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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

    751
    164
    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

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    85
    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

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    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

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    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

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    Sep 5, 2009

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

    Nanren888

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    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.
     
  7. Boure

    Boure

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    Nov 5, 2020
    Polarization refers to the alignment of electric field lines generated by the dipoles with respect to the earth's surface. The alignment of a conductor with respect to the ground would provide an idea regarding the nature of polarization. For example, a whip antenna held in an upright position/vertical to ground would be biased for horizontal polarization.

    For short distances/line of sight, the ground is essentially straight hence if the transmitter has a horizontal polarization(as is the case for most TV stations), efficient reception would require a matching horizontal polarization.

    With an increase in distance, the earth's curvature comes into play( for even greater distances the atmospheric conditions would also be a considerable factor). This means that at the transmitting station the polarization starts off horizontal but gets to the reception area with a different polarization.
    For even greater distances/ outside the line sight, reflection and refraction become a considerable factor, hence the polarization might be significantly altered by the time the signal gets to the antenna. It is therefore true that over large distances the antenna can perform differently at alternative polarization angles.

    For UHF loops, a loop with large sections placed horizontal to the ground is horizontal polarization biased regardless of the loop shape. A perfect circle held upright/ normal to the ground would be biased for both vertical and horizontal polarization since there are segments that are vertical and horizontal to the ground are every angle in between.
    Rabbit ears fully upright are designed for vertical polarization whereas if the rabbit ears are placed horizontal to the ground they are fully horizontal polarization biased.
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009

    VERTICAL polarisation
     
    hevans1944 likes this.
  9. Boure

    Boure

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    Nov 5, 2020
    Yep......it's supposed to be vertical. Sorry for that mistake.
     
  10. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    What?! No digression into circular polarization?
     
    Boure likes this.
  11. bertus

    bertus Moderator

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    Nov 8, 2019
    Hello,

    With circular polarization there ts also a difference between left and right turning polarization.
    It is for instance used with satelite communication.
    The difference in signal strentgh will be about 30 dB when you use the wrong polarization.

    Bertus
     
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