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Discussion in 'Electronic Basics' started by Kit, May 22, 2006.

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  1. Kit

    Kit Guest

    Hi all,
    I have an idle question. How do receiving antennas work. I have looked
    at Wikipedia and Google but I cannot find an answer. So what is
    actually picked up by the antenna? and how?
  2. BobG

    BobG Guest

    I think its Ohm's law. The carrier has a field strength measured in
    volts/meter. The antenna impedance is 300 ohms, and some small voltage
    in microvolts is picked up to be amplified and demodulated.
  3. Tim Auton

    Tim Auton Guest

    Did you find this page on Wikipedia:

    The first section, "Radio waves", has a decent enough introduction.

  4. Electromagnetic radiation impinges on the antenna, invoking a small
    voltage. Do you know about the three finger rule? You should look
    for literature regarding this, resonance and impedence in antennas.
    This will then lead you to concepts such as gain and the decibel. In
    essence, the antenna is a transducer. On the one hand, it is very
    simple. On the other, there are many details that can take you
    years to learn.

    It is unfortunate that when I tried to learn this from my first
    year college physics books, I was left with many questions. One
    of the other posters pretty much hit it on the nail, as near as
    I can tell. However, there are many details about this you should
    be made aware of if you are thinking to build your own. For
    instance, this equation is only an approximation for the length
    of an antenna:

    Wavelength = speed of light in a vacuum (C) / frequency (F)

    The correct length for an antenna will be somewhat shorter. Director
    and reflector elements will vary from this by a small percent. And
    then, there is a vast amount one can discuss about signal strength
    in various directions and front-to-back ratios in omni vs uni-
    directional antennas. A more thorough answer can be found in the
    ARRL "Technician/general class license manual for the radio amateur".

  5. Don Bruder

    Don Bruder Guest

    OK, you got my curiousity up with that one - I've never heard of a
    "three finger rule" that has anything to do with antennas (or antennae,
    if you prefer) so I went googling. Of the four (Count 'em... FOUR) hits
    for "+antenna +"three finger rule"" I found, two involved situations of
    the "Oops - the GPS on my boat broke, the compass is out, and I need to
    get home. But how far away from home am I?" type. In this case, the
    "three finger rule" involved using three fingers and the known height of
    a distant object (such as a radio broadcast antenna, smokestack, or
    lighthouse tower) to get an estimate of the distance between you and the

    Somehow, I doubt this is the "three finger rule" you're talking about...

    There were two other hits that looked semi-promising, but they were XLS
    files, and I don't run (or even posess any) MicroSoft software, so
    couldn't read them.

    Care to clarify?
  6. Tim Williams

    Tim Williams Guest

    The three finger rule is closely related, in particular, the orthogonal
    directions of a cross product (the two input vectors and their cross
    product), which is very important in E-M since the electric and magnetic
    fields are perpendicular.

  7. Don Bruder

    Don Bruder Guest

    Ahhh... OK. I "know" that one, but I've never seen/heard it named as
    such. The concept is nothing new to me, but referring to it as "the
    three finger rule" is something I've never encountered before.

    I use the term "3 finger rule" because different physics books
    use diffferent hands. Some books describe a "left hand rule",
    while others describe a "right hand rule". They both work as
    long as you make sure to orient the 3 vectors correctly.
    Still other textbooks describe this as a "3 finger rule",
    which is independent of what hand you use. I should warn
    that there are some terrible mistakes in the Wikipedia, and
    much more clear descriptions of this rule can be found in many
    physics textbooks.

  9. Kit

    Kit Guest

    Good, I think I am starting to understand this a little. But how can
    the AC current in the antenna be at say 550 Hz and 570 Hz at the same
  10. Errr, uhhh.... sorry you lost me. Not sure how we got to these
    two frequencies. Could you you send again, maybe re-word this?

  11. The antenna does get many different frequencies on it all the time, all from
    different sources. It is the receiver that will tune into the frequency you
    want to pick up. The rest of them are ignored. The different frequencies
    have there electromagnetic wave at different hights you might say or
    length's, so the length of the antenna elements are cut to best match the
    length of the wave created by the frequency you want to pick up. Most
    antenna elements are cut to a fraction of the frequency, usually 1/4 of it
    so the antenna is a manageable size. By cutting the elements to best match
    the frequency you want to pick up, then that frequency will have the highest
    voltage induced for your reciever to tune into. Hope this helps some. JTT
  12. Rich Grise

    Rich Grise Guest

    The "Left-hand rule" and the "Right-hand rule" are the same thing,
    but one uses conventional current flow, and the other uses electron
    flow. What you do is wrap your fingers around the wire, with your
    thumb extended. If your thumb is pointed in the direction of current
    flow, your fingers show the "direction" of the magnetic field.

