Output transformer for RF Preamp

Discussion in 'Homebrew and Kit Projects' started by AF4RK, Mar 13, 2017.

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  1. AF4RK

    AF4RK Ham Member QRZ Page

    I need to wind an output transformer for this circuit. It's for a receive loop antenna project 1.8 mHz to 7 mHz.
    I have used this website to get parts, but I would be guessing at how to wind it. I'm thinking about 1k center tapped with 50 ohms out. Maybe a .5 inch toroid but I'm not familiar with the types of toroids.
    any help would be much appreciated . Good in QRZ
  2. KL7AJ

    KL7AJ Ham Member QRZ Page

    A worthy project indeed. I am in the midst of writing an entire book on receiving antennas, such as active whips and loops.
    It's always a safe bet to wind a transformer like this as a trifilar. Use the smallest wire you can handle easily (30 ga magnet wire or so), and twist three 1-foot strands together like a rope. Lots of older literature used to do a big song and dance about the pitch of the twist....it's fairly irrelevant, you just want it tight enough to keep things easy to wind.

    Use a Mix 43 bead if you're in doubt. It works for just about anything H.F. Wind 9 turns of your "rope" through the core. Why 9 turns? Because it's halfway between 6 and 12. :)

    Be sure to mark the polarity, indicated by the dots on the schematic.

    That's about it.

  3. WW1WW

    WW1WW Ham Member QRZ Page

    Use a binocular core like a Fair-Rite 2843006802, conventional winding is best, start wit 4 turns on primary and 1 turn on the secondary.
  4. AI3V

    AI3V Ham Member QRZ Page

    Yes, all the above.

    Wind 2 "separate" coils for the primary, easy peasy center tap.

    Don't be afraid to try different # of turns.

  5. AF4RK

    AF4RK Ham Member QRZ Page

    Thanks, I think I have some of these!
    I like the trifilar idea. I have some binocular cores I got from kits and parts for a previous preamp that didn't have enough gain and the impedances were wrong. I am building one of these
    http://ece-research.unm.edu/summa/notes/SSN/note25.pdf. It's the theory for the Pixel RF Pro 1 B. I am building it for a friend, I bought mine. $500 is a lot of for an aluminum hula hoop! I can do it cheaper with PVC, LMR 400 for the loop and build a preamp. If the preamp blows up I can always build another one. Pixel antenna preamp is $300
  6. AI3V

    AI3V Ham Member QRZ Page

    Those are crazy prices, you can buy a bucket of transistors for that much $$.


  7. AF4RK

    AF4RK Ham Member QRZ Page

    I have gotten great performance from the Pixel RF Pro 1B, but the price is exceptionally high although the quality is also exceptional. But the common failure is a blown pre-amp, which is why I am attempting to build a copy for a friend. But it's still expensive to build. Hams don't count the cost of all the little pieces and parts and the shipping of same. It's an interesting experiment and a good way to learn about pre-amp circuits. the design of Dr. Baum is balanced, hence the discrete op amp design chosen. The first preamp I built failed due to lack of gain and improper transformer design. As you might expect, the schematic of the Pixel pre-amp was never published. "Secret Sauce!"
    AF6LJ likes this.
  8. WW1WW

    WW1WW Ham Member QRZ Page

    If you want easy and high performance then use and op-amp. The dxe-yccc-preamp kit is $27 from dx engineering, pcb, amp, transformer included. Detailed documentation is on the DXE website. If you really feel the need for a true FET front end then you can use something like an ADA4817 amp.
  9. AF6LJ

    AF6LJ Ham Member QRZ Page

    Given proper installation I am not sure how you would blow up the preamp?
  10. AI3V

    AI3V Ham Member QRZ Page

    You could literally build a RX loop and preamp from trash put out at the curb.

    Its only expen$ive if you don't know how to make a loop of wire.

    Good luck sir.


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