Woud a disguised Cobweb work the same?

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

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  1. AK5B

    AK5B Ham Member QRZ Page

    Clothesline by day, skyhook by night.

    (Tip: get your xyl or seamstress to fabricate some really lllllllllloooooooooooonnnnnnnnngggggg pants, overalls or socks that require extra clearance height above ground that will likely result in better daytime operation):D
    KM2K and G1YBB like this.
  2. G1YBB

    G1YBB XML Subscriber QRZ Page

    I have had exactly the same! I could watch the swr rise as I transmitted. I guess the toroids were overheating?
    I left it a while and swr was back to start point.

    I think I may have even damaged it as it was doing weird things on a couple of bands.

    I am seriously considering building a more QRO version of the diamond version discussed with the feed balun at the centre.
  3. SP3L

    SP3L Ham Member QRZ Page

    Increasing Cobweb dimensions and thus increasing the space between the elements ends makes the Cobweb less omnidirectional and increases its bandwidth. But it can not solve the problem with unstable SWR. IMHO the suspects are:

    1) the ferrite cores used for the balun are not what G3TXQ adviced: they are either smaller than required (1.4" for up to 200 W) or made of other material than the material #61

    2) the spreaders are made with lossy material like a mixture of fiber glass and carbon glass; it should be the pure fiber glass without any admixture of carbon glass

    3) the twine used to connect the ends of the elements is not a good insulator; check out the twines made of different material

    Cause no. 1 seems to me to be the most probable one.
    As to the cause no. 2 - if you still have some piece of spreader material left, you can test it in you microwave oven. Put in a small piece of it and a half full glass of water next to it and turn the oven for 30 seconds. If the spreader material gets warm, it is not good for the purpose.
  4. G1YBB

    G1YBB XML Subscriber QRZ Page

    I did use the ferrites Steve recommended in mine, but I think I was overloading them in 100% duty cycle.
    I think the fibreglass is OK, I think I would have done that test.
    I hadn't considered the twine, though it's nylon cord, should be OK? I can't recall now if it was a damp day when I had my last issues.

    But I do fancy building a diamond formation one like we discussed earlier in this thread.
    I'm guessing making the choke out of 2x FT240-61 would give greater than the 200W Steve suggests for the FT140 rings?
    So choke centrally then 4xRG316 (for 12.5ohm) up one spreader and back to 20wire then some more to 17m wire etc, with 5 wires all in squares, not bunched up to a common feed point like the 5 spoke version. This sounds correct to our previous discussion?
    I'm guessing this would be easier to simulate in 4NEC2 too?
    Would heavier wire gain a little more bandwidth?

    One of the main reasons I'm looking to make the choke as QRO as possible is my choke was fine at minimum SWR but was more stressed at the bottom of the band where I was operating, with a higher starting SWR it was (I assume) heating up. Is there a guide to power ratings on the ferrites? I haven't been about to find one though I have found people selling 3KW choke baluns that appear to have FT240 ferrites in them.
  5. AK5B

    AK5B Ham Member QRZ Page

    G3TXQ personally recommended using 2x FT240-61 toroids for the QRO cobweb I built back in 2011; I did so using a little bit longer piece of RG-316 for the increased size and it all worked FB. (I ran 600 watts PEP)


    G1YBB likes this.
  6. 2E0VSS

    2E0VSS Ham Member QRZ Page

    There is one for sale from the originator, Steve Web on the buy and sell Facebook site this morning In Leicester at £100.00.

    G1YBB likes this.
  7. G1YBB

    G1YBB XML Subscriber QRZ Page

    Dan do you happen to have a 4NEC2 file for this version?
    I downloaded the EZ file from your previous model of this one but it throws an error when imported into 4NEC2.
  8. AC6LA

    AC6LA Ham Member QRZ Page

    I'm afraid I've lost track of what is meant by "your previous model of this one". If you could upload the EZ file in question it would be a great help.

    Dan, AC6LA
  9. G1YBB

    G1YBB XML Subscriber QRZ Page

    What I'm actually after is a starting point simulation of the diamond cobweb with the feed point at centre and going up one arm with the 12.50ohm lines to the 10m wire first and up to 12 etc and 20m last.
    The model I downloaded (but wouldn't work) was the diamond with the feed at centre and 12.5ohm line going to 20m first then down the bands to 10m.
  10. AC6LA

    AC6LA Ham Member QRZ Page

    I have a half dozen or so different Cobweb models from 2015. I may be able to guess what you have in mind but, to start, I'd first like to determine what "throws an error" and "wouldn't work" actually means when attempting to open a given EZ model file with 4nec2.

    So I'll ask again. Please upload the EZ file that is giving you problems with 4nec2. After we determine how to correct or avoid the 4nec2 problem we can then move on to the order in which the elements are fed.

    Dan, AC6LA

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