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Thread: Building an CANTENNA

  1. #21
    Captain abcd567's Avatar
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    Quote Originally Posted by edmundhei View Post
    Hi sorry for the late reply, it is one meter long.
    Oh, your reply got burried in theoretical discussions. I noticed it just now.

    One meter coax cable has almost zero attenuation, whatever the type of coax.

    Try relocating the antenna to another spot. The current spot may have very weak signal. Few meters change of location may improve signal.

    Also make another cantenna and try it.

  2. #22
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    Quote Originally Posted by Strix technica View Post
    Right. Antennae are passive devices, they don't amplify anything so antenna 'gain' is not like amplifier gain.
    Of course, but antenna gain is defined as directivity x efficiency. As the calculation is purely theoretical, it should not be called gain at all since there is no such thing as an infinite, super-conducting ground plane.

    The real-world ground planes has quite bad efficiency. A 1/4-wave whip on a car roof has nowhere near 5dBi gain. A standard type of antenna, a 1/4-wave GP with sloping ground radials, normally has 2dBi gain like I said.

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  3. #23
    Captain abcd567's Avatar
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    CANTENNA SIMULATION BY SOFTWARE MMANA-GAL

    Environment: Free Space





    Last edited by abcd567; 2017-05-13 at 03:18.

  4. #24
    Captain abcd567's Avatar
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    SPIDER SIMULATION BY SOFTWARE MMANA-GAL.

    Environment: Free Space







    Last edited by abcd567; 2017-05-13 at 03:19.

  5. #25
    Flight attendant Strix technica's Avatar
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    Quote Originally Posted by MrMac View Post
    Of course, but antenna gain is defined as directivity x efficiency. As the calculation is purely theoretical, it should not be called gain at all since there is no such thing as an infinite, super-conducting ground plane.
    By that same argument, one cannot speak of gain of any sort of Rx antenna in a theoretical context which means one cannot speak of efficiency (because there's nothing with which to compare real-world instances).

    For the purposes of comparing different antenna designs, efficiency is irrelevant because, unless one has good reason to know that one design is more efficient than another, one must assume the same efficiency for each comparand and therefore the efficiency term drops out of the equation.

    In any case, there are likely sufficiently many variables in real-world performance that even in any two theoretically equivalent antennae (whether same pattern/efficiency/type), there is a good chance that efficiency will be dominated by those other variables. Even two identical antennae will likely have different efficiencies depending on where and how they're set up, especially 1/4w antennae which depend on external factors such as the size, characteristics and distance from ground. Therefore I suggest that, for the purposes of comparative analysis of the performance of 1/2w dipoles, 1/4w monopoles and abcd567's cantenna, efficiency is a red herring.

    abcd567 insists (though not consistently so) that his cantenna design has certain properties and I'm trying to draw attention to practical considerations that are relevant when trying to understand how the cantenna performs and why when compared with other types (such as the kit antenna) and the theoretical context is the only one in which apples can be compared with apples and I have yet to see abcd567 address himself to a single point I've made in that respect.

    Chief among those considerations is the angle of incident radiation where it matters, ie a/c > 100 nm away at which distance is only a few degrees to the horizion, essentially perpendicular to the cantenna. At that point and contrary to abcd567's assertions, the cantenna will almost exclusively behave like a dipole with a phase shift of 1/8w in one half of it.

    For the same reason, efficiency isn't particularly relevant in comparing performance as I did in those transferred power equations (which were only intended to illustrate an argument mathematically rather than by assertion) because there is no alternative but to assume that efficiency isn't significantly different between each case. It's the same antenna, after all, but with the influence of the ground plane considered. Those equations also provide for some basis to offer better rebuttal, if anybody is so inclined.

    Quote Originally Posted by MrMac View Post
    The real-world ground planes has quite bad efficiency. A 1/4-wave whip on a car roof has nowhere near 5dBi gain.
    It's true that there's no such thing as a superconducting ground plane, and it's also true to say that real-world ground planes are usually pretty terrible. But it's also true that a sheet of mild steel to which the whip is perpendicular is not comparable with a whip with declined ground radials.

    In practical terms, mild steel has a resistivity of ~150 nano-Ωm so a sheet of it has negligible impedance "essentially lossless" were my words compared with the chassis of a hand-held receiver, with actual terra firma or with ground radials (because the radials aren't continuous, incident radiation at azimuths other than where there is a radial is going to see a higher inductive impedance in the approximated ground plane) especially when declined (because they then begin to exhibit some characteristics of a dipole without a balun; c.f. Brewster's angle).

    Quote Originally Posted by MrMac View Post
    A standard type of antenna, a 1/4-wave GP with sloping ground radials, normally has 2dBi gain like I said.
    Sure, but 1/4w monos with declined radials aren't comparable to any of the antennae that we were considering, so the common real world gain of 2 dBi isn't particularly helpful.

    It's understood (and perfectly valid to point out) that real-world performance will differ from theoretical performance, but you can't compare any two real-world installations without actually measuring them.
    Last edited by Strix technica; 2017-05-13 at 14:44.

  6. #26
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    I just wanted to try a Cantenna yesterday.
    Mine is a Carlsberg-tenna I should say
    IMG_20170514_172253.jpg

    Cable is 75Ohm, random length.

    Well, once in place I could see that it performed a bit worse than my trusty 8 radials spider.
    I just had less planes.

    Maximum range seen was:

    Cantenna : 275 nm
    Spider 8-radials : 284 nm

    So Im sticking with the spider.
    Last edited by perseus68; 2017-05-14 at 16:43.

  7. #27
    Captain abcd567's Avatar
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    Last edited by abcd567; 2017-05-15 at 15:36.

  8. #28
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    Yes Captain, that one ;-)
    Today propagation must be better and I just hit 296nm distance with that little thing
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