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2nd freq to tuned ant

Discussion in 'Electronic Design' started by Nathan Minos, May 16, 2007.

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  1. Nathan Minos

    Nathan Minos Guest

    Can anyone tell me what happens if you power a dipole antenna with a
    continuous sinewave at its resonant frequency and then apply a second
    sinewave that is slightly offset?

    To quantify, let's say the resonance is 1Mhz and the second frequency
    is 100Hz less. Would a 100Hz beat be transmitted. Does the tuned
    antenna act as a modulator or is the product more complex?

    Thank you for any insights.

  2. Bruce Varley

    Bruce Varley Guest

    An antenna is a linear device ignoring extreme conditions, so it deals with
    each frequency component independently. If a component is not at the
    resonant frequency of the antenna, then the antenna will appear as a
    reactive load at that frequency, and radiation efficiency will be lower. You
    wouldn't notice 100 Hz difference in 1 MHz, it's tiny, so if you looked at
    the waveform of what's transmitted using a scope and a receive antenna, what
    you'd see would be the same as the waveform fed to the transmitting antenna.
  3. Guest

    The antenna itself is a linear device, so the two frequencies
    shouldn't mix and and you should not see any 100Hz difference tone if
    your drivers and receiver are all close to ideal

    The circuits driving the antenna probably aren't going to be all that
    linear, so you may get some mixing in their output stages, and your
    receiver may also have a slightly non-linear response (unless you over-
    drive it, in which case you will see loads of mixing).
  4. However, the signal is also exactly identical to a 0.999950 MHz
    carrier, double side-band modulated with 50 Hz. An AM receiver with
    the classical diode detector would produce a distorted 100 Hz, while
    one with a product detector would let you hear a pure 50 Hz tone.

    The antenna doesn't care. That much is true.

    Jeroen Belleman
  5. Don Bowey

    Don Bowey Guest

    No, regardless of the type of detector, both receivers will produce the 100
    Hz. Difference signal.

  6. Tom Bruhns

    Tom Bruhns Guest

    Looks like you didn't carefully read between the lines in Jeroen's
    posting. ;) Clearly, he had in mind a product detector driven by a
    1MHz-50Hz LO. On the other hand, he better be careful about the
    phases of the signals. If the product detector is driven by a LO of
    sin(2*pi*999950*t), and if the 1MHz is sin(2*pi*1e6*t), the difference
    is a cosine: A*cos(2*pi*50*t). If the 999.99kHz signal is
    sin(2*pi*999900*t) the difference is the same cosine term, so the
    total output is 2*A*cos(2*pi*50*t). But if the 999.99kHz signal is
    the opposite polarity, -sin(2*pi*999900*t), then the outputs from the
    two cancel and you get zero. I suppose most receivers that use
    product detectors either have some carrier to lock their LO to, or
    just receive the signal as a single sideband and suppress the other--
    or just detect the two sidebands independently.

    Although mixing in the output amplifiers is tough to avoid if you just
    blindly combine the signals, you can use a circuit to keep the
    signals out of the alternate amplifiers. One such circuit is a
    Wilkinson combiner. Problem: it wastes half the power. It could be
    done with filters, but Qu would have to be very high (incredibly
    high?) to avoid significant power loss and get good isolation, given
    such close frequency spacing.

  7. Don Bowey

    Don Bowey Guest

    He stated "...the signal is also exactly identical to a 0.999950 MHz
    carrier, double side-band modulated with 50 Hz." The OP's signal is NOT
    identical to what he described.

    Yes, but if you re-read the original post, you will see your comments are
    beyond the topic. Good, but not relevant.

  8. Nathan Minos

    Nathan Minos Guest

    OK, what if the antenna was non-linear? I am thinking here of a plasma
    tube, fed with two non-earth referenced sinewaves (single wire each)
    from either end. If one frequncy is resonant and one is not will they
    modulate, and to what extent?

    Or, each frequency could be equidistant from the resonance. For
    example, if the resonance is 1MHz, one would be 100Hz more and the
    other 100Hz less.

  9. Don Bowey

    Don Bowey Guest

    OK, playing "what if," if the antenna were non-linear, multiplication would
    generate, among lesser amplitude signals. 200 Hz. This would not propagate
    well at all from a 1 MHz antenna.

    Have you been lurking on s.e.b? You're Radium, aren't you?
    The detected or demodulated signal of the two frequencies would simply be
    200 Hz.

    You should go away and study Amplitude Modulation.

  10. LVMarc

    LVMarc Guest

    non linear system, if you put in two sine wave you get a plethura of sum
    s and differences.

  11. But it is!

    sin(1MHz) + sin(1MHz-100Hz) = 2 * sin(1MHz-50Hz) * cos(50Hz)

    Elementary mathematics. I stand by my original statement.

    Jeroen Belleman
  12. oopere

    oopere Guest

    The antenna is fully linear. As a consequence, this antenna would
    transmit the two frequencies -nothing else: you end up with two tones in
    the transmitted spectrum. There are no mixing effects (at least not for
    any reasonable transmitted power)

  13. Don Bowey

    Don Bowey Guest

    Yours is only usable elementary math after you have clipped the posts to
    eliminate thee correct relevant information.
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