AM Antennas II

Discussion in 'Wireless & RF Design' started by PRS, Apr 23, 2014.

  1. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    The first thread about AM antennas fizzled out so I'm starting another one. I won't bore anyone with details of the AM radio I just designed and built. I'm working on the antenna.

    I have a tuner taken from an old radio having a variable 22pF to 470pF capacitance. For the AM broadcast band I calculated an optimum inductance of 300uH. I wound a loop stick to this value and experimentally discovered the math to be right.

    To get the whole HF range I will tap this inductor at various points. But here's my question: Once I put this capacitor and the loop stick in a metal box and that box is grounded don't I need an external wire to attach to the loop stick so that radio waves can accumulate on a conductor that isn't grounded or shielded? I mean couldn't I run the hot end of the loop stick winding out to the roof via coax and there install a metal rod or a long wire for an antenna? It seems to me the old radios had a loop stick plus a point of attachment for a long wire -- just a slotted screw usually.

    The car antenna comes to mind due to the fact the radio is shielded. Just a hollow metal mast to collect radio waves and feed the tuner. I'm wondering if the original radio designers didn't just discover by accident that the loop stick they had merely planned as an inductor was enough to be an antenna in and of itself? But the original plan was to attach an external antenna -- just some metal -- to the loop stick.
     
  2. Art

    Distinguished Member

    Sep 10, 2007
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    Hi Paul,
    It's my understanding that (ferrite) loopsticks aren't too useful outside of AM broadcast band.
    You'd better look into that one before you become too invested...
    (why shortwave radios also have telescopic antennas.).

    Where the loopstick is the antenna, it shouldn't be shielded.
    They were making plastic radios by the 60's when they came in.

    If you use an external wire antenna with inductive loop on a ferrite loopstick,
    then I don't think there's much point in the loopstick to be in the chain at all for HF if it's shielded.

    Some loop antennas are built into enclosures made of PVC to stick them outside,
    but I can't comment on what effect the capacitor mounted inside will have,
    and of course, you loose access to all of the taps in the coil that way.

    Does it really need to go outside though?
    For HF up to 30MHz yes, but I do think you need another arrangement for HF
    (above medium wave)... you might as well go back to the long wire for that.
     
  3. Art

    Distinguished Member

    Sep 10, 2007
    785
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    It's not boring I'm learning as I go and can be corrected on anything I'm wrong about.
    I only learned what an Automatic Gain Control was and what it does here.
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the external antenn ashould not be connect directly to the hot end of the loopstick winding. use a few turns of wire to couple the external antenna, one end grounded, and the other to the external antenna. if y=ou connect to the hot end of the loopstick, it will affect the tuning of the coil, adding lots of capacitance where it will detune the loopstick,
     
  5. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
    989
    35
    Do I understand you right to be saying I should couple the loop stick by way of a small transformer winding? And if so do you mean like maybe 10:1? One end of this winding goes to the external antenna and the other goes to ground? Does this mean actual earth ground?
     
  6. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
    989
    35
    Antennas are cheap. I already have lots of magnetic wire and various ferrite cores. A quarter wavelength is too long, though. I don't have the yard for a dipole. But a loop stick, tuned with a variable capacitor is within my means. I realize I shouldn't shield the loop stick but if I'm to put it in a grounded metal box with a tuning capacitor it gets shielded. I was wondering if car makers got around this by using the external antenna to collect rf waves. That antenna is not tuned, it's far too short; it must be a piece of metal to collect EM energy to feed the tuned tank in the radio. Tell me if I'm wrong on this. I Googled the subject but found a variety of information leading me to wonder if anyone really knew.
     
  7. Art

    Distinguished Member

    Sep 10, 2007
    785
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    The ferrite wire antenna coupling is 3-4 turns right next to the first coil.

    Yes :)
    [​IMG]

    This is inside a can, but the ferrite antenna for MW reception replaces
    the coil on the right in this image.
    The coupling coil on the ferrite replaces the coil to the left in the image.
    Then from the 60's onwards the ferrite loop is always outside the metal chassis acting as the antenna
    and also half of the tank circuit, so the antenna doesn't have to be tuned separately with every station.
    It does sound like AM car radios would be doing that antenna coupling.
    They could go back to using a canned transformer because they know the radio is shielded.

