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Far-field antenna polarization...

Discussion in 'Radio and Wireless' started by Externet, Aug 5, 2020.

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  1. Externet

    Externet

    743
    162
    Aug 24, 2009
    Hi.
    Read somewhere that a television receiving antenna should be horizontal polarized. Also read that many miles away, it does not matter if the signal is received with a vertical polarized antenna, will perform the same :confused:
    Can anyone explain how it works ?
    What polarization are old 'rabbit ears', UHF loops ?
     
  2. Nanren888

    Nanren888

    283
    61
    Nov 8, 2015
    Polarisation depends on the transmitter. Look up for what's used where you are.
    Yes, horizontal is common.
    .
    Scattering.
    When the horizontally-polarised waves hit that randomly angled roofing iron, or buildings, or hills they scatter at rather unpredictable angles and polarisations.
    So, in a rich scattering environment, yes, reception is likely at almost any angle.
    .
    If you have line of sight, then the intended polarisation is the most likely one to receive.
    .
    Rabbit ears, are a dipole, with adjustable dipole length. So if you pull them out horizontally, you get a horizontal dipole, hence directional in azimuth.
    .
    Loops can be viewed as a dual of a dipole. That is a magnetic loop can be treated as a magnetic dipole through the middle, so the pattern will look similar to that. Not really used loops. Maybe someone else can fill you in on them.
     
    davenn and Martaine2005 like this.
  3. davenn

    davenn Moderator

    13,622
    1,882
    Sep 5, 2009
    Only if the transmitter is


    rubbish :) TX and RX need to be the same
    There is in the order of 25-30 dB difference between polarisations which would mean large losses of signal strength when using the wrong polarisation
     
  4. ramussons

    ramussons

    362
    69
    Jun 10, 2014
    The Discrimination due to Polarization is noticeable only on strong signals. Once you go far away from the transmitter and your receive signals go below -40 dBm or so, the Polar Discrimination in practice does not really matter. It does not matter which way the antenna is placed for weak signals.
     
  5. davenn

    davenn Moderator

    13,622
    1,882
    Sep 5, 2009

    having worked on DC to daylight, I would strongly disagree with that statement
     
    hevans1944 likes this.
  6. Nanren888

    Nanren888

    283
    61
    Nov 8, 2015
    Weak from no LOS and abundant scattering or weak from just being a low-level signal, eg range for power and sensitivity.
    .
    Even if there is line of sight and the range, powers and sensitvity is such that the signal is small, all the normal polarisation rules hold, including that you can basically null the signal completely by being the "wrong" way, whether it be horizontal, vertical or the wrong CP.
     
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