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Impedence matching: antenna-line-load

Discussion in 'Electronic Basics' started by Dominic-Luc Webb, Jun 28, 2006.

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  1. This regards antenna for radio.

    There is something not clear from info online and books on
    antenna design. Impedence matching is understood as necessary
    to optimize power transfer. However, it seems that the designs
    typically have a single point of impedence matching, but two
    interfaces at which power must be transfered: antenna to
    transmission line and transmission line to receiver. The designs I
    have seen add impedence matching only at the antenna to transmission
    line interface. Maybe someone can clarify how this works?

    Dominic
     
  2. blah

    blah Guest

    You are correct that you must match between the transmitter and the
    transmission line and between the transmission line and the antenna.
    However there are some considerations you probably aren't taking into
    account.

    Transmission lines are typically of a standard characteristic impedance
    and are relatively constant with frequency, this is not the case of an
    antenna. Some sort of matching circuit is typically required between the
    transmitter and antenna (such as a transformer at lower frequencies or an
    LC style match at higher frequencies) however this is typically fairly
    straightforward because the impedances involved are known. The impedance
    of the antenna may vary significantly based on geometry, location, and
    condition of the antenna, not to mention frequency. So matching to the
    antenna is a bigger issue.

    A mismatched antenna will cause the impedance of the transmission line to
    appear to change and in the case of any quality transmitter there will be
    a built in antenna tuner which will adjust the drive impedance at the
    transmitter/feed line interface. A remote antenna tuner, while a cool
    idea may not be practical for a variety of reasons.
     
  3. Phil Allison

    Phil Allison Guest

    "Dominic-Luc Webb"

    ** It is important to *impedance match* an antenna to the transmission line
    supplying it with RF power - otherwise
    only some of the power flowing down in the line is radiated while the rest
    is reflected back to the source.

    When the *characteristic impedance* of the transmission line is equal to
    the antenna's impedance at the transmission frequency ALL the energy is
    transferred to the antenna - without loss.

    However, at the source end the game is quite different:

    If the transmitter's *source impedance* is matched in value to the
    transmission line, then exactly half the RF power being produced by the
    transmitter is lost ( as heat in the transmitter itself).

    Often, with low powered RF devices such loss is tolerated, but where hundred
    or thousands of watts of RF energy are involved it cannot be. So the
    transmitter's source impedance is made lower than that of the transmission
    line.

    The currents flowing in the transmission line and transmitter output stages
    are the same, so power sharing is proportional to the relative impedances
    of each.




    ....... Phil
     
  4. Guest

    Guest Guest

    I have an opinion that In order for all the transmitted power to leave the
    antenna, the antenna has to be tuned /trimmed/lengthened to the wavelength
    of interest. If the antenna is not tuned the power that does not leave the
    antenna returns back into the radio and can cause damage to the final
    amplifier. A receiving antenna will receive better if it is tuned to the
    wavelength but it won't do any damage to the Detector circuitry if it isn't,
    as no power transmitted to be reflected back into the radio.
    Just my opinion. Lets hope we will find out. It is normal to use the same
    antenna for transmitting and receiving, a switch will change from receiver
    to transmitter. I expect there will be an antenna newsgroup that specialise
    in this kind of thing?
     
  5. Jamie

    Jamie Guest

    they call that SWR (standing wave ratio).
     

  6. That is essentially my understanding. I think it is actually called
    reflection, and this can be measured in order to tune an antenna. I
    have the Joseph J Carr book "Practical Antenna Handbook" that shows
    some testing techniques using an oscilloscope. I still have not
    found a simple way to test for this without a scope.

    Dominic
     
  7. Brian

    Brian Guest

    The transmitter and receiver usually has an output/input impedance of 50 or
    75 ohms. You would use a transmission line that matches that impedance. So
    that takes care of the impedance matching at the transmitter/receiver side.
    So that only leaves matching the impedance at the antenna side.
    Brian
     
  8. Brian

    Brian Guest

    Also, if you don't match the impedance at the antenna side, it will affect
    the impedance of the line.
    Brian
     
  9. Hi, and nice to to see you dropping in Brian.

    Please consider the case of a portable so-called international radio
    with a broken dipole and/or (a working) jack for an external antenna....

    Basically, my question regards the very common "novice" experience
    of being confronted with a simple battery operated portable radio (a
    receiver) in which no antenna or transmission line initially exists.
    There is the common quick fix of "sticking a random stretch of common
    insulated wire where the antenna should go". In this case, we make
    no assumption that the antenna or transmission line are matched. The
    working environment is a typical European garden "collective" situated
    adjacent to a farm in an essentially flat landscape. Some gardeners
    with no understanding of electronics sometimes wish to listen to medium
    or even long wave radio. So of course, their natural inclination is to
    stick a wire onto the (often broken) dipole or the external antenna
    connector, which typically yields less than professional results. I
    plan to improve this situation, starting with antennas of sane length
    for frequency desired (albeit undecided regarding long wave).

    :)

    I investigated this problem, looking at the Carr book for instance,
    and realized that these sources assume readers will use a co-ax
    cable, in which case impedence is typically specified, as you
    point out.

    And so, I began asking questions of how to pursue this when specs
    are not known, and neither line nor antenna impedence are likely
    to conform to the load ratings for the receiver. This is now
    an intellectual question (I could just buy the correct things). I
    have an oscilloscope and could probably come up with some tricks,
    but don't otherwise know a simple test for impedence more than
    maybe trial and error tuning. The dedicated test devices I have seen
    are sophisticated and quite specialized (presumably expensive).

    Dominic
     
  10. Christopher

    Christopher Guest

    Hello Dominic,


    This company produces add-on amplified AM antennas that could meet
    your need if you just want to buy a super AM antenna.


    http://www.ccrane.com/antennas/am-antennas/index.aspx


    Good Luck,

    * * *
    Christopher

    Temecula CA.USA
    http://www.oldtemecula.com
     
  11. Brian

    Brian Guest

    Have you used Google, to try and find the model radio that you have (on the
    internet)? You might be able to find out what the antenna input impedance is
    (for your radio), that way.
    Brian
     
  12. Brian

    Brian Guest

    Take a look at http://www.northcountryradio.com/Articles/actant2.htm
    ACTIVE ANTENNA FOR PORTABLE SW RADIOS
    Brian
     
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