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Reflecting and standing waves in wires

Discussion in 'Electronic Design' started by Danny, Dec 30, 2003.

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  1. Danny

    Danny Guest

    Hi

    I have been told by numerous people of the first idea opposed to the
    second here: Radio waves are transmitted by standing waves in the wires,
    and not because of current flowing up the wire sue to its capacitance
    relative to earth causing electromagnetic waves which then form the
    radio transmission.

    Can someone please explain whether i have just been horribly mislead all
    the years or whether even the understanding the i have been persuaded of
    is wrong in which case the real explanation would be really nice.

    The main flaw that i have the the idea i have been afflicted with is how
    can the charge be reflected back down the wire for form this standing
    wave. Also, how do standing waves work with electricity and stuff like.

    Danny
     
  2. I read in sci.electronics.design that Danny <>
    This is true for resonant antennas, such as dipoles.

    Standing waves occur because when the current injected at the driving
    point reaches the end of the driven element it has nowhere to go but
    back along the element.
    This is true for non-resonant antennas, much shorter than a wavelength.

    So you have been misled by being told that two different explanations
    are *always* true, whereas they are true under different conditions.
     
  3. Mac

    Mac Guest

    I just want to point out that there are varying degrees of standing wave.
    If you have a source, transmission line, and antenna that are all
    perfectly matched to the same impedance, then there will be no standing
    wave at all in the transmission line. But if there is an impedance
    mismatch, then there will be a standing wave. If the standing wave gets
    too big, then you are not delivering much power to your antenna, and you
    could even damage your transmitter. You could also get very high voltages
    that could cause arcing in your transmission line.

    Mac
     
  4. Wayne Shanks

    Wayne Shanks Guest


    RF energy flowing in a transmission line is ideally a traveling wave,
    not a standing wave. A Standing wave is actually the superposition of
    two traveling wave, traveling in opposite directions (as you implied),
    thus the standing wave component on a transmission line represents a
    zero net transmission of power. Standing waves are generally to be
    avoided. In resonant antennas a standing wave is used to increase the
    oscillating charge, thus increase the launched electromagnetic power.
    Power splitters and impedance transformers use standing waves to move
    power on Tx lines from one characteristic impedance to another, so
    standing waves are useful, but power is transmitted on traveling waves.

    Like electromagnetic waves in air, electromagnetic energy in a Tx line
    is carried as the energy stored in its electric and magnetic field.
    This assumes that the wavelength in the tx line is much smaller than the
    Tx line length. If the Tx line length is much shorter than the
    wavelength, then it can be treated as a wire conductor that carries current.


    Wayne s
     
  5. Robert Baer

    Robert Baer Guest

    Actually, the waves are standing for two reasons:
    1) they cannot sit down
    2) the FTL component was stolen by the idiot selling FTL cables (how
    else do you think he does it?)
     
  6. rick

    rick Guest

    It's certainly not true that the source, line and load ALL need be matched
    to prevent standing waves. Only if there is a mismatch between the
    transmission line and the load will there be a reflection and hence standing
    waves - so to prevent standing waves, you only need to match the load to the
    line - the source is irrelevant.
     
  7. That's what I was brought up to believe, IIRC. The idea being to trim
    your dipole (for example) to a resonant length so that the voltage
    nodes (or is it current nodes) are at minima at the tips of the dipole
    (IIRC it's the *voltage* nodes thust must be minimised) so that all
    the available power is radiated and none reflected back towards the
    transmitter. I also STR that standiing waves on antennas can cause
    radiation from the feeder as well but it's not my field so I'll shut
    me cakehole.
    x-posted for a better take on the subject.
     
  8. Mac

    Mac Guest

    You are quite right. My apologies. RF sources typically need to be
    designed for a specific load (usually 50 Ohms) but the source doesn't
    actually need to have a particular impedance to avoid a standing wave.

    On the other hand, if there is a standing wave, the severity of the
    problem will be somewhat reduced if the source impedance matches the line
    impedance.

    This is relevant to 75 Ohm video signals as well. The datasheets for
    video drivers always seem to show a 75 Ohm series resistor. The purpose of
    the resistor is to make sure that reflections coming back from the load
    don't get re-reflected at the source.

    Mac
     
  9. Reg Edwards

    Reg Edwards Guest

    What is your ultimate design objective - maximum radiated power?

    In any ordinary relatively low loss system, from transmitter to free-space,
    it is necessary only to match impedances at one point. When this done an
    impedance match occurs automatically, simultaneously, at every other point
    in the system. To think in terms of minimising standing waves and
    reflections can be misleading.

    Wait for it - wait for it !
     
  10. Take it easy, Reg. This is only a Usenet forum. Don't let your
    Pacemaker get unduly excited. :)
     
  11. Active8

    Active8 Guest

    It's certainly not true that the source, line and load ALL need be matched
    to prevent standing waves. Only if there is a mismatch between the
    transmission line and the load will there be a reflection and hence standing
    waves - so to prevent standing waves, you only need to match the load to the
    line - the source is irrelevant.[/QUOTE]

    You are quite right. My apologies. RF sources typically need to be
    designed for a specific load (usually 50 Ohms) but the source doesn't
    actually need to have a particular impedance to avoid a standing wave.

