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70Mhz FM amplifier circuits..

Discussion in 'Electronic Basics' started by Erwin Maes, Dec 26, 2004.

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  1. Erwin Maes

    Erwin Maes Guest


    does anyone have a link to a circuit for building a 20W RF amplifier
    (FM) for my RF plane?I'd like to fly with the live cam on beyond my line of sight, but the
    transmitter has only about 0,8W power, so the range is about 1000 metres.
    I'd like to extend it to about 5000 metres.
    Most FM amplifiers use the radio frequencies of 88-108MHz...

    Any links, tips are welcome!

    A merry christmas and a happy new year to you all!
  2. SioL

    SioL Guest

    There are other users of that frequency band out there. Imagine everyone
    else with such power/range, it would be a mess (knowing there is a limited
    number of channels available).

  3. Erwin Maes

    Erwin Maes Guest

  4. John Fields

    John Fields Guest

  5. Erwin Maes

    Erwin Maes Guest

  6. The FM amplifiers should work at that lower freq with lower output, of
    if you retune it or pad the capacitors, it should put out more power
    than in the FM band. Remember, you may be violating the power limits
    for the band.
  7. Miles Harris

    Miles Harris Guest

    Ground-to-air 20W will get you *much* further than you either need nor
    want! Your 800mW should be plenty; you simply need a more efficient
    antenna system! Take out the telescopic whip and loading coil and make
    up a simple half wave dipole. That should get the job done. Also check
    out your receiver/antenna set up in the plane isn't 'deaf' - that's
    the other side of the story that's equally important!
  8. Erwin Maes

    Erwin Maes Guest

    Ok, thanks!
    I'll try that!
    Will be more legal too I suppose ;.)
    I also thought of replacing the antenna of the transmitter with a sort
    of dish, pointed towards the plane.
    Don't know if 70MHz can be directed with a dish, as sattelites use GHz
  9. The higher the frequency, the more radio signals start to behave more
    like light, which can be reflected and directed with mirrors.

    At 70 MHz the radio signal are basically sent out in all directions and
    you cannot use dishes to create a beam like you can at 10 GHz.
  10. Sure you can. The wavelength and reflector size just have to remain
    in the same proportions.
  11. Alright, let's say we build a parabolic reflector for 70 MHz, a kilometer
    or so in diameter, do you say we can create a 70 MHz beam which can
    travel to Saturnus and it still keeps together as a beam?

    Won't it diffract a lot worse than a 10 GHz beam?

    What kind of feed horn would you recommend for such a kilometer size
    parabolic dish?
  12. That is what I understand to be the case.
    Any beam of plane waves that is launched at X wavelengths wide will
    have the same divergence angle.
    Not if the beam width is proportional to the wavelength.
    One sized proportionately to wavelength, of course.
  13. I have always had the impression that electromagnetic waves change
    behavior and characteristics with the frequency, because I had heard that
    radio waves become more particle-like at higher frequencies.

    At visible light frequencies we talk about wave-particle duality.

    So the waves would have to be more directive at higher frequencies, and
    behave more and more like particles.

    Now I realize these photons can very big, so the long wave radio
    signals consist of kilometer sized photons.

    Okay, you are probably right, it is just a question of scale.
  14. SioL

    SioL Guest

    You can, but a dish would be the size of the palomar radio telescope...

  15. Miles Harris

    Miles Harris Guest

    Yeah, but the diameter of the horn would have to be _enormous_ at
    70Mhz. He could always try a multi-element Yagi, of course, and just
    point it towards the plane.
  16. Now we get to the practical intersection of physics and the range of
    materials available versus the size of our opposable thumbs. For some
    wavelengths, in the range of of centimeters to hundreds of nano
    meters, we have tools and materials to create efficient and reasonably
    expensive reflectors and launch mechanisms to form plane wave beams
    many wavelengths wide. For bigger wavelengths, the size of the
    mechanism becomes large and expensive and for smaller wavelengths, the
    materials that reflect well and the difficulty in making a reflector
    that is smooth on the scale of wavelengths is difficult and launching
    organized waves gets tough.
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