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Ferrite rod antennas in AM broadcast receivers

Discussion in 'Electronic Design' started by George, Sep 3, 2012.

  1. George

    George Guest

    I have been trying to find info on designing ferrite rod antennas into AM broadcast receivers. Googling so far has just produced general loop antennaprinciples and characteristics of the rod material itself.

    Does a ferrite rod antenna have to be tuned to resonance when changing frequencies with the 0.5 - 1.6 MHz band? Or can decent performance be realizedacross the band without re-tuning? Are there gain vs. frequency curves available for given ferrite rod antenna designs?

    And I'd like to understand the antenna's phase (delay) performance across the AM band for a given design.

    I'd appreciate if someone can point me to this info.

    Thanks.
     
  2. Phil Allison

    Phil Allison Guest

    "George"

    I have been trying to find info on designing ferrite rod antennas into AM
    broadcast receivers. Googling so far has just produced general loop antenna
    principles and characteristics of the rod material itself.
    ** Yes.

    Pre-selection of the antenna signal is crucial to getting good results with
    the AM broadcast band, otherwise the receiver will suffer badly from cross
    modulation effects and images of the IF frequency.

    ** No.

    ** The length of the rod is the main thing, longer = better signal strength
    and directionality.


    ** Irrelevant.


    .... Phil
     
  3. George

    George Guest

    [OP here]

    To clarify my questions:

    I'm building an AM broadcast receiver to be used in a non-standard application. It will use an existing wideband COTS software defined radio product that does not provide tuning information to the ferrite antenna. So I'd like to be able to get enough antenna gain across the broadcast band from theantenna to avoid having to tune the antenna to resonance on each frequency.. But space limitations dictate use of a ferrite.

    This non-standard application DOES require knowledge of the antenna delay vs. frequency.
     
  4. Winston

    Winston Guest

    Can you create a voltage (say from a DAC) that
    you could use to resonate your loopstick using
    a voltage variable capacitor?
    http://en.wikipedia.org/wiki/Varicap

    --Winston
     
  5. Tim Williams

    Tim Williams Guest

    A tuned antenna is important in a superhet, where with an IF of 455kHz, an
    LO frequency of 1005kHz could pull in two channels, 550 and 1460kHz. But
    since your detector is different, this doesn't need to matter.

    So you need a Q of about 1. Size the L and C accordingly. That's the
    best you can do, and of course your antenna gain will be terrible (far
    below isotropic).

    Could you define "antenna delay", and quantify how it must be known?

    In general, a radio has absolutely no phase reference, and because
    bandwidth is generally narrow (e.g., 10kHz out of a >1MHz band), a radio
    is very insensitive to relatively sharp changes in phase vs. frequency
    (even if that phase shift lands in the middle of the channel)*. Is this a
    diversity application of some sort?

    * Obvious exception: analog TV, which is notably sensitive to multipath.
    One could argue, because this signal is wideband, and more time- than
    frequency-domain oriented, it doesn't count.

    Tim

    --
    Deep Friar: a very philosophical monk.
    Website: http://webpages.charter.net/dawill/tmoranwms

    [OP here]

    To clarify my questions:

    I'm building an AM broadcast receiver to be used in a non-standard
    application. It will use an existing wideband COTS software defined radio
    product that does not provide tuning information to the ferrite antenna.
    So I'd like to be able to get enough antenna gain across the broadcast
    band from the antenna to avoid having to tune the antenna to resonance on
    each frequency. But space limitations dictate use of a ferrite.

    This non-standard application DOES require knowledge of the antenna delay
    vs. frequency.
     
  6. Phil Allison

    Phil Allison Guest

    "George is a TROLL "

    [OP here]

    To clarify my questions:

    I'm building an AM broadcast receiver to be used in a non-standard
    application.

    ** That is not helpful.

    You are obviously yet ANOTHER idiotic code scribbler wasting our time.


    It will use an existing wideband COTS software defined radio product that
    does not provide tuning information to the ferrite antenna. So I'd like to
    be able to get enough antenna gain across the broadcast band from the
    antenna to avoid having to tune the antenna to resonance on each frequency.
    But space limitations dictate use of a ferrite.

    ** Low Q and high gain are incompatible goals.

    Your ideas are totally stupid.


    This non-standard application DOES require knowledge of the antenna delay
    vs. frequency.

    ** Bullshit.


    .... Phil
     
  7. George

    George Guest


    You may be right about going with an untuned antenna. It's worth a try. If a high-gain amp in lieu of a tuned stick will give me the receive gain ofa traditional ferrite, that's all I need. And it avoids phase non-linearity issues which my app is very sensitive to. Besides, I'm sampling the whole AM broadcast band at once and can't tolerate a narrow-band front end dueto the high-Q ferrite antenna.

    Two concerns:

    1. The Websites I have seen talk highly about the performance that is achievable with high-mu ferrite over ordinary loops in the same board space. Makes me wonder if I can get to that antenna gain without ferrite, even witha good amplifier. That amplifier is going to have its own problems like 3IM overload, etc. I understand the high environmental noise argument.

    2. The ferrite loop would have reduced the near-field impulse noise E-field pickup which now may become a problem.

    Well, all I can do is to try it.
     
