Cobwebb antenna

I just put a cobweb antenna in my attic. My house is a wood frame so the attic is fairly electrically transparent.

During recent range testing of a hexbeam I needed a horizontally-polarised resonant antenna covering 20m thru 10m which could be assembled and disassembled quickly and easily. The cobweb antenna meets the requirement nicely - it comprises 5 parallel dipoles bent into a square shape and mounted on a support structure made of fibreglass poles. The turn radius is a very compact 6. Computer modelling of this arrangement shows that one result of bending the dipoles is that the antenna is close to being omnidirectional in azimuth, with a Free Space eccentricity of about 4dB; a less attractive result is that the feedpoint impedance drops to around 12 ohms at resonance. The commercial CobWebb antenna marketed by Steve Webb G3TPW - and most of the home-made derivitives - overcome this problem by using folded-dipoles constructed from Figure-8 twin lead as the elements.

Cobwebb antenna

JavaScript seems to be disabled in your browser. For the best experience on our site, be sure to turn on Javascript in your browser. In stock, shipped in 1 to 2 days. The Cobweb antenna operates on the five upper shortwave bands 20, 17, 15, 12 and 10m. The five folded dipoles are fed via a common balun, only one feed line is required. The arrangement of the elements to each other and the ground effect results in a small gain compared to a dipole. The antenna is designed for a power up to W. The usable SWR bandwidth covers a large part of the respective band. The SWR minimum can be shifted as needed by changing the length of the dipoles. The unfolding of the dipoles results in very compact dimensions of about x cm.

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

The Cobwebb is an antenna that has gained in popularity in recent years. The horizontally polarized omnidirectional antenna is suitable for the 10m, 12m, 15m, 17m and 20m band. All elements are fed through one single feed line. At only half wave length above the ground, it will outperform any vertical antenna. Measuring only 2.

When you subscribe, you receive only messages for the product you have subscribed to. You can check your current subscriptions and remove yourself from subscriptions at any time by visiting the Reviews Home page and clicking on the 'here' box under Subscriptions. If you have comments, questions, or problems with this procedure please write to the Forums Manager. This project involves a management team of volunteers who each take a topic of interest and manage it with passion. The site will be something of which everyone involved can be proud to say they were a part. Toggle navigation. Forgot Password. Reviews Home. The CobWebb is a full size half wave dipole on each of the 5 amateur bands, 14, 18, 21, 24 and 28 MHz.

Cobwebb antenna

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. 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. 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.

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I can't find that information or the way to calculate measures. This raises the feedpoint impedance by a factor of 4 and provides a good match to 50 ohm feedline; the feedline is usually coiled to form an air-cored common-mode choke close to the feedpoint. Since mine lies on thick fiberglass insulation batting in my attic, I need minimal support structure. 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 SO and the feedline, using the remaining braid and inner. Now bind the two Chokes together - again, I used a couple of plastic tie-wraps. SWR on the 20m band the horizontal dotted red lines are and So as a further project and from personal interest I have decided as an experiment to have ago at building another Cobweb using G3TXQ design and will compare against the G3TPW original. Swapping Over the Cobwebs Last Friday evening in a bit of madness during a bout of boredom, I decided to dig out the dismantled G3TXQ cobweb from the attic and set it up outside in the garden. At first I was at a loss as to what I had done and so eventually decided to do a total rebuild and took the main box apart to check that everything was in order and to clean out all the silicone which was causing a mess everywhere. During recent range testing of a hexbeam I needed a horizontally-polarised resonant antenna covering 20m thru 10m which could be assembled and disassembled quickly and easily. I have not tried to adjust it but the SWR on mine is 1. The five folded dipoles are fed via a common balun, only one feed line is required. Trim the four ends to form short pigtails.

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Good luck in your build and please let me know how you get on. I suspect that some metalwork in the roof of my house is the cause. I'm a new ham, and I am just now setting up my base station, and the CobWebb will be my starter antenna. Frequency range MHz. The commercial CobWebb antenna marketed by Steve Webb G3TPW - and most of the home-made derivitives - overcome this problem by using folded-dipoles constructed from Figure-8 twin lead as the elements. Overall the antenna will be about 17' square but that is still smaller than a lot of 40m beam antennas which I know this will not be. The horizontally polarized omnidirectional antenna is suitable for the 10m, 12m, 15m, 17m and 20m band. Are the two twin wires shorted together on each side of the rectangle, to provide the gamma match. Hi Bob, Having the Cobweb in the attic does cause some issues as I found out. The G3TXQ is so much lighter; certainly the single wire and smaller aluminium central clamps make all the difference. If you want further information look at the Cobweb Revealed website or alternatively please ask and I'll be happy to help where I can but I warn you now I don't understand the theory, hi hi. Now repeat all of the above steps to form a second Choke.