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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. G1YBB

    G1YBB XML Subscriber QRZ Page

    How do we feel about knots in the ends of the wire elements?

    I have made a link dipole where to be ultralight I have used nothing but string to make the support for the links as so (left side wire hidden under white string):
    [​IMG]

    I was thinking once my cobweb elements are approaching tuned length I could add in my knots and do a similar thing like so:
    Capture.JPG


    The knots seem to have no detrimental effect on my dipole (great SWR and seems to work fine) so does anyone see an issue doing this on the cobweb elements?
     
  2. G3TXQ

    G3TXQ Ham Member QRZ Page

    You might find that adding the knots shifts the tuning somewhat - they will increase the tip-to-tip capacitance.

    Steve G3TXQ
     
  3. G1YBB

    G1YBB XML Subscriber QRZ Page

    Would it be OK if this was done before final trim? (which is how I did the dipole- get close, add knot, trim last bit for tune at desired freq)

    I see some people have used the inner brass piece of a terminal block to join the strings to elements, presumably these would have the same effect?
    On your page you have a slimmer but similar idea with the (what looks like) brass crimp ferrules.
     
  4. G3TXQ

    G3TXQ Ham Member QRZ Page

    It's OK if they are present before the final trim. But if you got it on frequency first, and then added the knots, I think you'd get quite a shift in frequency.

    Steve G3TXQ
     
  5. G1YBB

    G1YBB XML Subscriber QRZ Page

    OK cool thanks.
    It does shorten the effective electrical length a bit for sure. And can be a right PITA to get the knots spot on, but once done it's mega lightweight and pretty much fault free.

    My 3D printed centre hub (spider as we call it) has had a redesign as the one piece print the machine estimated 95 hours! So my mate has broken it into piece parts to print then assemble and glue, and we had a little epiphany so I think we have shaved a bit more weight off. I am hoping to get a working 5 band cobweb, disguised, coming in under (or around) a kilo. It's not going to survive a Canadian ice storm, but I'm not expecting it to see one :)
     
  6. AC6LA

    AC6LA Ham Member QRZ Page

    Pictures! We want pictures! :)

    Dan, AC6LA
     
  7. G1YBB

    G1YBB XML Subscriber QRZ Page

    Working on it :)
    I've started a draft page which will describe everything. While the spider is being printed (literally as I type this. 4 main spreader tubes are printed, feed box pole tube is printing now) I'm collecting bits and bringing the feed box together.
     
  8. G1YBB

    G1YBB XML Subscriber QRZ Page

    Just a note to say this is still ongoing. I have essentially built the disguised cobweb but I am needing several hours of daylight coupled with no rain and little wind in order to string out the wires and tune it up.
    Calm dry days not at work have been in very short supply for several weeks.
    What reminded me about this is it is a cracking day for antenna work today, first for some time, but I am at work, doh!
     
  9. AK5B

    AK5B Ham Member QRZ Page

    That was how things often were for me when I was up in the rain forest in KH6---every time I had time or went out to work on my antennas it would start to rain. Occasionally we had visitors from the U.K. staying at the lodge where I worked and I always said to them, "I bet you really feel at home here with this weather and all." While they usually smiled at this I think they were quietly dismayed at the damp, cold and dismal weather they found in the otherwise "sunny paradise of Hawaii."

    Good luck in finding a break in the weather combined with time off---I really know how that can be. (Murphy's Law of Antenna Construction).

    73, Jeff
     
    G1YBB likes this.
  10. EB5KT

    EB5KT Ham Member QRZ Page

    Hello all.
    Thanks for your simulation work. I just build a SP3L MK2 cobweb variant. You can see the details in my QRZ.com EB5KT.
    I upload the measured data with about 30m of RG213 line.

    73's and a happy new year.
     

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    G1YBB and SP3L like this.

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