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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

    A Cobweb sounds the ideal answer to my problem of working HF at home, where I can't really have an antenna up.
    Currently I am using a wire dipole for 20m that I put up after dark on a telescopic fishing pole.
    I'd planned to build a 20, 30, and 40m trapped inverted V dipole which I can just squeeze in using a 10m fishing pole. As I understand it, each trap will add loss (I was planning on tuning in band for simplicity which is worst loss case scenario) and also narrow the bandwidth of the following dipole(s).

    However the Cobweb sounds good, in particular I like the @G3TXQ design. I was wondering if it would still work the same if I built it to look like the rotary washing line already installed in my garden (pic attached) rather than the usual flat construction. I could have that low by day and sneak it up at night.

    Just to be clear, I am thinking of a fresh build made to resemble a washing line, not using a washing line as the basis of a build.

    typical washing line like I already have in my garden, with upswept 'spreaders'.
    line.jpg
     
    Last edited: Sep 8, 2015
  2. KB1CKT

    KB1CKT Ham Member QRZ Page

    In my mind, a washing like is about as ugly as a Cobwebb; or to say, if an HOA disliked one, they must dislike the other. They both look the same. But I guess one emits evil radiation and reeks of low lifes or something.

    Seems to me to be a good plan. I don't know how you could raise it easily; nor do I know if you could get the upswept part. But it strikes me as good solution to a need. I think it's a worthy attempt. [Maybe use bamboo spreaders? Heck, buy one of these contraptions, and replace the spreaders with bamboo, and go from there?]

    One thought though: would it be made simpler (much simpler!) to just do 20m? At the moment the bands are headed down, and if you are an evening operator then I wonder if those upper bands are really critical. If it was me (and it's not) then I wonder if I would want to build a 20m-only Cobwebb, fake in whatever else is required, and set that up. Then later on make a 20 thro 10m version, and replace the 20m-only version. Reason being, it's nicer to be on air quicker, than it is to make the ultimate solution right up front.
     
  3. G1YBB

    G1YBB XML Subscriber QRZ Page

    You have a valid point about getting on air quicker, and one worth pondering over.

    There is an existing actual washing line installed like in the picture which is why I thought it would make a good cover to replace it. I can build and test elsewhere.
    I would make it to fit a 10metre fishing pole I have for the dipole and use that, and have a small pole for off air back at head height.

    Now you have got me thinking about sticking with plan A, trapped 20m,30m, and 40m dipole, and making a cobweb for 17m and higher, which would also make it a smaller size, more similar to the washing line itself.

    Actually building it I would find away if it'll work.

    My question about swept up spreaders would still stand of course.
     
  4. WA4SIX

    WA4SIX Ham Member QRZ Page

    Most of those clothesline units are aluminum & I would expect some interaction. See if you can find a fiberglass one.

    Ed
     
  5. KB1CKT

    KB1CKT Ham Member QRZ Page

    I was thinking the spreaders could be replaced with bamboo or fiberglass. Drill out the rivets (I'm assuming they are rivetted) and use screws with the new spreaders. I don't think the mast in the middle matters too much. On a 20m-only version, the cross braces in the center can likely stay (may need to, in order to hold up under the weight).

    Actually... on a 20m-only version, I wonder if the plastic caps on the ends could be popped off, and short lengths of dowel shoved in. Wild guess says a 20m folded dipole won't fit around the standard clothesline (maybe I'm wrong). If so, maybe that would be quick/fast to pull off. The metal in the middle won't help, but perfection is the enemy of the good enough. What is the distance on the ends of the existing clothesline? I am wondering if the spreaders are long enough already.
     
  6. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    That's true, but (a) losses in trapped dipoles need not be excessive, and (b) lots of QRP operators and others set up inverted vees where the ends can be easily raised and lowered and simply clip on extra lengths of wire to allow efficient multi-band operation without traps. Wider bandwidth and no appreciable losses as long as the clip system is properly maintained. Not very practical in, say, Alaska, though.
     
    G1YBB likes this.
  7. G3TXQ

    G3TXQ Ham Member QRZ Page

    It should work fine provided the spreaders are non-conductive. You need to think about how you will support the balun box, and how it will connect to the wires which will now be at different heights for each band.

    Steve G3TXQ
     
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  8. AJ5J

    AJ5J Ham Member QRZ Page

    Aha! That balun does present a problem, doesn't it? Otherwise the upswept design would be fine as I see it. I found that the best performance on my cobweb was noted on 20 and 17. With Cycle 24 on the wane I would aim for those two bands alone (or maybe 15M added in) and forget about 10 and 12 which aren't so hot on a 'web ((too much interaction between them, IMO).

    IIRC the distance between the spreader arms was about 5-1/2 or 6 feet; really not much larger than a clothesline. With only one or two bands you could probably use the upswept design and possibly support a lightweight balun between the spreaders with "clothesline" rope.

    73, Jeff
     
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  9. G1YBB

    G1YBB XML Subscriber QRZ Page

    I wasn't planning on modifying an actual drier line, as mine has 3 arms anyway, and it will likely be weight inefficient.


    I have a link dipole already for 20/30/40m that I use for SOTA activations, but for the colder nights when I am in the house, I want something that doesn't need going outside and lowering the mast to reach the links each band change.



    Thanks for the reply. I was thinking of making/getting made something along the lines of that plastic piece at the apex of a slot together gazebo, but with a 5th socket for the arm holding the balun and maybe even a 6th to fit onto the pole.
    Although the faux drier style does offer itself up to a centre guy like you use on very long boom yagis to stop it sagging.

    Main thing is the concept sounds like it should work so I can get the thinking cap on.
     
  10. WA7DU

    WA7DU Ham Member QRZ Page

    I did not see height discussed in earlier posts in this thread. Height above ground is a critical parameter for this antenna (and most others). A low antenna will mean a steep take-off amgle, signal loss in the ionosphere, and short propagation. How high were you planing to fly the antenna?
     

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