    The three-finger rule is used differently - that's for a wire
    moving through a fixed magnetic field, where one finger is the
    direction of current flow, the next finger is the direction of
    the magnetic field, and the third represents motion.

    Applying either of these to an antenna seems somewhat of a stretch
    to me, since they're interacting with electromagnetic fields at
    a distance, but it's pretty much the same principle. The radio
    wave induces a current in the antenna, which causes a voltage to
    appear at the receiver end.

    Hope this Helps!
  13. Rich Grise

    Rich Grise Guest

    That's not hard at all - have you ever heard two instruments playing in
    harmony? It's exactly the same thing, except in current, not sound

    It happens all of the time in your radio antenna - it's picking up
    ALL of the signals from the air, but then you select which one
    you want to listen to, with the tuner.

  14. Bill Bowden

    Bill Bowden Guest

    But how can the AC current in the antenna be at say 550 Hz and 570 Hz
    There is only one AC current flowing which may be the sum of several
    signals. But if the antenna is tuned, it will accumulate energy at the
    tuned frequency and the AC current will be predominately at the tuned
    frequency. Sort of like pushing someone on a swing. You give a slight
    push at the right time and the swing goes higher and higher, which is
    similar to the AC current in the antenna going higher at whatever
    frequency it's tuned to.

  15. BobG

    BobG Guest

    I thought the three finger rule was invented by Bill Gates to restart
    hung up programs.....
  16. Guest

    I always wondered why transmitting antennas act like
    receiving antennas and vice versa. "Reciprocity" certainly
    works, but I've never seen a good explanation of the

    Here's my own explanation of antennas. I've yet to
    encounter similar things elsewhere, so I can't compare it
    against textbooks for accuracy. (The textbooks go about
    things differently.)


    One way to understand antennas is to look only at the
    EM fields and waves surrounding them.

    Suppose we could *see* EM waves. If we illuminate
    an antenna with a parallel beam of radio waves, so
    the antenna is encountering a pattern of plane waves,
    what would this look like?

    Well, first we'd notice that EM waves behave much like
    light, and an antenna would both reflect the waves and
    also cast a shadow. Even if the antenna only scatters
    the incoming waves without absorbing any, there'll still
    be a region of shadow behind the antenna. We might say
    that the antenna "punches a hole" in the planewave
    pattern, leaving a long fuzzy slot in the waves moving
    past the antenna.

    Physics has a simple description for such a process, and
    it applies both to light and radio. Whenever a small
    opaque object casts a shadow, we can describe the object
    as being a wave-emitter ...where the emitted waves are
    out of phase with the incoming waves. The small object
    scatters waves in all directions, like a concentric
    bullseye pattern. During the wave-scattering process,
    the object absorbs incoming waves and then re-emits them
    in a spherical pattern. And downstream from the object,
    the two waves partly cancel, forming a shadow region.
    In this shadow the sphere-pattern of scattered waves is
    being subtracted from the incoming planewaves. Or in
    other words, there is an interference pattern in the
    waves surrounding the object, with one of the minima
    forming the shadow in the wave pattern behind the object.

    The above explanation contains a central concept for
    understanding receiving antennas:

    In order to absorb waves, a receiving antenna *must*
    emit waves.

    This might sound impossible. But as long as no energy
    comes magically from nowhere long as the total
    energy passing out of a closed surface surrounding
    the antenna is zero or negative, we're not breaking the

    In order to receive, antennas must transmit.

    See what's coming next?

    Because a receiving antenna interacts with incoming EM
    waves via an emission process, therefore the physics of
    transmitting antennas is a subset of the physics of
    receiving antennas. If we can figure out how an antenna
    can emit waves, then we'll know how the same antenna
    can act as a wave-absorber.

    The explanation isn't complete yet, since our antenna
    could very well be a perfect conductor which only redirects
    waves without absorbing any. A superconducting antenna
    would still radiate a sphere-wave pattern and still cast a
    shadow, but it could only send out as much energy as it
    absorbed, so it wouldn't receive any EM signal on average.

