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Advice for an AM receiver

  1. Nov 27, 2014 #1
    My 18 year-old students have just learned that an LC circuit can be used as an oscillator or as an tuner, so I would like them to build either an FM transmitter or an AM receiver as a homework. However, the only trimmer capacitors I have right now are 3-40 pF. They could build their own coil with 22 gauge wire, which is fine for the FM oscillator, but not so much for the AM receiver since they would need something around 500 μH to be able to pick up an AM station (too many turns!). Should I buy bigger caps (hard to find variables with more than 100 pF)? Should I buy 500 μH fixed inductors?
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  3. Nov 27, 2014 #2


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    Last edited: Nov 29, 2014
  4. Nov 27, 2014 #3


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    There's also a old trick to make a AM receiver called a Foxhole or POW radio. The combination of antenna capacitance and the coil make an LC circuit in the right range to receive a strong signal.

    If you find the parts for a tunable set you might want to also try this for extra credit.

    Last edited: Nov 27, 2014
  5. Nov 29, 2014 #4
    Thank you for your help davenn and nsaspook. I will definitely order the tuning cap and try the foxhole radio. For now, I have to find a way to make it work with the material I have.

    I made a AM tuner with a 3-40 pf variable cap and a 800 μH inductor I had and I was only able to pick up a station at 1280 kHz. I think this is due to the fact that the selectivity is poor because of the 10 Ω resistance of the inductor. I thought I could improve selectivity by using a 50 μH inductor (lower resistance), which would force me to increase the capacity to keep the same resonance frequency. Could I just put three 100 pF caps in parallel with the 3-40 pF to have a capacity in the range 300-340 pF?

    Also, I noticed that when I put my multimeter on the capacitance measurement mode, it reads around 0.100 nF with the leads connected but not touching each other. Should I subtract 0.100 nF from every capacitance measurement?
  6. Nov 29, 2014 #5


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    your meter should have a know for zero'ing out the lead capacitance before you connect to the cap under test

    Do you already have the detector diodes you need for the receiver ?
  7. Nov 29, 2014 #6
    Most likely, this will not improve selectivity
  8. Nov 29, 2014 #7
    Yes, I use a germanium diode, a LM386 as an amplifier and a 8 Ω speaker. So with my 3-40 pF trimmer cap I can hear an AM station at 1280 kHz and a strong FM station when I turn the cap, which I assume is a slope detection.
  9. Nov 29, 2014 #8
    Even if the resistance of the 50 μH inductor is much lower than that of the 800 μH?
  10. Nov 29, 2014 #9


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    The inductor DC resistance is not much of an important factor here for either selectivity or sensitivity. The selectivity is limited by the low Q possible with a closely coupled antenna and detector likely being completely impedance mismatched. This same factor affects the sensitivity by not optimizing the energy flow from the antenna to the detector.

  11. Nov 29, 2014 #10


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    I would recommend not building a transmitter -- just focus on building receivers. This is because most of the EM band requires a license to transmit. There are some bands where a license is not required, but you also have to know what you are doing to even confine your TX signal to such a band.
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