Cobwebb Design.pdf Cobwebb Design.pdf
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Type : pdf

A few years back I acquired a homebrew Cobwebb antenna, which I used mainly for portable use with 40ft Trailer mast. Being very pleased with the performance of the antenna and its convenience of use, the one downside for me, was the difficulty in transporting it, being approx 8.5 feet square in size. So the idea was born, to design and build a foldable Cobwebb antenna, which would fit in the back of the car to ease of transportation.

The first step was to build a suitable hub. From discussions with various people, I arrived at a stainless steel designed hub. A friend with excellent welding skills duly made the hub for me. I was worried about how this mass of stainless steel would play with the performance and tuning of the Cobwebb, but "nothing ventured, nothing gained", so I went about experimenting anyhow.

The Stainless Steel Hub is shown below, and consisted of hub (35mm inside diameter) for mounting on a mast, with 5 hinged stainless steel tubes for mounting the fiberglass rods. The tubes can be locked in the spread position from underneath, by threading a bolt through the plate steel and into th e steel tube.

Stainless Steel Hub
Cobwebb Dimensions.xlsx Cobwebb Dimensions.xlsx
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Type : xlsx

Researching the original G3TPW CobWebb Antenna for the 14, 18, 21, 24 and 28 MHz Band (file shown attached above), and variations of it, I settled on generally following the design shown below in the sketch. This design used twin speaker wire as the elements. The speaker wire should ideally be 42 strand, available from any electrical wholesalers.

The dimensions I used are shown in the attached Excel sheet. Note these dimensions are approximate and don't need to be too exact.

The fiberglass rods were sourced locally, and come in 8ft lengths. As the rod profile is tapered, I trimmed the length on the larger diameter side of the rods, so that they sleeve snugly into the stainless steel tubes. The rods ended up approx 7ft long after this trimming, which is more than enough. You will notice that four fiberglass rods are green in colour, and one is white in colour. The white rod has a thicker wall thickness, which provided more strength for mounting the junction box. It also provided a "design feature" hi hi.

Next step was to make the junction box. I sourced the junction box sized 4.5" x 6" x 3" high, and fitted the two x 10 way terminal strips and choke balun and mounted the box on the white fiberglass rod, using two stainless screws drilled through box and rod. (Note: the open side of box faces down to prevent any entry of water/rain). Nuts were mounted on the upper/rod side and cut flush with a junior hacksaw. Speaker wire lengths were cut (allowing 10-15 cms extra length for tuning later), and shorted in appropriate positions (see attached excel sheet with dimensions). Holes were drilled neatly through the sides of the junction box, to allow entry of the speaker wire. Each element speaker wires were connected to the terminal strips, ensuring good solid connections.

The choke balun was made by winding 6 turns of RG58, on 3 inch diameter. I used a 3 inch diameter paint can. I then slipped cable ties under the windings to hold them temporarily. Then I used hot melt glue to hold the form of the winding. Then slip the coil off the paint can. Connect coax to terminal strips. Ensure to put cable tie on coax inside the box, to act as a strain relief on the coax.

Once the junction box is completed, then next step is fitting the speaker wires to the fiber glass rods, securing them with cable ties. I also wrapped electrical insulation tape to ensure the cable ties don't slip on the fiber glass rods later. The position of the wires along the rods are shown on the attached excel spreadsheet, as measured from Cobwebb centre.

From centre of Cobwebb, the wire positions on each of the four support rods are as follows,

35" (10M), 41" (12M), 48" (15M), 56" (17M) and 70" (20M)

I also put electrical insulation tape to secure the fiber glass rods to the stainless steel tubing. This would allow easy replacement of the fiber glass rods, if required, at a later date.

Fully assembled foldable cobwebb shown below in the spread position

Shown below in the folded position. Total length is 6.5 feet. This fits nicely in a ski bag or fishing rod bag for transportation.

Cobwebb up in the air. Checking with antenna analyzer reveals resonance on 10-20M bands, as well as 6M, 4M, 2M and 70 cms bands, as well as other non-amateur bands. So nine useful amateur bands on one antenna is not too bad !!

Note the Cobwebb is omni-directional, so no rotator required, and no tuner required on all bands.

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