    The above explanation makes lots more sense if you can *see*
    what's going on. Get a couple of Moire transparencies, one
    with fine dark parallel lines, and another with a bullseye
    pattern of concentric circles with the same spacing as the
    parallel lines. Overlap them to produce the interference
    pattern, then slide them a bit so you get a minimum or
    "shadow" which extends behind the center. This shows
    how an opaque object blocks waves by (re)emitting a wave
    pattern which produces a shadow by wave cancellation.

    In the shadow, where did the missing waves go? Clearly
    they're part of the maxima lobes going off in other
    directions. But if our antenna was actually absorbing
    energy on average, the shadow would be a little bit bigger,
    or the forward-scattered waves a little bit smaller.
    The missing energy would end up inside the radio receiver
    sitting at the center of our antenna.

    ((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))
    William J. Beaty Research Engineer
    UW Chem Dept, Bagley Hall RM74
    Box 351700, Seattle, WA 98195-1700
    ph425-222-5066 http//
  17. Tim Williams

    Tim Williams Guest

    Indeed! But[1] the case can also be made that a transmitting antenna can
    also recieve: consider that external waves cause a voltage/current, well
    energy on the resonant antenna element(s). If there is no loss, then the
    energy will be in equilibrium with the incoming waves. But since it's a
    transmitting antenna, this energy is also being radiated. In effect, we
    have a diamagnetic system, reflecting the incident signal, like a magnet
    floating on a superconductor.

    This isn't that bad an analogy, since magnetic fields do play a role, and
    the conductors in an antenna are effectively diamagnetic to AC signals
    (Lenz's law).

  18. Tim Williams

    Tim Williams Guest

    [1] "But" because you present your statement in the inverse direction. That
    I present my case opposite is just semantics, of course, and only serves to
    further prove the reciprocity of the case. ;-)

  19. Rich Grise

    Rich Grise Guest

    Think of the two antennas as the primary and secondary of a HUGE air-core
    transformer, with a very small mutual inductance. Electrically, it doesn't
    matter which is the "primary" and which is the "secondary" - the EM field
    can go either way.

    Hope This Helps!
  20. Guest

    Diamagnetic levitation is also great for illustrating another part
    of antenna theory.

    We might *say* that an antenna absorbs EM waves and then re-
    radiates them. But in fact the absorption and radiation processes
    are simultaneous. In diamagnetic levitation, whenever a magnet
    approaches a conductor, the conductor essentially responds
    instantly: as the magnet approaches, the current in the conductor
    rises. And as the current rises, the conductor creates its own
    b-field which repels the magnet. So whenever an externally-
    produced magnetic field hits a conductor, the conductor's own
    field and current appears at the same time.

    The same applies with voltage, charge, and metal mirrors: when
    an e-field impinges on a perfectly conductive metal plate, the
    movable charges within that plate will smoothly change their
    position to produce an exactly opposite e-field, in order to
    "short out" the part of the incoming external e-field that's
    parallel to the metal surface. The charges move in such a way
    that they zero out any voltage measured parallel to the metal
    plate, so as the incoming e-field changes, the charges move at
    the same time, keeping the voltage always zero.

    If all of this obeys the conservation of energy, then the EM waves
    emitted by the metal plate must be out of phase with the incoming
    waves, so they subtract from the incoming waves to create a shadow
    behind the plate. In other regions they produce what looks like
    "reflected" or "scattered" waves.

    When we think of mirrors, we think of reflected waves. But to be
    accurate, we should be thinking of "simultaneously-re-emitted
    waves." The mirror emits radiation that magically creates a
    shadow behind the mirror, and also creates something *resembling*
    reflected/scattered waves ...but which actually is some waves
    emitted by the mirror.

    So to understand receiving antennas, we have to see them as
    emitters, emitters which try to match the fields of the incoming

    Here's another piece of the puzzle. If the antenna only
    emits part of the energy it receives, and keeps the rest inside
    itself, then it's acting as an absorber. And if it swallows
    up *exactly half* of the incoming energy and radiates the rest,
    then in that case it absorbs the maximum possible EM energy.

    See equation 989 on this page:

    Antenna directivity and Effective Area

    So any simple dipole receiving antenna must, at best, throw away
    half the incoming EM energy in the form of "scattered waves."

    ((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))
    William J. Beaty Research Engineer
    UW Chem Dept, Bagley Hall RM74
    Box 351700, Seattle, WA 98195-1700
    ph425-222-5066 http//
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