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Question about antennas

Discussion in 'Electronic Basics' started by bench, May 18, 2004.

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  1. bench

    bench Guest

    Hello all,

    I have a question about how do antennas actually work. My problem
    is this : normally a voltage source has a plus terminal and a minus
    terminal. Now, ... if we look at a standard fm telescopic radio antenna
    all what we have is a piece of metal, and the end bit which physically
    connects to the radio is also the voltage source. So do we have here
    a one terminal voltage source?
     
  2. John Miller

    John Miller Guest

    Could it be.........that you're overlooking the relationship of the antenna
    to ground?

    --
    John Miller
    Email address: domain, n4vu.com; username, jsm

    It is easier for a camel to pass through the eye of a needle if it is
    lightly greased.
    -Kehlog Albran, "The Profit"
     
  3. bench

    bench Guest

    But the radio is not connected to ground, the ground of the radio is the
    negative of the battery, isn't it ?
     
  4. Don't get caught up in terminology !!
    There is a big difference between a circuit 'ground' and a radio 'ground.
    I would think it best for you to read up on the theory rather than just ask
    "how" here. The questions to ask here would be along the lines of help in
    understanding what you are studying.
    Best of luck
     
  5. Joe

    Joe Guest


    Hi bench,

    I am not sure if this is an answer to your question, because I just started
    studying RF stuff. I have my amateur radio tech license (just this past
    Feb), and I have a copy of the Radio Amateur Handbook for 2004 (very
    reasonable priced at US $24.95). I got my license so I could homebrew RF
    stuff without getting into trouble with the FCC.

    What you are describing is a 'vertical' antenna. I have used them in my
    latest remote control project which uses 433MHz modules ( a ready made
    transmitter and receiver which can be mounted on a pc board along with some
    other stuff). So far I have tested the system with a loop type antenna and a
    vertical (as you described).

    The loop antenna is sort of a directional antenna and is just a loop of 20ga
    wire with a capacitor coupled to the 'hot' end (the end which is connected
    to the antenna pin on the module), and the other end connected to the
    battery ground. I get pretty good range with it, but I get better range with
    the vertical, which is a piece of 20ga wire sticking straight up out of the
    transmitter box, and coupled to the antenna output of the module thru a
    capacitor (per the AR handbook). The wire length overall is 15.5 cm which is
    supposed to be close to a quarter of the wavelength of the radiation. I
    experimented with the transmitter at different heights above the ground in
    the backyard. The higher the transmitter antenna was, the better the range.
    I think the 'ground' is actually the earth when you use a vertical antenna.
    The closer it is to the earth, the less signal gets to the receiver. So the
    earth is the ground, at least it seems that way from what I have observed.

    In both experiments I used a vertical antenna (same, quarter wave) on the
    receiver and kept the receiver at a constant height. The receiver flashed a
    10mm jumbo LED when it received the signal so I could measure the distances.
    I conducted most of the range measurement experiments close to dusk and
    after dark so I could see the LED.

    I am a hobbyist, so if my brain didn't interpret the results I observed
    correctly, I hope someone on this forum who is more knowledgeable about
    these things will correct me. I am still experimenting and open to opinions.

    Also, you may want to post your question to a group called
    'rec.radio.amateur.homebrew' if your ISP provides it.

    hth,
    Joe
     
  6. bench

    bench Guest

    My question is very general, not about radio amateurs. Do you have
    a portable fm radio, if you do you will know that it has got what is
    known as a telescoping antenna (I assume because it is opened
    like a telescope). When I opened my radio, I saw one wire coming
    out of the antenna and connecting to the circuit board. Now, ...
    my question is this, if we want to apply a signal source to some
    amplifier, we need a plus and a minus i.e. any voltage source
    should have two connection points. So if the antenna is considered
    as a signal voltage source, how can it have one wire. I consider
    my question to be really focused so I think most people would
    agree it is relevant here.

    Thanks
     
  7. Jay Davis

    Jay Davis Guest

    http://www.electronics-tutorials.com/antennas/antenna-basics.htm
     
  8. Joe

    Joe Guest

  9. bench

    bench Guest

    Please share your answer to my question
     
  10. MG

    MG Guest

    The common node of your portable radio is an extended surface. Is the "GND"
    plane of the PC board to which a lot of metal parts like connectors are
    attached. All this stuff has a capacity just by sitting in the middle of the
    empty universe, also has a capacitance to earth.
    At RF frequencies this capacitance is sufficient to close the circuit to
    earth or to establish a reference voltage anyway should earth be far away.

    At 200kHz Long Waves a better GND is needed but at FM 100MHz even a small PC
    board is enough.

    MG
     
  11. L. Fiar

    L. Fiar Guest

    I see your problem, it seems to be about perception. The antenna
    signal does use a ground, and that ground is part of the antenna
    system... you just have to recognise it.

    A loop antenna should be easy. Being balanced, it does not have a
    ground, the two ends are equal but opposite and DC coupled.

