One of the interesting things about ham radio is that you can experiment. For those of us who like to ‘learn by doing’ there are a whole host of projects you can get your teeth into.
The Cobwebb antenna is a multiband antenna that takes up very little room and has a reasonably good performance over 5 bands. Most choose the ones between 14Mhz and 28Mhz (14, 18, 21, 24 and 28 MHz). There is a wealth of information available at a number of different sites. The notes I followed are available here (http://test.g0mtd.co.uk/CB1.pdf) it’s about all you need. I’m also fairly sure that this is the site that a number of constructors have used before and followed with ease.
Being a ‘clanky’ the only critical point I could make is that the drawing dimensions could be a bit more helpful to those slightly less mechanically minded. Some useful numbers long the length of the poles would be handy so that you don’t spend a long time putting things together only to adjust then adjust again. So in order to be as helpful as possible the attached CAD files should be helpful.
So, what’s on the shopping list.
1. To do it bang on the money and you need 36.82m of 6A twin 12v cable. Found mine on eBay – 100m for £36
2. Some chocolate blocks
3. Thin bar 2-3mm worth- This is for the connections in the box. I used copper welding rod
4. Fibreglass poles – eBay trader spratreader sells exactly what you need for about £25
5. A centre block to hold everything in place. I’ve made my own out of a block of nylon. Just a piece of scrap material laying about in the workshop
6. Space to lay things out
7. Some spare time and an antenna analyser of some sort to make adjustments.
Follow the instructions in the pdf to the letter and you’ll get a useful antenna. If you’re felling clever then you can adjust things to a la Steve (http://domain1809176.sites.streamlinedns.co.uk/cobwebb_features.htm).
So I got things right I made a simple mock up using bell wire to test the principle in my QTH. The photo’s below show it in all its glory.
Next step was to build the real thing. Again following the instructions in the pdf is more than satisfactory. But they can be summarised as follows.
1. Cut the wire into the right lengths
2. Cut back the insulation and solder
3. Make up the centre section
4. Lay out the antenna without the wires attached.
5. Mark out the intersection points
6. Fix antenna cables and enjoy – this takes a while to get it right!
The whole build took me about 4 to 5 hours in total, most of that time was ‘thinking’ time on how to adapt some of the instructions for my antenna. particularly fixing the poles into the centre section, see below
Ignoring the ‘bodge tape’ the hollow poles a fixed to the centre section using a M12 cap head screw that was bonded into the tube using Devcon epoxy. A thread has been cut into the block to accept the bar. The poles can then be screwed into the nylon block and the centre hole is for a portable mast (painter poles as you’ll see later).
Essentially it is not a difficult antenna to build, the use of a good workshop that can help with the mechanical aspects is very handy otherwise buying things like taps (as opposed to dies) can be expensive for such a small amount of use. the most time consuming aspect it laying out the cables to make them symmetrical. I can suggest elastic bands and patience to allow you to adjust things. See below for the finished set out (note the temporary connection set up at the bottom of the picture.
All that needs to be done is check the tuning and away you go. The photo below is set up in the garden using an FT817 with 2.5Watts which allowed me to have a QSO with an HB station who was also portable and running QRP. I have plans to make the antenna my portable HF one for when I go up and down to Scotland as it works very well and fits into the car without too much bother. I need to work out a way of simply attaching the cables to the poles and to finish off the connection plate as well as making up a protective bag of more likely tube that I can use to transport it in.
So here it is in all its working glory for that first QSO
To make the laying out a bit easier here is an AutoCAD file (.dxf) with the layout in it. The file should be able to be opened by a number of 2D CAD packages as well as simple to use 3D packages like Google Sketchup