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Thread: Which antenna have more coverage?

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    Last edited by edmundhei; 2017-05-05 at 16:59.

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    Captain Anmer's Avatar
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    Unless we have someone here who has conducted simultaneous tests of all three with comparable setups, you're not going to get a conclusive answer.
    Mike


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    He can always buy them if he doesn't own those antennas yet and do the tests

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    Captain Anmer's Avatar
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    Quote Originally Posted by SoCalBrian View Post
    He can always buy them if he doesn't own those antennas yet and do the tests
    Good idea. It would have saved him asking the question too.
    Mike


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    Captain abcd567's Avatar
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    Quote Originally Posted by BendiT View Post
    https://shop.jetvision.de/epages/648...Products/67305 this one. A friend of mine has tested it. Works good.
    He has asked "which antenna has more coverage" which means comparison, not just just experience about a single antenna.

    If it is experience about one antenna only, I can also say:

    https://www.amazon.com/gp/aw/d/B00WZL6WPO this one. I have tested it. Works good.

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    Yep, I've just got a Flightaware antenna as well, to replace a Radar-Rama. It only arrived a couple of days ago so I haven't had time to put it up or test it yet, but thought I'd post a UK link for it.
    https://www.modmypi.com/electronics/...nna-66cm--26in

  7. #7
    Flight attendant Strix technica's Avatar
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    Quote Originally Posted by edmundhei View Post
    Which antenna have more coverage?
    That's a complex question. Even if you had all three to test, which will perform best will, to a large extent, depend on your own circumstances and location (especially how built-up the area is and how high your antenna is mounted), so nobody else's experience is going to help you very much.

    Here are a few points to consider:

    • Gain quoted by the manufacturer is usually BS. Make your decision based on the type, construction, cost and your own circumstances.
    • Familiarise yourself with the different types of antenna (1/4 wave monopole [aka whip], 5/8w monopole, 1/2w dipole, J-pole, colinear), the different patterns they have (see below) and what other requirements they have. For example, monopoles require a ground plane. An RF ground can be approximated on an antenna mounted atop a mast, but it's only an approximation.
    • Antennae don't amplify signal (how can they, they're just bits of metal!), so antenna "gain" probably isn't what you think it is — rather, an antenna's gain tells you something about its pattern.
    • Avoid anything with a loading coil in it such as the Radar-Rama you linked. Why? A loading coil makes the antenna resonant when it's too short for the nominal frequency. That gives them an okay VSWR for transmit, but the amount of received power they can collect is less than what a correct length antenna would be capable of.
    • This application involves receiving weak signals from afar, therefore your range will have a lot to do with the antenna's pattern and its ability to collect RF power. You might think 'the bigger the better' but, unfortunately, an antenna's size is directly linked to its wavelength. But you can combine multiple antennae together to increase RF power collection, which is why colinear antennae are of such interest in this application.
    • To illustrate the last point, consider an a/c at FL400 (and ignoring curvature of the earth and atmospheric lensing effects): directly above you, it is only 6.6 nm away from you; at 45°, it is still only 10 nm away; at 100 nm away from you (along the diagonal), it makes an angle of only about 3° with the horizon. Therefore, nearly all of the a/c of interest are going to be on the horizon relative to you, and any a/c that make any significant angle with the horizon are going to be so close (relatively speaking) that reduced sensitivity at that angle isn't important. Moreover, there are going to be many more a/c at distance on the horizon than in the relatively tiny airspace above you at higher angles of inclination.
    • Because you are interested primarily in signals at the horizon, there is no substitute for height. An average quality 1/4w with crappy virtual RF ground that is atop a high mast will almost certainly outperform the best colinear mounted on the ground surrounded by brick buildings.


