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Sensitive AM radio

Discussion in 'Electronic Design' started by Leeper, Oct 9, 2003.

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  1. Leeper

    Leeper Guest

    It has been 30 some years since I fiddled with AM radio, and was wondering,
    does anyone have any pointers to AM radio circuits that are very sensitive
    for picking up very weak signals, and have excellent selectivity?

    Since many hobbyist sites don't have "typical specifications", when they
    refer to good/excellent/great, it is hard to determine which project to
    build.

    Just looking for something to have some "fun" with for a change.

    (remove nospam)
     
  2. Don Pearce

    Don Pearce Guest

    The problem with trying for high sensitivity in AM is that in those
    low frequencies, it is external (mostly man-made) noise that
    dominates, and there is really very little point in trying to make a
    receiver particularly sensitive.

    If you want better reception, there is really no alternative to a
    better antenna. There are plenty of good designs out there - mainly
    top-loaded monopoles, but in the end there is no substitute for height
    (and distance from interference sources). Add a really good ground for
    the receiver and you are on your way.

    If you have a radio with a ferrite antenna and no external socket,
    make a big wooden frame (about a metre square) and wind about 20 or 30
    turns of wire round it. Connect this to the terminals of a variable
    capacitor taken from an old radio. Stand the transistor radio inside
    the frame, which is standing on its edge. Tune in the station you
    want, then adjust the tuning capacitor on the frame. Mutual coupling
    will give you the benefit of the increased pickup area of the big
    frame antenna.

    d

    _____________________________

    http://www.pearce.uk.com
     
  3. The problem is less a sensitve receiver, but the signal to noise.
    AM is not that great in that respect.
    You don't want to operate it in a faraday cage ony, do you ?

    Rene
     
  4. Active8

    Active8 Guest

    yes! a classic passive re-radiator. how would it compare with a good
    active antenna design other than price and complexity?

    brs,
    mike
     
  5. Don Pearce

    Don Pearce Guest

    Depends on the size of the active antenna - if known good and bad. The
    results from this are pretty amazing, though. And of course if you
    have a reasonable junk box, it costs nothing.

    d

    _____________________________

    http://www.pearce.uk.com
     
  6. Active8

    Active8 Guest

    i'm sure. it's a big resonator boosting the signal kinda like a regen
    RCVR. less noise, too, not that it matters with all that QRM/QRN.

    i mentioned once that my old tube SW RCVR had 2 ant terms and two
    speaker terms. the ant was a 5 or 6 turn loop around the inside of the
    wooden case. connecting the spkr terms to the ant in some way i cn't
    recall gave me a much louder outout. can't say for sure if it helped the
    reception, but i recall that it did. someone gave a suggestion as to
    what may have caused this phenomenon. wish i had that old thing. if i'm
    lucky it's in my step dads, shed. dunno.

    brs,
    mike
     
  7. Don Pearce

    Don Pearce Guest

    Absolutely, of course with short wave it is a lot easier to make an
    antenna that is a sensible fraction of a wavelength long - much harder
    for medium and long waves.

    d

    _____________________________

    http://www.pearce.uk.com
     
  8. default

    default Guest

    Like they say sensitivity ain't the problem. You can have some fun in
    the evenings picking up AM DX signals with a good receiver. I use a
    Radio Shack DX200 radio and have added a long wire antenna and simple
    antenna tuner so I can peak the signal I'm interested in. I can
    frequently pick up stations >800 miles away. (ONLY AT NIGHT)

    Check out tuned loop antennas. With AM sometimes just getting a
    directional antenna in a good location will do a lot more than
    sensitivity.
     
  9. default

    default Guest

    Something like a large tuned loop antenna with an electrostatic shield
    on it may work wonders. That should cut out a lot of noise. It would
    attenuate the signal too, but a simple low noise preamp may cure that.

    Anyone have experience with this approach?
     
