Watt
    12m : 4.8 Watt
    10m : 5.5 Watt
    At 10 Watt the cores warm up so 10 Watt is the max. pep power for the BN43-202
    More loss than i expected, about 50% at 2 cores
    Here is the output on 20m.

    This means a loss of about 25% in the transformer.

    20m 10 Watt input.jpg
    2x BN43-202 back to back.jpg
    Last edited: Jan 27, 2017
  2. G3TXQ

    G3TXQ Ham Member QRZ Page

    Do you have the equipment to repeat Owen's measurement? These back-to-back tests are not representative of what happens when feeding an EFHW.

    Steve G3TXQ
  3. PD7MAA

    PD7MAA Ham Member QRZ Page

    Hi Steve

    Unfortunally i don't have the same Equipment. Doesn't this test say something about the cores ?
    Last edited: Jan 27, 2017
  4. G3TXQ

    G3TXQ Ham Member QRZ Page

    It tells you what the loss is if you run two of them back to back :) But if the load impedance when they are tested back-to-back is not the same as the load impedance when feeding the EFHW, there's a possibility of large errors.

    In the back-to-back test, the load impedance is not under your control; with Owen's method you can set the load impedance to anything you like.

    However, the various numbers - WW1WW's, Owen's and yours - are broadly consistent with each other when you correct for the load impedances. So Owen's loss figure of 3.7dB (57% of your applied power) when feeding an EFHW looks about right to me, and consistent with what you are measuring.

    Steve G3TXQ
    WB5YUZ and PD7MAA like this.
  5. WW1WW

    WW1WW Ham Member QRZ Page

    Hi Steve, how did Owen have a good match with 3300 ohms and a 5.5 to 1 transformer? He measures 45 ohms on the input with a 3300 ohm load. What am I missing?
  6. G3TXQ

    G3TXQ Ham Member QRZ Page

    Actually, Owen measured 45+/-j22 - there's a big difference - the SWR is 1.6:1

    But not just Owen - the OP measured an input SWR(50) better than 1.5:1 with a 3300 Ohm load, rather than the 2.18:1 you would expect from an "ideal" transformer.

    My guess is that it's due to insufficient winding reactance - the same thing that is causing the high losses. But that's just a guess - I haven't measured one.

    Steve G3TXQ
  7. WW1WW

    WW1WW Ham Member QRZ Page

    For the hell of it I made a new transformer with a 2861006802 core. 10 turns on the secondary 2 turns on the primary using 30ga Teflon covered wire. The primary inductance is 2.8uH and the secondary is 70uH. I ran the transformer with a 1.12k resistor into the VNA 2180 and measured -13.81dB S21. Correcting for mismatch loss 0.041dB and analyzer input mismatch 13.69dB that leaves a transformer loss of only 0.08dB. Much better than the 0.7 dB I measured on the type 43 core.
  8. G3TXQ

    G3TXQ Ham Member QRZ Page

    Interesting! I'm surprised you achieved such a low input VSWR with only 70uH of winding reactance. What frequency was the measurement made at?

    Steve G3TXQ
  9. WW1WW

    WW1WW Ham Member QRZ Page

    The data I quoted was at 7MHz. Attached are sweeps from 1 to 30 MHz with a marker at 7MHz.

    Attached Files:

  10. G3TXQ

    G3TXQ Ham Member QRZ Page

    What was puzzling me was the very low input SWR.

    At 7MHz your 70uH winding inductance represents a reactance of +j3079Ohms; this appears in parallel with the load of 1170Ohms to produce a composite 1022+j388 on the secondary side or 40.9+j15.5 on the primary side. That would be an input SWR of 1.48:1 and a mismatch loss of 0.168dB rather than the 0.041dB you measured.

    Steve G3TXQ

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