I have been desperately wanting to build a CobWeb (or is it Cobbweb) antenna for many many years as it looks like a very compact and clever design. My QTH is not that big and having large beams or long wire antennas is not really possible. The Cobweb looks like one possible solution.
The width dimensions of the Cobweb antenna looks fine for me, and the idea of folding the dipole legs is nothing new, but I keep on getting stuck at exactly the same point every time: Lack of spreaders. I was thinking of using fishing rods at one point because according to the one website (image above)(full acknowledgement to Kerry Amateur Radio Group in the links below) one could get away with 2.5m spreaders, something that’s affordable. However I also wanted to add a 40m capability, and then the dimensions just went out the window so that project is on pause at the moment until I can raise a bond on my house to buy proper spreaders.
If you are willing to forego the 40m requirement, the 20m dimensions look reasonable. Having said that, to complicate matters a little bit there are also two basic variations on the 20m design. In the one case they suggest making folded dipoles from twinflex speaker wire (aka “Ripcord” or “Figure-8 wire”) and matching the impedance by what appears to be a shortened gamma- or T-match. More about my concerns about that in a moment.
The second variation proposed by G3TXQ uses a single wire, one for each band, maybe similar to a fan dipole with closely spaced elements, folded in a square like a crab’s claws. He motivates his choice quite well and I expect that he will have fewer losses than the twinflex dipole, but I haven’t actually done any modelling or measurements. Making a balun for this variation to match the impedance is also easy.
I’m aware of a third design approach based on the Halo antenna idea, but I haven’t had a chance to study the details. Solid idea and it seems to bridge somewhere between the first two options.
Practical methods of construction – if spreaders or fishing rods are available – is easy with many variations and options on the web. However, a common comment is that tuning is absolutely necessary and it seems like it can take some time on the roof. The awkward tuning could be due to the close proximity of the dipole elements.
Which brings me to my earlier comment about efficiency, and I am referring here to comments by Callum McCormick M0MCX founder of DX Commander when he talked about what he observed when he folded back the ends of his 40m antenna to make a shorter vertical (see link below) and I’m left wondering if that same principle is applicable here. Furthermore, some people suggest that in an emergency one could erect a reasonably successful antenna using twinflex speaker wire (some people call it figure-8 wire) because the characteristic impedance of the twin lead is around 110 ohms, it appears that the losses from 14MHz and upwards could be significant – and that is exactly where the Cobweb is intended to operate.
But remember, any antenna is still better than no antenna, and I encourage you to at least try building one of the designs.
Links and Acknowledgements:
Kerry Amateur Radio Group https://www.kerryamateurradiogroup.com/ei9fvb-foldable-cobwebb-antenna.html
Solving the Mystery of a Shorter Antenna – End Effect & Velocity Factor https://youtu.be/L9HX7352ukw
g4zfqradio Making a Cobwebb https://sites.google.com/site/g4zfqradio/making_a_cobwebb
M0PZT CobWeb Aerial https://www.m0pzt.com/cobweb-aerial/
Karinya Amateur Radio (G3TXQ) – Cobweb Antenna http://www.karinya.net/g3txq/cobweb/
A different approach to tuning the Cobwebb antenna by Jean-Paul Suijs – PA9X https://www.pa9x.com/tuning-the-cobwebb-antenna-with-a-different-approach/
And that is my story for today. As always, comments on WhatsApp or Telegram to +27 82 573 5580 are always welcome.
73 de Leon Uys ZR6N (AG5RH when I visit the kids in Texas)