  8. Jamie

    Jamie Guest

    I think you may want to concern yourself also with front end saturation
    if you're sampling the whole AM band at once. Agc in the IF stage works
    good because it only acts on the selected pass freq.

    I suppose if you are doing this a with a high res DSP you could pull
    out the weak ones with DFT's.

    Jamie
     
  9. Phil Allison

    Phil Allison Guest

    "George is a TROLL "


    You may be right about going with an untuned antenna. It's worth a try.

    ** He says, clutching at straws.


    If a high-gain amp in lieu of a tuned stick will give me the receive gain of
    a traditional ferrite, that's all I need.

    ** It will not.


    And it avoids phase non-linearity issues which my app is very sensitive to.

    ** Your app is bullshit.


    Besides, I'm sampling the whole AM broadcast band at once and can't tolerate
    a narrow-band front end due to the high-Q ferrite antenna.


    ** My god , what is this idiot doing ?

    He won't answer, so you KNOW it is 100% stupid.



    ..... Phil
     
  10. josephkk

    josephkk Guest

    Read thread, disappointed by how many could not avoid resonating the
    antenna.
    Well maybe, but with a Q of no more than 1/2. The biggest problem after
    that is amplify a bit first then filter down to AM band or filter first
    then amplify?
    In either case the filter design will be at least 6th order to be useful
    and that gets you into possibly ugly phase/frequency/delay issues. And
    very likely difficult implementation problems. Inverse Chebychev or
    Bessel may be possible choices.
    After that most of the tough stuff is done, just an serious 16-bit, 10
    MS/s digitizer, and analytical software (oops maybe i spoke too soon,
    sounds like medium sized FPGA).

    ?-)
     
  11. George

    George Guest

    Agree an un-tuned loop or rod followed by an amp is the way we should go. Any suggestions on who to contact for engineering services in order to sub-contract the design?

    You asked about signal levels. The signals are ordinary AM broadcast carriers which should be plenty strong for our needs. And yes, external noise predominates in this application.

    George
     
  12. Phil Allison

    Phil Allison Guest

    "George = IDIOT "Agree an un-tuned loop or rod followed by an amp is the way we should go.
    Any suggestions on who to contact for engineering services in order to
    sub-contract the design?


    ** Try the tooth fairy.

    You asked about signal levels. The signals are ordinary AM broadcast
    carriers which should be plenty strong for our needs.

    ** Horse poo.


    And yes, external noise predominates in this application.


    ** No, insanity does.

    Cos it is all your mad idea.
     
  13. Phil Allison

    Phil Allison Guest

    "John Larkin"
    "Phil Allison"

    ** They were NOTHING like what the mad OP is on about.

    ** Nothing like what the mad OP is on about.

    **** knows what the colossal idiot is really trying to do - radio location
    maybe.

    Can't you tell a TROLL when you see one ?

    ( Rhetorical question .. )

    Hint:

    They ignore all questions and criticisms and only pick the one reply that
    seems to back their fuckwit, secret idea up.


    .... Phil
     
  14. Guest

    While I have not seen the original message from the OP, there are some
    issues.

    1.) A small (relative to the wavelength) loop antenna will have a very
    low radiation resistance (in milliohms), well smaller than the 50/75
    ohm input impedance assumed by most receivers.

    2.) The atmospheric noise level at VLF/LF/MF bands is extremely high,
    so good results can be obtained with very poor antennas (such as
    loopsticks with gains in the -30 .. -40 dB range).

    Tuned loopsticks work quite OK due to the impedance transformation
    between the resonant LC and the pick up coil.

    Trying to use untuned (ferrite) loops at VLF/LF/MF would require some
    very low input amplifier stages (such as hefty common base stages) to
    actually take advantage of the current available in the loop.
     
  15. josephkk

    josephkk Guest

    Better than you might think, it is the dominant property in image
    rejection. Resonated with another section of the variable capacitor that
    generated LO. Q of maybe 50 most likely, more than that would hurt RX
    bandwidth.
    A Hallicrafters is a bit better than an average AM receiver.

    ?-)
     
  16. Phil Allison

    Phil Allison Guest

    "John Larkin"

    ** The OP is crapping on about a device that fits in the palm of the hand,
    samples the whole AM band at once and does some kind of magic trick.

    I would say he was " off with the fairies" but that is a big insult to
    fairies.



    .... Phil
     
  17. Phil Allison

    Phil Allison Guest

    "John Larkin"
    "Phil Allison"<

    ** You are merely crapping on about your own, false imaginations.

    The one thing you are uniquely expert on.

    Yawwnnnnnnnnnnnnnnnnnn.....................



    .... Phil
     
  18. George

    George Guest

    Sounds right. Our spec an measurements agree (in the ballpark) with predicted receive signal level of -80 dBm using a -35 dBi antenna receiving a 1 kW transmitter at 100 km distance.
     
  19. Guest

    This is done (software defined radio) but I think the problem with it is
    overloading the receiver with a strong signal. It's sorta the same problem
    that FFT based spectrum analyzers have.
     
  20. Guest

    But once the front end is saturated, you're dead. There's TOO MUCH noise in
    the AM band.
    FPGAs are *way* too expensive for anything on AM. ;-)
     
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