    A bit of explanation about gain and pattern:

    Antenna gain is quoted relative to the ideal, omnidirectional, hypothetical point-source antenna. No such thing exists, especially not for receive, but it's a useful theoretical "ideal" reference (hence dBi [actually, the 'i' stands for 'isotropic' but 'ideal' is easier to remember]). As I said, antennae don't amplify, so if an antenna's gain is > 0 dBi, then that additional signal is coming from somewhere else, ie spacial direction. A basic dipole, for example, has a nominal gain of 2.15 dBi because it has a sensitivity pattern that is greatest along the plane perpendicular to the antenna and least along its axis. See the diagrams here to visualise what I mean.

    The higher the gain, the tighter the pattern. This can be a good thing because it means you can focus your sensitivity on where the weakest signals are. This is what TV antennae and satellite dishes do: they have directional elements in them that give them decent gain in a specific direction so they can reach further (and, handily, improve rejection of images and multipath signals).

    But, gain doesn't tell you everything about the pattern. For example, the chord of a 1/2w antenna's pattern is at 0° to the horizon because it is (nominally) equally sensitive above and below. But the pattern of 1/4w and 5/8w antennae, which require ground planes, is somewhat elevated with respect to the horizon which isn't really what you want.

    I've never tried a commercially built monopole, but I've had quite respectable results from a home-built colinear. Colinears are fiddly to assemble and unforgiving of inexact dimensions, but they're very cheap to build, don't require a ground plane and can have as much gain (ie directionality) as you want just by adding extra pairs of 1/2w elements.

    Remember that a piece of wire of any length will pick up something, the rest is fine-tuning and optimisation with ever-decreasing returns on effort and investment. And there are other factors oft forgotten that can dwarf tweaks to the antenna itself: for example, impedance matching, feeder coax shield decoupling and feeder coax loss, most of which requires that you learn something about the theory behind its construction and operation.

    A more specific example of what I mean: there's no point in fussing over a few dB of loss in impedance matching or antenna sensitivity when you're using crappy RG58 coax with a loss of 10 dB per 10 metres over a 10 metre feeder run. You'd do much better to replace that RG58 with good quality RG6 (~2 dB/10 metres loss) despite being different impedance because you'll lose less in the coax than you will to the impedance mismatch (if any).

    TL;DR- Broadly speaking, the differences between different types of antennae are incremental by comparison with factors like height and line-of-sight. So decide whether you want to invest the time to learn about this stuff to eek out the last dBm of rx power, or just go for an affordable, reasonable quality commercial solution (that includes ground radials where required), do your best to get it as high as possible and be happy.

  8. #8
    Captain abcd567's Avatar
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    RADIATION PATTERN OF FULL WAVE DIPOLE (1/2 Wavelength + 1/2 Wavelength)
    Also Applicable to Half Wave Monopole with Ground Plane



    RADIATION PATTERN VS GAIN
    Radiation pattern flattens as gain increases. As a result faraway airplanes (which are at nearly horizontal line-of-sight) are picked up better with higher gain, which has a flatter pattern.



    DIPOLE PATTERN vs LENGTH
    Also Applicable to Monopoles with Ground Plane, Length 1/2 of Length for Dipole
    Last edited by abcd567; 2017-07-16 at 17:35.

  9. #9
    Flight attendant Strix technica's Avatar
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    Quote Originally Posted by abcd567 View Post
    DIPOLE PATTERN vs LENGTH
    Also Applicable to Monopoles with Ground Plane, Length 1/2 of Length for Dipole
    It should be noted that anything larger than 1/4w or 1/2w has funky impedance characteristics and will require a matching network. Without it, you'll lose any benefit you might get from increased power collection ability, if not actually do much worse than a plain 1/4w or 1/2w.

    Colinears are (or so I understand) a different story because they're made up of 1/2w elements put end-to-end in opposing phase. Though I don't think I've ever seen anyone put a colinear on a VNA to test that, commercially made colinears don't seem to bother with matching networks so I guess their impedance is comparable with that of coax.

  10. #10
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    Some one (TomMuc) has tested 5 different antennas, and has posted his conclusion here:

    http://discussions.flightaware.com/p...2.html#p209072

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