    The basic dipole antenna is also balanced, with two sections, each
    having an equal but opposite signal upon it.

    With the ground plane antenna, this may appear to have just a single
    vertical element, but has an image reflected in the ground. The
    reference point is the ground.
    Your problem is in the concept of that ground, which may be true
    ground or an artificial ground made up of a plate, radials, etc.
    In the case of a car, the vehicle body is that ground. There are
    antennas that have radials pointing out from the base of the
    antenna - this is then the ground.
    Efficiency is traded off against practicality and, in some cases, the
    ground may be as small and simple as the radio ground. Not the most
    efficient situation, but practical for portable equipment.

    There are several radio handbooks available, which may go into more detail
    on RF radiation, propagation, and antenna theory. The Internet is fine, but
    nothing beats a good technical book.
     
  12. L. Fiar

    L. Fiar Guest

    antenna.

    It can be the earth, but there are antennas that use an artificial ground...
    maybe a plate, radials, or something similar.
    There can be a couple of reasons for this...

    First of all, view point - the antenna will be clear of obstructions that
    may cut down the signal, and you can see further around the earth from
    higher up.

    Secondly, the distance between an antenna and it's ground will affect both
    radiation resistance and radiation pattern. At certain heights, more signal
    is radiated up into the sky, but at the correct height more goes out where
    you want it - across the earth.
    Also, there are many more Amateur groups, like:
    rec.radio.amateur.antenna
    rec.radio.amateur
     
  13. bench

    bench Guest


    Believe me I have looked in books, they do not have
    what I am asking. Even the link posted here talks
    about antennas but does not answer my question.
    Again, I have seen many interesting
    answers here so I will reply here rather than reply to
    all the senders. Firstly people seem to be talking about
    loop antennas. Can we get a fix on terminology, is
    a loop antenna the same as an fm telescopic antenna
    because if it isn't then sorry but I am not really
    interested in loop antennas.

    Now, let me explain, ... if you connect a coax cable
    to a t.v. vhf antenna, the antenna has two points and
    this is very sensible, since the voltage is induced on
    the two ends of this ellipse-like loop. Now, ... with
    my fm radio there is this piece of extendable antenna
    and it seems that there is one wire coming out of it
    if one opens the cover and looks inside. Supposing
    I wanted to connect a coax to this antenna, I have
    only one wire, this is my question, where is the other
    wire because a voltage must be between two wires.

    Now, ... I think your answer was that the circuit
    board, being made up of a lot of copper wires,
    becomes the other polarity of the antenna, is that
    what you mean, if so, this may make sense
    because also in the Half Wave Dipole Antenna
    as seen in Jay's link the two halfs of the antenna
    are not physically connecetd.

    I think the thing that is confusing me is that in t.v.
    vhf antennas both sides of the output come from
    the same piece of metal, i.e. they are physically
    conneceted, whereas with a Half Wave Dipole
    Antenna the two sides are physically isolated,
    there is something not intuitive about this.
     
  14. John Miller

    John Miller Guest

    I understand your perplexity. When you try to figure it out based on an
    understanding of how DC behaves, it makes no sense at all.

    RF is a different breed of cat. A length of conductor, continuously excited
    by RF of a sufficiently high frequency relative to its length, will have
    different AC voltages and currents along its length, ranging all the way
    down to zero volts or amps. So you can see why DC thinking can mislead you
    in an RF world.

    --
    John Miller
    Email address: domain, n4vu.com; username, jsm

    I get my exercise acting as pallbearer to my friends who exercise.
    -Chauncey Depew, who lived to ~94 years of age.
     
  15. I think L. Fiar answered your question very well. You have it exactly
    right. There is no monopole antenna. If your FM radio were reduced to
    the size of a pea, but the telescoping antenna stayed the same, it
    would cease to be effective as an antenna.

    If you put a piece of metal floating in the air, it is passive: it
    resonates at multiples of it's length, just like an acoustic
    resonator. If it is a perfect conductor, it absorbs no power. At 1/2
    wavelength, the voltage is highest at the ends (and 180 degrees out of
    phase), and the current is highest in the middle.

    If you want to make an antenna out of it, you neeed to break it in the
    middle and tap into that current flow. The current flows into a load
    (the receiver) and now, the antenna absorbs power.

    In the case of a 1/4 wave antenna with ground plane or ground radials
    .... look at it like this: Place a wire vertically on a reflective
    surface and look at the wire and it's reflection. You will see the
    dipole. At radio frequencies, the radials (or your radio) look like a
    mirror.

    hth

    Frank Raffaeli
    http://www.aomwireless.com/
     
  16. Joel Kolstad

    Joel Kolstad Guest

    That web site doesn't really answer his question.

    Jay: The 'other wire' (ground) is some ill-defined mash of the circuit
    ground in the radio. If you're familiar with the idea of an antenna above a
    ground plane (and a 'whip' antenna as a voltage source with its ground
    referenced to that plane), they just start thinking of taking that ground
    plane and making it finite and smaller and smaller until it's, uh, all the
    ground traces of the radio's circuit board. At that point, the exact
    radiation pattern is anyone's guess, although often times whip antennas are
    electrically short and the radiation pattern is still something resembling a
    bagel.

