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Thread: Homemade FM Receiver - 75 vs. 300 ohm

  1. #1

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    Default Homemade FM Receiver - 75 vs. 300 ohm

    Greetings,

    I made an FM receiver for my desk at work so I could utilize an external antenna located some 40 feet away I had planted in a window for better reception. The indoor antenna kit I got came with an F connector that gave me a signal and a ground wire connection, I'm assuming the red wire is the signal and the black is ground, anyway this is the way I hooked it up and it seems to work well. I connected the ground wire to the brass fasteners, I'm assuming the corner bushings on the circuit board can serve as a ground, is this correct? I'm assuming a single antenna connection point implies a 75 ohm impedance on the receiver side?


    https://drive.google.com/file/d/16zo...ew?usp=sharing


    https://drive.google.com/file/d/1EQs...ew?usp=sharing

    I'm trying to build a duplicate FM receiver in the same fashion for one of my jealous co-workers. The issue is that the only F connectors I can find for sale are the double lead 300 ohm variety, the one I bought is "matching". How do I get this double lead F connector to work on my single point circuit board?

    1) Twist leads together and solder both to single point circuit board input?
    2) 1 lead to circuit board and 1 lead to ground?
    3) 1 lead to circuit board and 1 lead disconnected?


    https://drive.google.com/file/d/1JiT...ew?usp=sharing


    https://drive.google.com/file/d/1CKl...ew?usp=sharing

  2. #2
    Super Moderator 5B4AJB's Avatar
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    Default

    Quote Originally Posted by DMack View Post
    corner bushings on the circuit board can serve as a ground...
    Probably... look carefully at the 3-5V input (bottom right), see the ground connection (right-hand pin) goes to the big groundplane all around the board? if the tinned corners are connected like that, then it should work fine.

    Quote Originally Posted by DMack View Post
    implies a 75 ohm impedance...
    Yes, F-connectors are normally 75Ω

    Quote Originally Posted by DMack View Post
    I'm trying to build a duplicate FM receiver in the same fashion for one of my jealous co-workers. The issue is that the only F connectors I can find for sale are the double lead 300 ohm variety, the one I bought is "matching". How do I get this double lead F connector to work on my single point circuit board?

    1) Twist leads together and solder both to single point circuit board input?
    2) 1 lead to circuit board and 1 lead to ground?
    3) 1 lead to circuit board and 1 lead disconnected?
    The matching transformer is also known as a balun, it will change 300Ω to 75Ω or vice-versa. You can use it for a 75Ω receiver with a 300Ω ribbon antenna.

    RF connectors are not super critical for receivers, you can use anything that will fit on the board provided they have 2 conductors...

  3. #3

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    May 2019
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    Quote Originally Posted by 5B4AJB View Post
    ...you can use anything that will fit on the board provided they have 2 conductors...
    I'm not clear on your instructions, what do you mean by "fit"?

    How should I wire up the F connector to my circuit board? Option 1, 2, or 3 that I described earlier?

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