    On the other hand, if there is a standing wave, the severity of the
    problem will be somewhat reduced if the source impedance matches the line
    impedance.[/QUOTE]

    All true, but just so no one gets the idea that matching the source
    to the line is never important, remember that it's done for maximum
    power transfer. Also, the transducer gain of the amplifier is
    dependent on the match and the input impedance of the amp is
    dependent on the output impedance and the reverse transfer
    admittance or reverse transmission coefficient depending on what
    perameters you're using. That's just the $20 way of saying that if
    you change the design load, you change the gain and input
    impedance.

    Of course, if you need less gain than is available with a given
    device, you can purposely mismatch the output to the load

    With power transistors in the final, you'd deal with the large
    signal impedance parameters.
     
  12. Reg Edwards

    Reg Edwards Guest

    Old Wives' Tales.


    The SWR meter located at the output of the transmitter does NOT measure
    standing waves on the transmission line to the antenna.


    Most of you are fooling yourselves.


    The instrument indicates only WHETHER or NOT the transmitter is loaded with
    the correct design value of resistance, usually 50 ohms. Which, under
    normal operating conditions, is all anybody needs to know anyway.


    And the internal impedance of the transmitter has nothing to do with it.
    Even the transmitter designer has no interest in what the internal impedance
    is. It is NOT the designer's objective to maximise output power by matching
    the internal impedance to the load.


    The instrument is mis-named. It doesn't measure anything. Its correct
    description should be TLI, Transmitter Loading Indicator.
    ----
    Reg.









    --
    ............................................................
    Regards from Reg, G4FGQ
    For Free Radio Design Software go to
    http://www.btinternet.com/~g4fgq.regp
    ............................................................
     
  13. It certainly curious that this subject more than any other seems to
    generate a huge amount of confusion. There are so many conflicting
    views on this issue it's hardly surprising some folk get all mixed up
    over it. Perhaps you should write a definitive essay on it and stick
    on your web site, Reg!
     
  14. Rick

    Rick Guest

    Unless your SWR meter happens to be a scalar reflectomer.
    It's cheaper than paying someone to do it.
    So your SWR meter "doesn't measure anything", but at the same time it
    tells you all you need to know. Spooky.
     
  15. Reg Edwards

    Reg Edwards Guest

    So your SWR meter "doesn't measure anything", but at the same time it
    ======================================

    Measuring is not the same as merely distinguishing between good or bad.


    Delete the SWR meter scale and replace it with a coloured band, changing
    from green at one end, via brown, to red at the other.


    One of these miniature, 1/2" square, units will do fine.


    Or just a red and a green LED, both in one package.


    Get yourselves into the 21st Century.


    But then, it must be admitted, there'd be only the weather to argue about.
     
  16. Within my limited knowledge on the subject, the SWR meter simply tells
    you the ratio between forward and reflected power at the point in the
    line the measurement is made. The smaller the degree of reflection,
    the better; ideally 1:1 but in practice ISTR that even at 3:1 most of
    the power is still usefully radiated.
     
  17. Reg Edwards

    Reg Edwards Guest

    The SWR meter located at the output of the transmitter does NOT measure
    ===========================

    Done! See one-liner above.
     

  18. Sorry, Reg. Can you repost the link? Nothing appeared here.
     
  19. Hello Paul,
    just read carefully what Reg has written, and if you happen to
    come across a 100 metre drum of coax cable try this experiment.

    Tx-VSWR meter-----------long coax cable------------short/open/50

    Use an old cheap VHF taxi radios or whatever you have.
    Put the VSWR meter at the transmitter end of the long
    coax cable and then mess about with the far end, by attaching
    say a good antenna, 50 ohm load, known bad antenna,
    a short circuit, open circuit, 25 ohm load.
    Take note of what the VSWR meter reads in all cases.

    Next, move the VSWR meter to far end of cable and try this,

    Tx---------------------long coax--------------------VSWR meter-load

    Put the various good loads and bad loads, short circuit,
    open circuit and note what the VSWR meter reads in all
    cases and compare these readings with the first experiment.

    Read again what Reg has written and what you have
    written about "assumed" radiation.
    Hint: you get little radiation from coax cable with a dead short
    at the far end.
    What did the VSWR meter tell you, when it was at the TX end
    and an open circuit or dead short was on the coax at the far end?

    Regards,
    John Crighton
    Sydney
     
  20. Thanks, John. There are a lot of guys here whose 'advice' just goes in
    the recycle bin, but you are *not* one of them. BTW, did you see the
    post from the guy who wanted to control a "DC motor" - he could really
    use that great circuit you referred me to for PWM speed control!
     
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