Antennas

Homebrew 5 Band Cobwebb Antenna – Resounding Success

cobwebb assembledThe cobwebb antenna originally designed by Steve Webb (G3TPW) is a fairly simple antenna to build, requiring only a few hand tools & simple skills.

This antenna project has come about due to a friend entering into the amateur radio hobby and wanting a more “hands-on” approach to getting his first station assembled, rather than just going out and buying everything ready to go.

Without question, the most difficult part of building the cobwebb antenna is the metalwork, and if like me, your engineering skills could be better then a full kit of pre-drilled parts is available from Aerial Parts in Colchester for a reasonable price, this project was built using this very kit.

Various articles with slightly differing designs can be found in plentiful supply online, the G3TXQ version of the cobwebb is the one my friend and I chose to construct – download the instructions.

Some designs of the antenna require the use of twin core insulated wire for the elements, and when using this method, at a given length, the insulation has to be stripped and the cores shorted together to allow matching.

A major disadvantage of using this method is that once the short has been made, should there be any error, it’s a lot more difficult to resolve the problem.

Parts Required

As mentioned above, the metalwork for the cobwebb was obtained commercially, although it can be fabricated by someone with fairly good engineering skills & tooling.

The rest of the components required are:

  • Approx 36m wire to make the elements, 1mm stranded works just fine and is not too heavy.
  • Lengths of 22mm PVC/fibreglass tubing or similar for the spreaders – we found that the standard overflow pipe stocked by most plumbing suppliers is a very good fit in the centre pieces.
  • 2 x ferrite cores, the G3TXQ design states FT140-61, we used FT240-43 as these were already available at hand, these work perfectly in this project.
  • Approx 2m of RG174A mini coaxial cable
  • Suitable waterproof casing to house the balun, the one used here is 120mm x 185mm x 80mm, IP65 rated.
  • Terminals for wires, we used 4mm stainless steel bolts, washers and nuts to construct the terminals, and crimp terminals to terminate the wires.
  • Panel mount cable ties similar to these

Cobwebb Construction

Construction is fairly straightforward if following the instructions.

The most complicated part of construction is the current balun, but is fully explained in the downloadable pdf mentioned above.

For completeness, the following excerpt is taken from the file regarding balun construction:

Cut two 510mm lengths of RG316 miniature coax (or RG174 if you do not intend running more than 100W). Trim the four ends to form short pigtails. Lay the two coaxes alongside one another and connect centre-to centre and braid-to-braid at each end. Although not strictly necessary, I find it helps keep things neat if you slide a few narrow pieces of heat-shrink tubing over the coaxes to keep them together. You have now created a single transmission line with a Characteristic Impedance of 25 Ohms.

Now wind 8 turns of this twin cable onto a FT140-61 toroid to form a 1:1 Choke. The “crossover” winding is not an electrical requirement – it’s just an easy way to get the coaxes to emerge at opposite ends of the ferrite core. Use a couple of plastic tie-wraps to hold the coax to the core at each end.

Now repeat all of the above steps to form a second 1:1 Choke.

Now bind the two 1:1 Chokes together – again, I used a couple of plastic tie-wraps. Ensure that the two Chokes are aligned – i.e. the coaxes emerge at roughly the same positions.

At one end of the assembly (left side of the ‘photo) connect the coaxes in parallel: braid-to-braid, inner-to-inner; this is the end which connects to the antenna elements. At the other end of the assembly (right side of the ‘photo) connect the coaxes in series: braid from one Choke to inner of the other; this is the end which connects to the SO239 and the feedline, using the remaining braid and inner.

This is the method used in this project and it works flawlessly.

Tuning

cobwebb balun boxThis is the completed balun box, terminals and wires of the completed cobwebb antenna. This photo was taken with the antenna mounted on a short pole while testing.

Starting from the outside wires (20m) tune the pairs of wires by folding back the ends on themselves to obtain a reasonably close SWR on each of the bands in turn remembering to fold each wire in the pair by the same amount to maintain electrical balance. An antenna analyser really makes this part of the project much easier.

With the exception of the 15m & 10m bands it has an acceptable SWR curve wide enough to cover each entire band. With the 15m & 10m bands being quite wide bands, it was tuned for centre of band.

Once all is correct, you can mount the antenna on it’s final mounting , re-check the SWR again as it will most probably change as the antenna is raised or lowered from the testing position.

Final Thoughts

While the cobwebb was being built it appeared as it was going to be an ugly mess of wires mainly due to the spreaders not having any weight on them and the element wires not being under slight tension.

Also, moving the antenna during construction, it felt rather flimsy and, probably due to it’s size, awkward to handle. However, once the antenna was completed to the testing stage, any concerns regarding the construction faded.

When the cobwebb was in it’s final position, as noted above, the SWR changed from resonance in the centre of each band to being resonant at the top of each band.

Results: considering the poor band conditions at the time of initial testing, I can say that it appears to receive fairly well with a considerable amount lower noise than the vertical antenna that was previously being used.

How does it perform at transmitting? The first contacts were Russia, Turkey, Serbia & Canada. Hopefully this is a sign of contacts to come…

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