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Building an CANTENNA

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  • #16
    Originally posted by abcd567 View Post
    Cantenna is neither a pure 1/2 WL Dipole, nor a pure 1/4 WL Monopole with ground-plane.
    It is a hybrid.
    I agree, and that's what I said — the cantenna's characteristics depend on the angle of incident radiation.

    However, the other day you insisted that it was a 1/4w monopole:

    Originally posted by abcd567 View Post
    Cantenna is NOT a 1/2w dipole.
    It is a 1/4w monopole, with a ground plane disc (bottom of the soda can), and a 1/4w long decoupling sleeve (cylindrical wall of the soda can).

    Originally posted by abcd567 View Post
    Due to very large diameter of the soda can (66mm, almost 1/4 WL), the bottom of soda can makes a substantial ground-plane.
    If the soda can has a diameter of 66mm, then the radius is 33mm. That makes the ground plane a bit under 1/8w.

    Originally posted by abcd567 View Post
    As a result, unlike Coaxial Dipole, the Cantenna does not behave as a pure 1/2 WL Dipole.
    Perpendicular incident radiation can't see the ground plane, so it'll look like a 1/2w dipole albeit, as I said before, with some degree of decoupling/symmetrisation that you don't get with a coaxial dipole. In fact, because of the lip and the concave shape of the ground plane, its 1/4w properties probably don't begin to kick in until the angle of incidence to the perpendicular is fairly substantial. Even when its 1/4w wave properties are apparent, you've got to consider that the ground plane isn't a full 1/4w given the geometry of where the active element is in relation to the edge of the disc.

    Originally posted by abcd567 View Post
    These are 3 Cantennas with different diameter can/pipe. All other dimensions are identical.
    The worst performer was the one with smallest dia (20mm dia copper water pipe with end cap), and best performer was the one with largest dia (66mm dia standard soda can).
    I believe it, especially for a/c overhead (where the gain is least). But even FL400 directly overhead is only 6.6 nm away, increasing to 9 nm at 45° (6.6/cos° 45). In fact, you have to get to 3.8° to the horizon before the plane at FL400 is 100 nm away (90 - arccos° 6.6/100) by which point you're well into dipole territory.

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    • #17
      Originally posted by abcd567 View Post
      Simulation of Cantenna
      Gain = 1.71 dBi
      Vertical plane pattern looks like a 1/2w dipole to me, not remotely like a 1/4w mono, and yet its expected gain is less than that expected of a dipole.

      Perhaps this is not surprising, though, considering that this is a simulation of the cantenna transmitting rather than receiving. It would, indeed, look like a dipole if driven by a transmitter, and a tx simulator can't model different angles of incidence for rx.

      Also, the feedline doesn't appear to be modelled, but then I'm not familiar with 4nec2 so I'm unsure how important that is.

      ETA: the other cause of decreased gain might be that there is a 1/8w phase shift between radiation incident on the core and on the can. That's a quarter of a half wave so it's not insignificant (think of how the induced voltages will add up when the voltage maxima are at the ends of the dipole).
      Last edited by Strix technica; 2017-05-11, 09:27. Reason: mention 1/8w phase shift

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      • #18
        Originally posted by abcd567 View Post
        Since angle of tilt is not big in this one, the improvement by making another better cantenna is not likely to be big.

        It is easy to make a Cantenna. No harm making and trying another more accurate one.

        What is the length of coax between Cantenna and DVB-T? A long coax cable will result in reduction of reception.
        Hi sorry for the late reply, it is one meter long.

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        • #19
          Originally posted by Strix technica View Post
          So that's P = (0.5+0.5)^2/0.5 = +3 dBd or +5.15 dBi.
          But this only works for a whip directly on a perfect ground.
          That is directivity, not actual gain.

          /M
          F-ESDF1, F-ESGG1, F-ESGP1, F-ESNK1, F-ESNV2, F-ESNV3 F-ESSL4, F-LFMN3
          P-ESGR, P-ESIA, P-ESIB, P-ESGF
          mrmac (a) fastest.cc

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          • #20
            Originally posted by MrMac View Post
            That is directivity, not actual gain.
            Right. Antennae are passive devices, they don't amplify anything so antenna 'gain' is not like amplifier gain.

            Directivity, AIUI, is related to gain in antennae specifications. The tighter the pattern, the greater the gain relative to the ideal isotropic — in a given direction.

            If you think I've misapplied the theory and in particular the relationship between incident amplitude, impedance and transferred power, please do say how.
            Last edited by Strix technica; 2017-05-12, 16:33.

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            • #21
              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.

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              • #22
                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.

                /M
                F-ESDF1, F-ESGG1, F-ESGP1, F-ESNK1, F-ESNV2, F-ESNV3 F-ESSL4, F-LFMN3
                P-ESGR, P-ESIA, P-ESIB, P-ESGF
                mrmac (a) fastest.cc

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                • #23
                  CANTENNA SIMULATION BY SOFTWARE MMANA-GAL

                  Environment: Free Space





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

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                  • #24
                    SPIDER SIMULATION BY SOFTWARE MMANA-GAL.

                    Environment: Free Space







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

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                    • #25
                      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.

                      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).

                      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, 14:44.

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                      • #26
                        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 I´m sticking with the spider.
                        Last edited by perseus68; 2017-05-14, 16:43.
                        Orange Pi PC Plus + NooElec NESDR SMArt + homemade 6 ele CoCo

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                        • #27
                          Originally posted by perseus68 View Post
                          ..................
                          So I´m sticking with the spider.
                          You mean this one?

                          https://forum.flightradar24.com/thre...ll=1#post90878


                          GROUNDPLANE ANTENNA MADE OF COAX ONLY - Without SO239 or N-Conector


                          QUICK SPIDER - No Soldering No Connector




                          Last edited by abcd567; 2017-05-15, 15:36.

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                          • #28
                            Yes Captain, that one ;-)
                            Today propagation must be better and I just hit 296nm distance with that little thing
                            Orange Pi PC Plus + NooElec NESDR SMArt + homemade 6 ele CoCo

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