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Woud a disguised Cobweb work the same?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by G1YBB, Sep 8, 2015.

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  1. EB5KT

    EB5KT Ham Member QRZ Page

    Hi again.
    I share with you an old measurement of the G3TPW cobwebb built with speaker wire.
    73's - EB5KT
     

    Attached Files:

  2. G1YBB

    G1YBB XML Subscriber QRZ Page

    Glad you finished yours. My disguised cobweb (start of this thread) is still ongoing.
    I went out earlier this year to do final assembly and measurement and tuning but the feed box arm had a manufacturing fault and failed before I could start. It is now repaired and ready again, but I have been too busy building VHF yagi equipment and contesting.
    I will get to finish this project soon I hope!
     
  3. SP3L

    SP3L Ham Member QRZ Page


    I am very glad that you have built the Mk.2 variant. As I can see in you measurement results the bandwidth on the 10 m band is noticeably better than in my MK.1.

    I was very glad with the Cobweb performance. When I was comparing it to the my multiband GP (similar to AV-640 design), the Cobweb was almost always better or equal to it. The Cobweb was installed on a 11 m high mast and the GP on a 5 m high mast. And the noise and QRN heard on the Cobweb were down when compared to the vertically polarized antenna. I hope you will be equally glad with the MK.2 version. If you work both SSB and CW, it should not be a problem for you to work 200+ DXCCs with the Cobweb at only 100 W output.

    Another approach to widen the Cobweb bandwidth is shifting its feedpoint. I once started here in this forum the thread "OCF Cobweb" where such approach is proposed. This solution is not ideal but perhaps somebody will want to construct a real thing based on it?

    Thanks again for trying out the MK.2 version and letting us know the results.

    Jacek, SP3L
     
  4. SP3L

    SP3L Ham Member QRZ Page

    Great! I was a little bit afraid that you gave up and the Disguised Cobweb was forgotten for good. Waiting for your final report and some pictures/drawings...
     
  5. G1YBB

    G1YBB XML Subscriber QRZ Page

    I definitely will update with info when done. I have a Web page describing it so far but not published until I get it finished.
    I have a yagi I'm trying to finish by 12th Jan, and two small projects and I am not planning anything new so it should be cobweb next!
     
  6. M0MNM

    M0MNM Ham Member QRZ Page

    Since there's a few experts on here...

    Quick question. Would lengthening the dimensions where the wires cross the spreaders, (in an effort to widen the space between the end of the elements) affect performance?

    As soon as I started to use an amplifier with mine, the swr used to creep up, so I upgraded the balun, and the wire, and slightly moved the wires out (around an inch per spreader), but it doesn't seem to perform like it used too. :(

    SWR is great across all bands. Any input appreciated. :)
     
    SM6CJB likes this.
  7. SP3L

    SP3L Ham Member QRZ Page

    Your question could not come in a better moment. I was just busy with tidying up my Cobweb models. I had improved accuracy of my models and re-ran the simulations again. I can compare the reference G3TXQ design with my own three designs from Mark 1 through Mark 3 without worrying about the models accuracy.

    Here are the antenna views.
    upload_2017-10-4_12-34-36.png

    upload_2017-10-4_12-34-52.png

    upload_2017-10-4_12-35-3.png

    upload_2017-10-4_12-35-13.png

    And the results. Pay attention to the SWR values shown in the table in the blue frames – these were the results I wanted to improve by changing geometry of the antenna. The penalty was the gain Gmin measured at the least preferable direction for the Cobweb (see the results in orange frame). When SWR improves, Gmin goes down. So, the goal was to improve the SWR in 15 m and 10 m bands without sacrificing too much antenna gain.

    upload_2017-10-4_12-36-16.png

    I think that this is the order of the antenna performance: Mark 3, Mark 2 and Mark 1. But Mark 1 and Mark 2 are quite easy modifications of the basic design while Mark 3 is more complex and is described in more detail in this thread:

    https://forums.qrz.com/index.php?threads/my-final-attempt-to-improve-cobweb-bandwidth.573588/

    I hope I helped a bit.
     
  8. AK5B

    AK5B Ham Member QRZ Page

    Are you taking declining propagation into account?
     
  9. M0MNM

    M0MNM Ham Member QRZ Page

    I have 3 other people locally with a cobweb, so I'm comparing it to theirs...mine is lower in height, but I always seemed to make the trip to people who could hear me, with 100 watts, now I have 400 and can't seem to be heard. :(

    I do take on board what you are saying though, I just can't shake the niggle that it's not quite right...

    SP3Ls modifications seem to suggest end spacing isn't as critical as swr though? I think....

    Maybe I'll just have to put it back and try it. Thank you. :)
     
  10. G1YBB

    G1YBB XML Subscriber QRZ Page

    Apologies for bumping this antique thread, but I am hoping to actually complete this project very soon!
    I have 3 weeks off work furloughed and am ploughing through the antenna projects full time!
    Just 4x19 ele for 432MHz and this to make!
    This only needs the wires stringing out and tuning, it's been built awaiting assembly for about 3 years. So many VHF/UHF yagis made and contests entered in between...
     
    AK5B likes this.

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