    Another way to think of this: Say you have a dipole antenna that you're
    using as a 'voltage generator' by taping the middle of it, Vant+ and Vant-
    (like figure 3 on the web site listed). Now start sliding the the tap point
    down... further, further, further, until you're all the way at one end!
    This is called an 'end fed' antenna (see:
    http://www.qsl.net/aa5tb/efha.html) and can be analyzed rigorously.

    ---Joel
     
  17. Joel Kolstad

    Joel Kolstad Guest

    A 'classical' way to think about this is that the loop antenna is being
    coupled to inductively (a current is flowing around the loop) whereas with
    the dipole the antenna is being coupled to capacitively. Think about what
    happens if you have, say, a sine wave generator connected to a capacitor,
    then a load resistor, and then another capacitor back to the generator: The
    sine wave will 'go through' the capacitors and appear across the resistor,
    even though there's one two plates involved in (each) capacitor. Now, move
    the plates a thousand miles away and everything is (to a very crude
    approximation) still the same other than changes in magnitude, phase, etc.

    The bottom line is that if you stick any 'bits of metal' in space and
    connect them together through a load, the fields in the air will start
    inducing currents through the load. Exactly how 'well' that occurs depends
    on the geometry of those 'bits of metal,' and the well-known cases of
    dipoles can be readily analyzed. You cannot realistically get anything more
    than an educated guess as to the _exact_ radiation pattern of, say, the
    antenna on a cell phone... although with a computer with lots of horsepower
    and memory and time on your hands, you can solve Maxwell's equations
    directly and figure it out.

    ---Joel
     
  18. Joel Kolstad

    Joel Kolstad Guest

    I beg to differ... end-fed antennas (just about) fit this description and do
    work well. See the link posted previously:
    http://www.qsl.net/aa5tb/efha.html . This doesn't stop people from claiming
    that end-fed antennas shouldn't work at all... the main difficulty being
    that, while the "ideal" end-fed antenna has infinite impedance and therefore
    can't accept/output power, real end-fed antennas (or any antenna fed at the
    minimum current/voltage maximum) tends to have an impedance more along the
    ballpark of some kilohms, which can actually be more desirable than 50 ohms
    at times.

    You can't get that 'some kilohms' answer from the 'ideal dipole' equations
    with simplifications, however (there's where the infinite impedance answer
    comes from)...you need to solve something like Hallen's or Pocklington's
    integral equation, which -- since they can only be done numerically
    anyway -- immediately leads one to just sit down and use NEC or a similar
    computer program to perform the analysis.

    ---Joel
     
  19. Hi Joel,

    It's true that my example did not account for end-fed antennas, where
    the feed is at (or near) the maximum voltage point; however, if the
    circuitry and feedline attached to the antenna were reduced to "pea"
    size, it would still cease to be an effective antenna because the
    voltage gradient along the "pea" would be nearly zero, thus giving the
    antenna nothing to drive against. In this very hypothetical example,
    the antenna would still be ineffective. In the real world, there is
    always something attached to the pea (or the receiver or transmitter
    has significant size relative to a wavelength), thus completing the
    total antenna. In the usual case, an end-fed antenna would be as
    _practical_ as a center-fed antenna.

    The feedline and radio contribute to the total aperture, and therefore
    become part of the antenna, unless the feed is balanced and matched.

    Frank Raffaeli
    http://www.aomwireless.com/
     
  20. L. Fiar

    L. Fiar Guest

    Yes.
    It is quite common to use ground rods or a car body
    as the ground element, but that is not practical with
    a portable radio. So, you make do with what
    you can carry around.
    No connection as far as DC is concerned, but there is capacitance.

    To compare with a loop antenna...
    As far as DC is concerned, you can see a short circuit. As far as the RF is
    concerned, that loop has an impedance.
    In the case of a dipole, you see an open circuit, but the RF sees 75 Ohms
    between the elements.

    For the ground plane antenna, at DC it is an open circuit, but the impedance
    to the RF signal very low, lower than a dipole. Where an antenna uses
    radials for the ground, the angle of these radials affects the impedance.


    I may have found a Web reference for what you are looking
    for...

    <quote>
    How can we axplain the fact that you can drive current
    into a dipole when the ends are open circuited...
    </quote>
    The document goes on to ask:
    "What about a monopole, does it need just one
    part to radiate?"

    http://www.hottconsultants.com/pdf_files/dipoles-1.pdf
    The site is directed at EMC issues, which is why it then goes on to refer to
    EMC.

    There's a couple more parts to the dipole document on that site:
    http://www.hottconsultants.com/pdf_files/dipoles-2.PDF
    http://www.hottconsultants.com/pdf_files/dipoles-3.PDF
     
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