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  • Coaxial sleeve antenna mystery !

    From tk5ep@21:1/5 to All on Mon Sep 20 22:02:14 2021
    Hi all,

    I wanted to build a simple coaxial sleeve antenna for the 402 MHz weather balloon band.
    I made it with a piece of RG58 coaxial cable and a 8 mm external diameter tubing for the sleeve. I cut both the sleeve and radiator length to the calculated frequency.

    I measured it with my VNA and i can't find any resonance !

    I checked my length and settings in adding a single wire to the bread to make a dipole, and all is just fine with a sharp resonance (RL about 35 dB) near the spotted frequency.

    I've tested almost everything i could guess. Adding some ferrites around the feedline, checking connections, etc... but nothing helps !

    Is there something i've overlooked ? Is there a minimum ratio between the coaxial outer diameter and the sleeve tubing ?
    I didn't find anything in all descriptions and explanations i've found... All authors are claiming their antennas are working perfectly... :-)

    This is really driving me crazy ! :-(

    73 Patrick TK5EP

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  • From i3hev, mario@21:1/5 to All on Tue Sep 21 13:19:11 2021
    Hi Patrick,

    On 21/09/2021 7.02, tk5ep wrote:

    Hi all,

    I wanted to build a simple coaxial sleeve antenna for the 402 MHz weather balloon band.
    I made it with a piece of RG58 coaxial cable and a 8 mm external diameter tubing for the sleeve. I cut both the sleeve and radiator length to the calculated frequency.

    I measured it with my VNA and i can't find any resonance ! ...

    This seems to be a quite peculiar behaviour, even a single piece of wire
    should show some kind of resonance...

    Could you share some pictures of the assembly?

    '73 es DX de i3HEV, mario


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  • From tk5ep@21:1/5 to All on Tue Sep 21 10:05:53 2021
    This seems to be a quite peculiar behaviour, even a single piece of wire should show some kind of resonance...

    Could you share some pictures of the assembly?

    Hi Mario,
    Thanks for your answer.

    At first I made a version in folding back the coaxial shield to make the sleeve.
    This version had an aluminium tubing and I've tighten the shield to the sleeve with a collar.

    This group does not allow sharing images, here 2 links :
    https://ibb.co/r0wLZd4
    https://ibb.co/S69Ybnv

    73 Patrick.

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  • From i3hev, mario@21:1/5 to All on Wed Sep 22 00:03:28 2021
    Hi Patrick,

    On 21/09/2021 19.05, tk5ep wrote

    Thanks for your answer.

    my pleasure :)

    At first I made a version in folding back the coaxial shield to make the sleeve.
    This version had an aluminium tubing and I've tighten the shield to the sleeve with a collar. ...


    The assembly seems just fine, but the VNA response seem to point to huge losses. However, since you pressed the braid to the aluminium tubing,
    the contact should be good enough.

    I think the aluminium may have some kind of transparent insulating
    coating (some kind of varnish, or passivating surface finish) so the
    contact with the coax braid is in fact a very bad one.

    As far as I know, there is no constraint between the tubing internal
    diameter and the coax external diameter. In fact, the tube interior is a virtually null-field region (almost... :)) - except of course for its
    end, where some kind of capacitive coupling with the coax braid external surface may absorb some power in generating braid currents (but that
    would not be nearly enough a reason for such a behaviour).

    All in all, I'd scrape the aluminium tube under the braid with sandpaper
    until it shines :) and then I'd try again.

    Good luck and let me know the outcome! ;)
    '73 de i3HEV, mario

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  • From tk5ep@21:1/5 to All on Sat Sep 25 01:03:48 2021
    Hi Mario,

    No problem with the connections or whatsoever.

    After numerous tests, i came to the conclusion that this antenna with a sleeve very close to the coaxial shield is far too tricky to be reproducible.

    There is a very strong influence and the sleeve has to be cut much shorter than theoretical length.
    And even if i could get a resonance, the RL is not good at all. The antenna seems broadband, that's maybe why some are claiming good results with it !

    There are also strong common currents with are difficult to eliminate. Even with a coaxial coil and ferrite cores, there are still present.
    I never could get it work without strong influences of the cable position and subrounding parts.

    The solution could be to make a much larger sleeve, but then the antenna loses most of its interests. And even then, you have to try and cut !

    Just for fun, I will try with a larger sleeve and/or smaller coaxial cable.

    73 Patrick

    All in all, I'd scrape the aluminium tube under the braid with sandpaper until it shines :) and then I'd try again.


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  • From i3hev, mario@21:1/5 to All on Sat Sep 25 14:23:08 2021
    Hi Patrick!

    On 25/09/2021 10.03, tk5ep wrote:

    Hi Mario,

    No problem with the connections or whatsoever.

    ah, OK...

    After numerous tests, i came to the conclusion that this antenna
    with a sleeve very close to the coaxial shield is far too tricky
    to be reproducible. ...

    Well, the sleeve ending is a point of maximum for the voltage, so the
    effects of border capacitance are as strong as possible - the voltage
    can be of the order of 50-100 times grater than that at the dipole
    centre - so a 1 pF or so of stray capacitance there, has the same effect
    of a 100 pF in parallel to the line.

    Since the stray capacitance depends heavily on the system geometry, I
    cannot but agree on both your conclusions, that the sleeve's internal
    diameter should be much greater than the coax, and that the system is intrinsically rather unreliable...

    However, there are times when one must settle for what is available -
    here we have a saying that goes more or less "Something is better than nothing..." :)

    '73!
    mario

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