Inductor in Series with RF Signal

Discussion in 'General Electronics Chat' started by Sparky49, Apr 28, 2016.

  1. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    Hi all,

    My understanding is that an inductor will tend towards blocking an AC signal from passing through it.

    I have recently constructed a circuit for a small radio IC (http://www.farnell.com/datasheets/96892.pdf page 17, figure 8) for FSK operation. However, there is a an inductor in series with the output. How does this not block the RF energy? Is it because of the use of a loop antenna, rather than a monopole?

    If I were wanting to use a monopole antenna, what would be the best configuration? Simply remove the inductor?

    Many thanks for your time,

    Sparky
     
  2. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    The inductor and series capacitor form a band-pass filter (and what looks to part of a matching network) to let the fundamental carrier frequency pass while blocking others. No, you can't remove it.
     
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  3. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    In that case, can I simply use a monopole antenna instead of the loop?

    Many thanks for your reply.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    It resists changes in current, yes. But I'm not sure that's a useful way to think about an inductor in all applications. Plenty of AC - all the power entering my house and powering this computer - passes through a transformer, for instance.
     
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  5. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Maybe but it's not a simple matter of just replacing it with a random wire if you need to maintain range. The loop antenna forms a component matched tuned circuit optimized to transmit the signal. A proper 1/2 wave dipole or 1/4 wave with ground plane should work but would be the size of a baseball. The loop antenna is physically small but electrically large at the the transmit frequency.

    http://ww1.microchip.com/downloads/en/AppNotes/00831b.pdf
     
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  6. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    Thanks, that example with the copper loop looks interesting, I will try to emulate that.
     
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