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Thread: best antenna

  1. #2661
    First officer
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    Quote Originally Posted by Rooster View Post
    There is really little need for building (or buying) antennae with "gain"
    Aircraft fly in essentially a hemisphere over your head...
    Actually they don't fly in anything like a hemisphere, I can detect aircraft out to 300+nm in all directions other than straight up! I suspect that if they flew at that height the ISS would be considered a collision risk��
    Antenna don't require much gain straight up as the distance to an aircraft overhead is much less than one on the horizon, that is where you need the gain.
    FR24 F-EGLF1, Blitzortung station 878, OGN Aldersht2, PlanePlotter M7.

  2. #2662
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    Quote Originally Posted by Rooster View Post
    There is really little need for building (or buying) antennae with "gain" Aircraft fly in essentially a hemisphere over your head... so a 1/4 ground plane is the best for overall coverage IMHO - antennae with "gain" dont actually have ANY gain whatsoever...
    Completely untrue, all of it.

    The definition of antenna gain is passive re-distribution of radiation pattern. Every dB that can be won from a direction where you don't need it, is added link margin for detection of weak signals, or overcoming feeder loss.

    A 5dBi antenna is often optimal. It will outperform your 2dBi GP every time, and still have enough gain straight up not to lose a/c above your head.

    Gain is not amplification, but it's most definitely real, and beneficial.

    /M
    F-ESDF1, F-ESGG1, F-ESGP1, F-ESSL4, F-ESNK4, F-LFMN3
    P-ESGR, P-ESIA, P-ESIB, P-ESNV

  3. #2663
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    Quote Originally Posted by rederikus View Post
    The antenna is vertical so there is no loss of signal anywhere in the hemisphere. There is an effect wheregenerally the more elements you introduce, the larger your hemisphere becomes at somewhat the expense of nearer signals. Thus an antenna of this type can pick up signals from further away than a simple ground plane antenna with just one receiving element. This is expressed as dBi (decibels relative to isotropic radiator). An isotropic radiator is (I think) considered here as a point source and I assume carries a value of 1. The multiple element signals are additive and, naturally since bigger numbers are easier to sell the marketing people tend toward using dB instead of dBi as a "gain" figure.
    It's not at the expense of nearer signals, it's compressing of the radiation that would otherwise go upwards and downwards. The more elements you add (if done optimally), the smaller opening angle towards the horizon you will get.

    An isotropic antenna is a theoretical point from where radiation goes exactly equal in a sphere. This is referenced as 0dBi.

    A 1/4-wave GP has 2dBi gain in it's sweet spot (normally towards horizon and slightly upwards), most of that is "taken" from what would have been the lower half of the spehere.
    A 1/2-wave dipole has 2.15 dBi gain, taken equally from top and bottom of the sphere.

    Collinears vary in gain depending on how they are built, but they all just compress the "sphere" further and further into a horizontal "disc".

    /M
    F-ESDF1, F-ESGG1, F-ESGP1, F-ESSL4, F-ESNK4, F-LFMN3
    P-ESGR, P-ESIA, P-ESIB, P-ESNV

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