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Thread: Building an CANTENNA

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    Building an CANTENNA

    Hi. I just build a cantenna today. But the signal is actually worst /similar result than the orgianl one come with the dvb-t dongle. Any suggestion?
    I have tested the orginal one and the cantenna outside and inside. They both are the same result.
    The photo are inside because it is more easier to take.
    Here are some photo.
    WhatsApp Image 2017-05-05 at 14.39.33.jpg
    WhatsApp Image 2017-05-05 at 14.39.33 (1).jpg

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    Flight attendant Strix technica's Avatar
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    Quote Originally Posted by edmundhei View Post
    Hi. I just build a cantenna today. But the signal is actually worst /similar result than the orgianl one come with the dvb-t dongle. Any suggestion?
    Remember that the theoretical gains for a 1/2w dipole and a 1/4w whip (on a decent ground plane) are about 2 and 5 dBi respectively.

    A cantenna is effectively a 1/2w dipole, so the difference in performance might be explained by that alone: your cantenna has less gain and the loss due to lack of resonance in the 1/4w whip is less than the extra gain it gets from being a 1/4w whip.

    This is complex stuff as you'll see if you read the hundreds of pages long threads about antenna design. A detuned antenna doesn't necessarily perform poorly, but the length of the feeder and impedance mismatches come into play in unpredictable way.

    You can try building a co-linear (aka "co-co"; again, look at the threads). They're tricky to get right, but they're inexpensive. Just be sure to use decent quality RG6 coax with a known velocity factor.

    Bottom line: try it and use what works best.

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    Quote Originally Posted by Strix technica View Post
    Remember that the theoretical gains for a 1/2w dipole and a 1/4w whip (on a decent ground plane) are about 2 and 5 dBi respectively.

    A cantenna is effectively a 1/2w dipole, so the difference in performance might be explained by that alone: your cantenna has less gain and the loss due to lack of resonance in the 1/4w whip is less than the extra gain it gets from being a 1/4w whip.

    This is complex stuff as you'll see if you read the hundreds of pages long threads about antenna design. A detuned antenna doesn't necessarily perform poorly, but the length of the feeder and impedance mismatches come into play in unpredictable way.

    You can try building a co-linear (aka "co-co"; again, look at the threads). They're tricky to get right, but they're inexpensive. Just be sure to use decent quality RG6 coax with a known velocity factor.

    Bottom line: try it and use what works best.
    I am also using an mcx to coax coax to f connector. Will this affect the coverage?

    DVB dongle - mcx to coax - coax to f

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    Flight attendant Strix technica's Avatar
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    Quote Originally Posted by edmundhei View Post
    I am also using an mcx to coax coax to f connector. Will this affect the coverage?

    DVB dongle - mcx to coax - coax to f
    Minimally, less than the difference in gain between the whip and dipole cantenna.

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    Quote Originally Posted by edmundhei View Post
    Hi. I just build a cantenna today. But the signal is actually worst /similar result than the orgianl one come with the dvb-t dongle. Any suggestion?
    I have tested the orginal one and the cantenna outside and inside. They both are the same result.
    The photo are inside because it is more easier to take.
    Here are some photo.
    WhatsApp Image 2017-05-05 at 14.39.33.jpg
    WhatsApp Image 2017-05-05 at 14.39.33 (1).jpg
    The ADS-B transmission is vertically polarized.
    Keep your Cantenna vertical. If it is tilted, you will get reduced reception. If it is horizontal, you will get almost zero reception.




    Quote Originally Posted by edmundhei View Post
    I am also using an mcx to coax coax to f connector. Will this affect the coverage?

    DVB dongle - mcx to coax - coax to f
    The MCX to F Pigtail has negligible effect due to its short length


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    Captain abcd567's Avatar
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    Quote Originally Posted by Strix technica View Post
    Remember that the theoretical gains for a 1/2w dipole and a 1/4w whip (on a decent ground plane) are about 2 and 5 dBi respectively.

    A cantenna is effectively a 1/2w dipole, so the difference in performance might be explained by that alone: your cantenna has less gain and the loss due to lack of resonance in the 1/4w whip is less than the extra gain it gets from being a 1/4w whip.
    Cantenna is NOT a 1/2w dipole.
    It is a 1/4w monopole. The 66mm dia bottom of soda can provided a ground plane disc, and soda can's cylindrical wall provides a 1/4w long decoupling sleeve (sleeve balun), and helps in suppressing common-mode currents on coax shield.

    Please see diagram below.

    Last edited by abcd567; 2017-05-05 at 21:24.

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    Quote Originally Posted by abcd567 View Post
    The ADS-B transmission is vertically polarized.
    Keep your Cantenna vertical. If it is tilted, you will get reduced reception. If it is horizontal, you will get almost zero reception.