    When you connect a tuned loop antenna to a prebiult radio of this circuit,
    you are connecting another loop across the coupling coil and need to tune the loop you added again
    with another tuning gang.
     
    Last edited: Apr 25, 2014
  8. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
    989
    35
    This method of coupling the antenna 'rod' to the ferrite inductor sounds good to me. Its a 1:10 transformer and that means ten times the voltage at the input amplifier. It also squashes the extra capacitance introduced by the rod and its wire by the square of the turns ratio, and thus reduces it by two orders of magnitude. I'll experiment with this. As it is I'm getting signals by putting a tuning capacitor in parallel with the loop stick on my desk. But I've got noise problems. Whether this be due to the radio itself or to the antenna remains to be determined.
     
  9. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    Paul,
    I have been thinking of the various posts on the subject of the AM loop antenna. It was mentioned by one poster that the ferrite rod would not work well at frequencies higher than the AM broadcast band. I think it depends on
    the composition of the ferrite rod, as there are ferrite materials that work well into the VHF region. One of the things I remember about the Ramsey signal magnet antenna kit was that they furnished components and instructions to wind the antenna for higher frequencies. It also had a partial shield around the loopstick to minimize static interference. It also had a varactor diode to tune it remotely, and had a transistor circuit to have an output impedance equal to the impedance of coax. The votage to run the circuit and the tuning voltage went up the coax from the remote location. I lost interest in the AM band about 7 or 8 years ago and I sold the signal magnet antenna. I still have my hombrew receiver, but haven't tested it in
    a while.
    Ned
     
    Last edited: Apr 27, 2014
  10. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
    989
    35
    Thanks for the information and for the post itself. I was beginning to think everyone but me lost interest in the subject. Here's where I'm at:

    With a loop stick of about 300uH in parallel with a 22-470 variable capacitor I got all of the stations around here. But those signals were weak. It was inferior to the ferrite loop antenna I had put on the roof. So I got to thinking maybe the roof antenna could feed the desk top antenna and sure enough I now have a strong signal but the added inductance made the variable capacitor of no effect. I discarded it and I've been listening to a country station all morning, loud and clear. All of the stations come in and are selected by the LO alone.

    What does this mean? I think it means the ferrite loop in parallel with a tuning capacitor is a means of discriminating between stations, not picking them up. The ferrite loop is the antenna all by itself. And I'm beginning to wonder how a steel rod wound with a couple hundred turns and set on the roof will do. Also I'm going to build an FM discriminator so I can see if I can pick up CB and emergency services.
     
  11. Art

    Distinguished Member

    Sep 10, 2007
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    There's a guy on YouTube that I can't seem to find now that demonstrates the use
    of 40 or so of the longest ferrite rods glued together like a big disc with windings
    around the circumference of that. He was comparing it with the outdoor tuned wire
    loop antennas that were also homemade, and seemed to get some good results.
     
  12. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    35
    Thanks for that. I'm working on an FM discriminator right now so I can pick up police and fire and CB radio bands.
     
  13. Art

    Distinguished Member

    Sep 10, 2007
    785
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    In my neck of the woods they are all >400 MHz, and I have a $50 handheld radio from the future :D
    It squishes the carrier wave horizontally with audio instead of vertically the crazy thing.
     
  14. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
    989
    35
    I know what you mean by horizontally verses vertically as per an oscilloscope. FM is horizontal, AM is vertical. I recently constructed two circuits, not of my own design, one an FM modulator, the other an FM discriminator, the first to create FM at 455 kHz, the second to detect the same. Neither one work.
     
  15. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Keep in mind that AM broadcast stations are always vertically polarized. Any antenna that is more or less deliberately vertically polarized as well, will be an advantage.
     
  16. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
    989
    35
    That's interesting KL7AJ, I'll keep it in mind.
     
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