Woud a disguised Cobweb work the same?

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

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  1. G1YBB

    G1YBB XML Subscriber QRZ Page

    I have actually considered leaving a few all plastic pegs on it :)
    Although I don't actually use the real rotary line we have outside now, prefer it dried indoors away from birds, rain, and spiders.
     
  2. G1YBB

    G1YBB XML Subscriber QRZ Page

    not sure this claim is entirely factual either!

     
  3. G3TXQ

    G3TXQ Ham Member QRZ Page

    No - that claim shows a fundamental misunderstanding. The cobweb's gain could be 6dBi when the antenna is placed over ground because it benefits from the ground reflection gain which is around 6dB. A dipole in the same position would be 7dBi to 8dBi. So the cobweb is between -1dBd and -2dBd.

    The author has mistakenly (or perhaps knowingly) compared the cobweb's gain over ground (6dBi) with the dipole's gain in free space (2.1dBi) In other words he's comparing apples with oranges.

    More "gems" from his cobweb description:

    "Folded dipoles construction made it very low floor noise antenna"
    "T-match impedance system adaptor (bazooka antenna)"
    "Performance over 90%"
    "Very low floor noise and more gain than any comercial vertical on perfect ground."

    Steve G3TXQ
     
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  4. G1YBB

    G1YBB XML Subscriber QRZ Page

    I thought it more than a little hopeful. Any physical shortening of an antenna (like bending it into a square) has to reduce its maximum gain I would expect.
    However, I think it's fair to say it (square) has more gain than the dipole in the nulls the dipole would have.
     
  5. G3TXQ

    G3TXQ Ham Member QRZ Page

    Yes - if your dipole isn't rotatable the cobweb will have a little more gain in certain directions.

    Steve G3TXQ
     
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  6. SP3L

    SP3L Ham Member QRZ Page

    If I may cut in...
    I think that Steve is a bit too conservative in the cobweb vs. dipole comparison. Before making my own cobweb, I did a lot of simulation with the 4nec2 program. And because a picture is worth a thousand words, I am pasting below a few radiation patterns printouts.

    Cobweb vs. dipole 11 meters above the ground, 20 m band:
    upload_2015-10-21_8-40-14.png


    Cobweb vs. dipole 11 meters above the ground, 12 m band:
    upload_2015-10-21_8-45-8.png

    Cobweb vs. dipole 11 meters above the ground, 10 m band:
    upload_2015-10-21_8-48-15.png

    Initially, I thought that the greater the ratio "height-to-wavelength" the better a dipole is vs. a cobweb. But after lifting the antenna models to 25 meters, the cobweb was still almost equal to the dipole on 20 m band. So, that is not true that at higher heights a dipole is better than a cobweb.

    I think it depends on the shape of a given band radiator. In my model of the G3TXQ cobweb, the 20 m band radiator is almost rectangular while the 10 m band radiator is significantly deformed. See the picture below:
    upload_2015-10-21_8-56-11.png

    So, if in your implementation the feeding point is somewhat closer to the antenna center, your cobweb should be very close to a dipole in its performance at phi=0 degress.

    Of cource, at phi=90 degress, a cobweb is much better than a dipole.

    I have finished my cobweb recently and I am very glad with its performance. You can see a photo of the antenna on my QRZ.com page.

    Jacek SP3L
     

    Attached Files:

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  7. G1YBB

    G1YBB XML Subscriber QRZ Page

    Interesting input Jacek.

    My planned design has the 17m element as a perfect square. It has been on a bit of a hold for a week or so, but I just need to decide on the bore size of the centre piece and get it printed by my friend.

    I was worried about the off central centre of gravity due to the feed point, but the ferrites and coax and their box is quite light. So far apart from the actual feeder coax I'm predicting a total antenna mass of about 1100grams. Possibly less as my solidworks ABS material seems to weigh more than the actual box I got (which was too short for the ferrites, waiting for bigger brother) though I have nothing in the model for fixing the wires to the arms yet, I will probably use very lightweight cable ties.

    No doubt I will have it finished in time for the bands to be dead again :)
     
  8. SP3L

    SP3L Ham Member QRZ Page

    I am impressed how light your version is. Post some pictures when you have it finished.
    I think you will be as happy with the cobweb performance as I am.
    73
    Jacek SP3L
     
  9. G1YBB

    G1YBB XML Subscriber QRZ Page

    I will definitely do photos as I build it.
    I have it modelled up in Solidworks 3D design package. For each part I can assign correct mass properties. I have weighed the wire and the fibreglass arms, and the bulkhead connector. I recently had a wall mounted coax box I designed in solidworks printed which was actually lighter in real life than solidworks predicted given ABS as the material.
    I'm aiming for ultra lightweight as it will be used on a 10m fishing pole. If it is successful, I may look at a revised version for SOTA too.

    Further up this thread is an animation from my solidworks model.
     
  10. G1YBB

    G1YBB XML Subscriber QRZ Page

    Just been checking out your QRZ page about your cobweb. and interested in your comments about the end spacings. How big is the space on 20m?
    I have set all mine to be 6 inches, but did wonder about making the spacing a bit bigger as it would mean the wire is a little further from the 17m wire.
     

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