    The MCX to F Pigtail has negligible effect due to its short length

    Hi. I have repostion the Cantenna, my max distance is 109 nm. When I drill the hole for the connector, i used the wrong drill. Therefore the can is in an angle. As show in the picture. Shall I make a new one?
    WhatsApp Image 2017-05-06 at 11.44.38.jpeg

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    Captain abcd567's Avatar
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    Quote Originally Posted by edmundhei View Post
    Hi. I have repostion the Cantenna, my max distance is 109 nm. When I drill the hole for the connector, i used the wrong drill. Therefore the can is in an angle. As show in the picture. Shall I make a new one?
    WhatsApp Image 2017-05-06 at 11.44.38.jpeg
    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.

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    Flight attendant Strix technica's Avatar
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    Quote 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).
    I'm not formally trained in antenna design so I claim no particular expertise with antenna physics however, from what I understand of the theory, the cantenna probably lies somewhere between, depending on the angle of incident radiation.

    Let's see whether we agree on what 1/4w monopoles and 1/2w dipoles actually are:
    • A 1/2w dipole is a balanced antenna (both 1/4w poles actively collect and contribute to signal), requires no ground reference and has a gain of ~2 dBi with a pattern perpendicular to the axis of the dipole. The natural impedance of a dipole is 72Ω but, being balanced, they require a 1:1 balun (into a 75Ω feeder, as is ideal with RTL SDRs) to work efficiently.
    • A 1/4w monopole is an unbalanced antenna that requires an RF ground plane that is at least 1/4w in diameter for efficient operation. The ground plane needn't be directly coupled (and often isn't) to the feeder shield, and it reflects incident radiation to the monopole for which reason its gain is ~5 dBi with a pattern that is slightly elevated relative to the perpendicular ground plane. Because there is only one driven element (the feeder shield goes nowhere), no balun is required and its natural impedance is 36Ω, half that of a dipole. Pole-mounted 1/4w monopoles include an RF reference, usually in the form of 4 or more straight rods which are often somewhat declined relative to the horizon to improve impedance matching into 50Ω coax.


    The cantenna's ground disc is not 1/4w wide, in fact it's probably < 1/8w for soda can of average diameter (~60mm), which means it isn't going to be a very efficient ground plane to the extent that it presents as a ground plane at all. Many soda cans have concave bottoms, though I have no idea what effect that might have.

    The 1/4w decoupling sleeve is DC coupled to the feeder shield at the bottom of the 1/4w core. It extends 1/4w back over the feeder and ends in an open impedance with respect to the core. Because of this, radiation (of any frequency) incident on the feeder will have less of an influence than might otherwise be the case.

    The core extends 1/4w in the other direction, also open impedance at its end. Radiation incident perpendicular to the cantenna will see two 1/4w conductors — which looks very much like a 1/2w dipole, albeit one where the feeder shield is somewhat decoupled from the active antenna. But the decoupling sleeve is not a balun because it's still connected to RF ground so, in that sense, it's more like a 1/2w dipole without a balun.

    You're right that if the sleeve ≈ coax , there would be some capacitive coupling between the sleeve and the feeder, but all that does is increase the influence of the feeder shield on the active antenna. Regardless of the diameter of the sleeve, the sleeve is still at RF ground, just as a 1/4w monopole's feeder shield and RF ground plane are, so only the core is active but without the benefit of a 1/4w ground plane that a monopole would normally have (I'm still talking perpendicular radiation for now, which doesn't see the disc at all).

    The above is also somewhat true for radiation incident at 45 to plane and core, but some of the incident radiation will also be reflected back onto the core as in the case of a monopole. The extent to which the cantenna has the characteristics of a 1/2w dipole or 1/4w monopole will vary according to angle but, as the angle to the core decreases, gain will drop because neither possess any gain on axis.

    The remotest of sources requiring the greatest gain will be on the horizon, so the cantenna probably looks like a 1/2w dipole without balun where it matters most.

    Leastways, that's my inexpert analysis. I could well be wrong but if I am, I'd like to know how I've misapplied theory.

    Don't get me wrong, I'm not saying your cantenna design is a bad one, on the contrary, you proved that it outperformed your dipole, but at least part of that is probably because your dipole construction didn't appear to have a balun in it in order to make the arm coupled with the shield an active element.
    Last edited by Strix technica; 2017-05-06 at 14:51.

  10. #10
    Flight attendant Strix technica's Avatar
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    Quote Originally Posted by abcd567 View Post
    What is the length of coax between Cantenna and DVB-T? A long coax cable will result in reduction of reception.
    So will the type of coax. From memory, good RG6 is about -2 dB/10 metres where RG59 is about -7–-9 dB/10 metres. Even RG8, if you can get it, is something like -4 dB/10 metres.

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