DIYing a QRO cobwebb

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AK5B, Feb 7, 2012.

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  1. AK5B

    AK5B Ham Member QRZ Page

    Recently I learned a bit about Steve Webb's (G3TPW) 5-band Cobwebb antenna and decided one might make a suitable transitional antenna during my upcoming QTH relocation (KH6 to W5).

    Since I will be taking down all of my antenna farm: Moxons, dipoles, Butternut, delta loop and single Yagi well before the big move I figure having a compact yet very efficient multiband antenna easy to take down at the last minute (and set back up quickly at the new, yet unknown exact QTH) would be simply great.

    I had originally been put off by G3TPW's Cobwebb as it requires using funky PVC covered speaker wire for the elements; but then I discovered the other
    Steve's (G3TXQ) single-wire elements version that seemed a lot simpler to construct. Since I like to run a bit of power usually when I'm trying to work DX I decided to build one that would handle the 500-700 watts PEP I frequently need to make the contact (It's a long way to anywhere out here in the mid-Pacific).

    I also decided to utilize as many parts for my cobweb from my junkpile as much as possible:

    A big, thick aluminum table-saw top piece laying around that I figured could be cut down for a sturdy center hub.

    Four large 9' long fiberglass telescoping poles that that I used for my 17 meter Moxon's spreaders---could be cannibalized for the sake of my new cobweb.

    Plenty of 1mm copper wire with 1.5mm insulation already on hand for the elements (same dimensions as G3TXQ used for his).

    The rest of the bits such as the hardware, PVC junction box and 1:1 current choke would be purchased and built as necessary (primarily Home Depot or online sources).

    That's the basic outline of my plan which has already been put into place. I have taken plenty of photos thus far and will start posting them here tomorrow---I must "QRT" now in the forum as it's time I head to my job---but since I have a few days off later this week I will try to post a lot more info as well as make more progress with my cobweb project then.

    73, Jeff
  2. G7DIE

    G7DIE Ham Member QRZ Page

    Good luck with the build, it would seem the Cobweb has gained another fan, it's the best 8'sq you'll ever build ;)
  3. AK5B

    AK5B Ham Member QRZ Page

    Roger, roger. That's what I gather from reading all about it (and KC8VWM's YouTube comparison videos helped convince me, too).

    I'll try to get to posting photos today or tomorrow of what I've done so far, too.

    73, Jeff
  4. AK5B

    AK5B Ham Member QRZ Page

    Finally got some of the photos ready to upload.

    This is the old Skilsaw aluminum table top just after I cut it in half with a hacksaw and my Dremel (burned up another Dremel switch and recent set of brushes in the process---found out the hard way that once it starts getting hot it's best to give it a rest!):


    After cutting it down further into a 12" square I taped it like so for the DXE resin support-block placement and drilling preparation (shown with one set of blocks in place):

  5. AK5B

    AK5B Ham Member QRZ Page

    By the way, the 12" hub plate weighed almost 4 lbs. as shown above so I decided to try to pare that figure down a good deal by extensive drilling, sawing and filing. Before doing so I drilled only the essential holes necessary for mounting the spreaders and balun support arm.

    Here's a couple of pix with my first assemblage of all the blocks and PVC spreader arms (the large fiberglass spreaders slip conveniently over the 1" PVC) in place:


  6. AK5B

    AK5B Ham Member QRZ Page

    A couple of weeks prior to fabricating the hub I took my 17 meter Moxon down so I could canibalize it for it's four 9' long fiberglass spreaders. Here it is shortly after lowering the telescoping aluminum mast it was atop and setting it on the ground for further disassembly:



    While I sort of hated to see it come down I figured the speaders would work nicely for my cobweb and I could still be QRV on 17m with a delta loop I have strung up in the jungle nearby.
  7. AK5B

    AK5B Ham Member QRZ Page

    After cutting numerous cable ties, disconnecting and removing the feedline that I'd snaked through part of the mast I then sorted out everything I would need for the basic cobweb frame.

    I gathered the speaders and headed over to a clear section of our lawn where I slipped them onto my temporary hub assemblage. One of our cats, Guido, came over to supervise the operation:


  8. AK5B

    AK5B Ham Member QRZ Page

    The whole assembly was now somewhat heftier than I expected despite the featherlight fiberglass spreaders. I was still a long way off from cutting and installing the elements, balun box, feedline and hub to mast bracket, too, so I got out my drill, hacksaw and set of files and trimmed the hub a lot more.

    This process was done over a week or two while I had bits of spare time (and the xyl wasn't around to have to put up with the racket I was making) but I eventually reduced the hub weight over two pounds (1 kg)---not bad, all things considered. It is still sturdy enough for the rest of the antenna (or so I hope)!

    I now present the New and Improved NH7RO Swiss Cheese Hub Plate:


    I also drilled several holes in a piece of the table top left over from my initial cutting that I fashioned into a sturdy hub-to-mast bracket:



    Of course the long U-bolts will also be trimmed and I also plan to make the bracket ready for a smaller mast attachment that snaps onto my telescoping mast as well.

    That's about where I am now with my cobweb project as of today. More photos and info to come as I continue to proceed (at my usual snail's pace, hi hi!).

    This is still quite a lot of fun besides being considerably more hard work than envisioned (but that's nothing new, really)

    Comments and suggestions always welcome.

    73, Jeff
    Last edited: Feb 9, 2012
  9. KC8VWM

    KC8VWM Ham Member QRZ Page

    Nice work Jeff.

    Making the hub and spreaders is the majority of the entire job. Looks like you got quite a bit accomplished. The hub plate looks pretty darn heavy duty. Not likely going to ever be a failure point. Those spreaders look pretty beefy too. Perhaps this is the heavy duty mil spec version huh? :) Should survive ice storms quite nicely. Keep those pictures coming. :)
  10. AK5B

    AK5B Ham Member QRZ Page

    Thanks, Charles. Yes, this is the MIL SPEC version cobweb, hi hi!

    In some ways I almost regret using the Skilsaw hub. It will be a heavy monster once the spreaders, wire, QRO balun, resin blocks, PVC arms and twenty+ long stainless steel bolts, U-bolts, hardware and mast bracket are all in place---as I could have opted for a truly lighter weight version as you did instead.

    On the other hand, I was itching to put that saw top to good use for a long time. Since I knew I'd be taking all my antennas down in the near future I also thought that those big Moxon arms would be perfect for a cobweb. I believe my hunch will play out correctly, too. At least I won't have to worry about tropical storm damage here or wind or snow failure once it's up in New Mexico later this year.

    G3TXQ's element lengths charts (with the same wire dimensions of the wire I have on hand) and excellent step-by-step balun instructions really helped clinch the deal. This'll be my last DIY antenna project before we leave Hawaii and I hope I can get on the air with it in time for the early spring propagation. I need to work only ten more DXCCs on 15 meters before I acheive the magic number of 100 on that band so that is kind of inspiring me as well. Any rate, I think it will be great to work some last dx from the island that's been home for me and the xyl for the last ten years---without having to continually run outside and "turn the beam." (I only have one rotator and that one's for my 6m Yagi).

    Thank you for the initial inspiration for building it, too.

    I plan to make a bit more progress tomorrow on it and will post more pix as I move along.

    73, Jeff
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