Calculations for smaller unun

Discussion in 'QRP Corner' started by KK4UTO, Oct 14, 2018.

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  1. KK4UTO

    KK4UTO Ham Member QRZ Page

    I have been a ham since 2013 and have almost exclusively used an EARC end fed antenna. It works great for my home and on a nice hike. I am also exclusively qrp being I only own an FT817nd and Z817 autotuner. So my question is this. I have built a 9-1 unun using an t80-2 toroid and 20awg magnet wire. I potted it into a 1.5 inch PVC pipe with two end caps. So I really want to see how small I can go with a 5 watt signal for an unun. I have some T50-2's and will probably try to wind them with 24awg. but can I use a T37-2 with say 30awg wire wrap wire. and if unknown then does anyone know where I could find a calculator for such a thing. Thanks for the help.
  2. N8TGQ

    N8TGQ Ham Member QRZ Page

    W7CJD and K5VZD like this.
  3. W4OP

    W4OP Ham Member QRZ Page

    Using powdered iron cores is going to result in a very narrow banded transformer.Ferrite 43 or 61 is the proper choice for a wideband transformer. If you are matching just a single band, then use an L or Pi Network. The result will be higher efficiency.
    Secondly, the 9:1 UNUN's you see on the internet are NOT UNUN's. They are impedance transformers. If you require common mode choking action, then you need to follow the impedance transformer with a 1:1 UNUN.
    Finally no 1:1 choke placed at the matchbox end of a half wave antenna will have virtually no effect as the common mode impedance of the antenna is likely going to be as great or greater then the choke impedance.

    Dale W4OP
  4. KK4UTO

    KK4UTO Ham Member QRZ Page

    Wow. I love that one. I am going to have to try that. I am printing the enclosures and then potting with 20 minute finish epoxy. The PVC works great but kinda hard to get the soldering iron into it without melting it some.
    K5VZD likes this.
  5. N8TGQ

    N8TGQ Ham Member QRZ Page

    Have to clear up my earlier post. The matcher I linked to is for a single band half-wave antenna, not a multiband end fed long wire. Sorry for the mixup!
  6. W4OP

    W4OP Ham Member QRZ Page

    Then get away from the powdered iron or even the ferrite transformer and go with an L network.

    Dale W4OP
  7. WN1MB

    WN1MB Ham Member QRZ Page

    The LDG-Z817 is a switched L-network tuner, albeit with a rather limited tuning range (6-600 ohms) on HF.
  8. W4OP

    W4OP Ham Member QRZ Page

    You do not need an L tuner for a monoband EFHW- just a fixed L and C. The feed resistance will be a function of radiator wire diameter and height above ground in terms of wavelength. One numerical solution is to treat the radiator as a single wire transmission line above ground. Typically this turns out to be in the 3000->4500 Ohm range.

    Dale W4OP
    Last edited: Oct 17, 2018
  9. KU3X

    KU3X Ham Member QRZ Page

    My 40 meter EFHW uses a 64 to 1 UnUn wound on an FT 150-43. My wire length is 65 feet. I shunt a 150 pf cap across the input (50 ohm) side of the UnUn. I feed it with 25 feet of RG 174 and place a line isolator at that point. From there I use a short length of RG 174 to the radio. I try to keep my UnUn about 15 feet or higher in the air. I don't need a tuner because it offers a very low SWR on 10, 15, 20 and 40 meters.

    It's good for 50 watts and that's plenty for me since I only use it for QRP.

    You can check the efficiency of you UnUn by placing two of them back to back and seeing how much power you end up with on the output side of the two UnUn's. I found this design to be very efficient. I have also compared this system to a 40 horizontal wire fed with coax and a balun. Using reverse beacon I found the two antennas to be equal in performance.


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