I built a Cobwebb 5-band antenna for HF. Here are my notes which should hopefully help anybody else heading down the same path.
Why?
It’s multi-band, omni-directional and doesn’t need to be located on a mahoosive-tall mast to get decent performance. It can be constructed from inexpensive materials and is (mostly) well-documented on the Internet how to make one. It’s lightweight and small-footprint so doesn’t present a big wind load so doesn’t need a very beefy antenna mast. I’ve had quite a few QSO’s with people with Cobwebb’s who are happy with them.
Disadvantages and Traps
It’s omni-directional so, obviously, you don’t have the gain / rejection benefits of something like a hexbeam or Yagi. 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. Some of the documentation on the Internet has some errors/omissions. 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.
Principal Documentation Sources
Unfortunately, the Cobwebb’s designer’s website (www.g3tpw.co.uk) is defunct – Steve did use to offer kits at GBP 279 plus shipping. Obviously, there’s the oracle, G3TPW’s original article. This is OK but has some errors/omissions. IW5EDI made his own and published his notes. G3TXQ has some very good information on the obligatory choke that you will need at the antenna feedpoint (box at the heart of the spider-web). M0MRR made a hand-drawn picture of his version that’s actually very detailed and good information on wire lengths and expected bandwidths.
Notes on G3TPW’s Documentation
I’m talking about the version that I downloaded in summer 2016 . The length of the 17m wire is wrong – it’s too long – it shouldn’t be 4400mm per leg, more like 4060mm (plus about 80 extra to fold back for the trimmable loop at the end). The coax choke says different things in different places in the document – 6 turns on 3″ diameter or 6 turns on 4″ or, as per the photos, about 6 turns on about a 5″ diameter? I’d just go to G3TXQ’s page and try 10 turns of RG58 on a 4.5″ diameter. The “unique G3TPW system” for
Construction Notes
Materials
Living in an agricultural region of Spain, off-the-shelf clamps and glass-reinforced plastic rods of every conceivable size are hard to come by. At the time of starting this project, I wasn’t aware of this Spanish provider of a basic kit and other materials at a very reasonable prices that would have saved me a lot of time. There’s no McMaster-Carr here! What I can get is aluminium alloy (some alloy, don’t ask which, probably 2000 series) in plate and bar, engineering plastics, fasteners, a limited range of GRP tubes and bars and that’s about it. I needed to resort to eBay UK to get decent twin-core speaker wire at a reasonable price. There is a GRP tube / bar factory not too far from me that doesn’t charge too much for delivery (which is fine as long as somebody doesn’t step on the tubes in transit). They pre-cut the GRP to lengths specified. I couldn’t find any of those nice two-part plastic clamps (e.g. Stauff brand) for the tubes so I machined 2-part clamps from aluminium alloy bar. I also couldn’t find large-sized pole clamps for the mast, so, again, I machined some (very, very) beefy clamps from aluminium alloy round stock.
Spreaders
I used short sections of galvanised steel tube in the centre with some Delrin bushes (home-turned) to support the next stage of GRP (tube) and then the final stage was solid GRP rod. The manufacturing tolerances didn’t allow the GRP rod to slide inside the GRP tube so I had to chuck each bar in the lathe and whittle down the OD a little with emery tape at the joining end in order to allow assembly. I made little steel jigs to enable on-centre, perpendicular drilling of the GRP ready to accept the stainless self-tappers. I splashed out on brand new split-point drill bits for the job. Since this is Spain and everything that is plastic or wood suffers hugely from the summer heat and UV, I sprayed the GRP black using rattle-can paint. And black looks cool.
On a general note, I know people who have made spreaders from aluminium, bamboo and PVC drain pipe. Aluminium (or any other metal) doesn’t sound like a Good Idea to be close to the antenna wire itself, I would have thought that would screw up the antenna’s characteristics pretty well. Bamboo sounds awesome although it wouldn’t last very long in the Spanish climate of heat, UV and occasional rain – 3 years maximum. PVC might last a while outside if you painted it. My experience with PVC here is that it lasts about 5 years before UV makes it brittle. Plus PVC pipes aren’t so rigid for the same diameter as GRP – not even close. So with PVC tubes you’d also be adding quite some extra wind load in order to get some rigidity for the spreaders. A hybrid of bamboo-reinforced PVC (bamboo on the inside) pipe might be cheap – and interesting!
Junction Box
This was made almost exactly to G3TPW spec except I used solid-core copper, insulated house wire for the jumpers. I attached the box to the GRP pole with a home-machined clamp from HDPE (chopping board plastic). I used 6 turns of RG58 around 4-inch former for the choke, but I expect that a few more turns would have been advisable according to G3TXQ’s investigations. I installed a SO-239 chassis socket on the box wall – in hindsight it would have been neater to mount that socket on the outside end, allowing the maximum length of shack-feed coax to be hidden and protected inside the tubes.
Home-Machined Clamps For Spreaders and Mast
The home-machined clamps are beautiful, even if I say so myself. But they took a ridiculous amount of time to fabricate. They’re also quite weighty, especially the mast clamps which are machined from solid 95mm round aluminium alloy bar.
Attaching Wire To Spreaders
The “unique G3TPW system” for attaching the speaker wire to the fibreglass spreaders isn’t exactly clear in G3TPW’s document. From what I can work out, you slip a couple of extra lengths of heat-shrink onto each wire before connecting into the junction box. Then, at each support point where there’s the screw into the fibreglass spreader, you attach a eyelet crimp connector. The crimp connector isn’t doing anything electrical at all, it just has a handy shape to make a hook. Once tightened down, the crimp connector can be bent upwards to make the hook. You can then slide down the heat-shrink to the support point – the idea of the heat-shrink, I think, is to provide some extra abrasion resistance. Then you pinch the wire into a bit of a ‘U’ at the crimp connector hook and use a small cable tie around the fibreglass spreader, the ‘U’ and the crimp connector barrel to hold the whole lot in place.
73, EA5IGC