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Thread: Minimal transmitter

  1. #1

    Default Minimal transmitter

    Hi everyone!

    I found this tx on the web:

    I was wondering whether if it might be made more minimal by skipping the 150pF capacitor. The transformer primary is about 9uH and the resonant freqency requires about 220pF, so something (the transistor?) must contribute with some 70pF to overall capacity. Maybe one could use a core that enables primary to have about 29uH and thus eliminate the 150pF capacitor. I am not an expert, just learning.

    What are your thoughts?


    Last edited by om1ava; Wed 4th Apr 2018 at 11:06.

  2. #2

    Join Date
    May 2018
    Roswell, Ga


    Actually, it is a bit more complicated than you think.

    Since the collector inductance is a tightly coupled transformer, only the "leakage" inductance appears in the collector circuit. This inductance is probably much smaller than the 9uH you cite.

    The capacitance in the area, on the other hand, is larger than you expect. Although the IRF510 is somewhat isolated by the series 15 ohms, that device has a large gate capacitance that does interact with the inter-stage tuning. Additionally, that reverse biased 1N4148 is being used as a varactor, which is a capacitor too.

    Likely, the 150pF value was determined experimentally. There are a number of reactive components at play there.


  3. #3

  4. #4


    Quote Originally Posted by om1ava View Post
    Hi everyone!

    I found this tx on the web:

    Could be as minimal as this!

    More at


    Last edited by vu2nan; Tue 2nd Oct 2018 at 13:33.

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