COBWEBB ANTENNA PDF

The main part of the CobWeb is the mounting-plate that holds the crossover poles and boom arm. After looking at the various examples online, I opted to re-engineer an existing design — Some of which used either plastic PVC conduit clamps or small U-bolts — I opted to use the rugged Stauff element clamps which are able to support the spreaders securely without damaging or stressing them. The mounting arrangement consists of 2 plates — The 1st holding the main crossover poles 1 on top, 1 underneath and the boom, the 2nd supporting the boom and fixing to a mast. This design permits the CobWeb to be mounted at any height up a typical 1.

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The hardware kit comprised two aluminium plates, V bolts for mounting on a pole, Stauff clamps to hold the aluminium tube, three lengths of aluminium tube and all the necessary nuts and bolts. The builder will need to add either plastic or fibreglass tubes, wire for the dipoles, and the connection box and balun.

If you are interested, please contact me via email. By the way, fibreglass can be used throughout if you prefer, without aluminium.

The Stauff clamps are great for this because they have a large surface area for clamping — much better than U bolts which can crush the tube. Hardware kit for the CobWebb Construction With a bit of spare time early in I decided it was time to put one together myself.

The metalwork was easy, but I found that winding the transformer and balun was surprisingly fiddly and required an unusual combination of strength and agility in the fingers. I took the precaution of testing each half of the transformer individually for shorts and continuity and that worked out fine. In retrospect it would have been easier if I had used a larger box to contain the transformer, but once it was drilled, I was committed.

Initially I assembled the CobWebb with only the 20m element, and placed it on a pole about 15ft high. To my relief, the analyser showed a match of right in the middle of the band at Temporarily, until better weather, my own Cobwebb remained a single-bander. It was also a droopy single bander. To keep costs down, while being a little uncertain that it would work, I used cheap lightweight plastic tubing to support the elements. At last, in June , I have found time to complete my CobWebb.

The plastic arms have been replaced with fibreglass tubes, and wires for all bands are installed. The photos below are probably self explanatory. Resonance was achieved first time in or very close to each amateur band.

Just a quick tweak of an inch or so was needed to complete the job. The network analyser plot nicely shows how resonance within each amateur band can be achieved. Note that in common with most aerials of this type, coverage of 10m is limited to part of the band because the span from You can easily chose the section that you want by trimming the dipole length.

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High Quality 5 bander CobWeb Antenna

Use the Contact page to talk about your project. Here are my notes which should hopefully help anybody else heading down the same path. It can be constructed from inexpensive materials and is mostly well-documented on the Internet how to make one. Being quite an efficient antenna, it has limited bandwidth. This is particularly noticeable on 20m and 10m where we are lucky to have quite a lot of spectrum available to us so you need to make some tough choices when trimming your antenna. Sourcing some of the parts to make it at a reasonable price in your country might be tricky which could lead to quite some extra effort to fabricate alternative parts.

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Cobwebb Antenna

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. 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. However, the folded dipole approach to matching has some disadvantages: The use of twin lead adds to the weight of the antenna A shorting link has to be placed at a position along the folded dipole, determined by the differential-mode velocity-factor of the cable. Variations in cable insulation can mean that the dimensions which work for one constructor may not work for another.

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