  10. You look at the schematic, and just by looking at it you can see whether
    it's a standard design, or has something more to it. Basic AM broadcast
    radios are a dime a dozen, and have always been so. Realistically, you
    are not going to see anything but an incremental difference between
    the various designs if they are using the same basic scheme.

    Most of them will be simply for the sake of building an AM broadcast
    receiver. Simplicity and low cost are the consideration, not good specs.

    You need to look for the out of the ordinary. Something with a stage
    of ampliciation ahead of the mixer might be an indicator, but that's
    not an absolute. SOmething with an oddball IF frequency likely would
    indicate better IF selectivity, since there's got to be a good reason
    to shift the IF away from the frequency where standard components
    are available. Car radios, for instance, used to have a 262KHz IF
    rather than the usual 455KHz, for better selectivity, and an RF stage
    for better sensitivity (and better AGC range) and image rejection.
    Upconverting to an IF above the AM broadcast band might indicate
    a better design (or at least an attempt at such) because one might
    have a better filter up there, and upconversion allows for doing things
    differently. A good IF filter would indicate better IF selectivity,
    be it a better than average ceramic filter or a mechanical filter at
    455KHz, or a crystal filter at the higher frequencies. And if someone
    is bothering to put in a better filter, they might be paying attention
    to the other things. Something with a better than a simple diode
    detector would also suggest a better design. Maybe something as
    simple as some forward bias on the detector diode, or some op-amp based
    detector, or better yet the use of a mixer to convert the IF signal
    to audio, with some sort of locally regenerated carrier (be it a limiter,
    a filter and limiter, or a PLL). A full blown synchronous detector
    complete with selectable sideband would have to indicate a good receiver.
    Nobody will put something like that on something that is otherwise average.
    Watch for little features that won't be on the average radio, like a
    Q-Multiplier or some sort of audio filtering. Look for better filtering
    at the signal frequency, with more than one tuned circuit there (which
    might also bring a stage of RF amplification, not for "better sensitivity"
    but to overcome losses in the tuned circuits). Look for a first mixer
    that is more than a simple transistor; some sort of balanced mixer suggests
    a better design, and something with a diode ring mixer has got to be out
    of the ordinary at those frequencies. Look for something with a good IF
    strip, indicating more IF gain, maybe better IF selectivity, and perhaps
    better AGC.

    And actually go looking in books and magazines, rather than merely looking
    on the web. Wireless World has published a number of very out of the
    ordinary AM broadcast receivers in the past. Who's to know how good
    they are, but the ones I saw were very definitely an attempt at different
    design. Or look through books and magazines for radio amateurs. You likely
    won't find AM broadcast receivers there, and may not find much in the way
    of AM receivers, but many will be better than the average AM transistor
    radio, and a bit of rework would get them to tune the AM broadcast band.

    No, you can't really compare specs of receivers in construction articles.
    But you should be able to easily look at them and tell whether they
    are the usual design or not.

    Michael
     
  11. Tim Shoppa

    Tim Shoppa Guest

    Assuming you're talking about AM broadcast band (540kHz - 1710kHz):

    1. Buy a GE Superradio

    2. Hook it up to a good antenna

    "Good" for the antenna will depend on how much room you have and what
    noise sources there are.

    Tim.
     
  12. N. Thornton

    N. Thornton Guest


    If you want fun with a.m., try a super-regenerative set.

    Regards, NT
     
  13. There are some radio designs using a digital IF decoder. It allows for
    bandwidth shaping, noise filtering, and gain control performance that
    wouldn't be very practical to do in analog circuits.

    Blaupunkt car radios do this since AM/FM/CD/EQ head units don't have
    room for all of the analog components needed to clean up mobile
    reception.
     
  14. Tim Shoppa

    Tim Shoppa Guest

    That's especially true of consumer AM MW receivers from the past decade
    or two. Few radio manufacturers put any effort at all into the AM section,
    always putting complete emphasis on the FM section.

    There are a few exceptions... OEM car radios still have good AM performance,
    and there are a few portable consumer receivers (like the GE SuperRadio)
    which try hard on the AM section.

    Tim.
     
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