I’ve made a little CAD drawing in dxf format of the dimensions used when making the Cobwebb antenna. Hope they are useful. They can be found here or a link to the pages on my site about my experiences with the antenna cab be found here.
I did notice that I had made a small dimensional error that had gone un-noticed and caused me quite a bit of bother when trying to tune the antenna.
I’ve enjoyed building the Cobwebb antenna and thought I’d do a little bit of modelling to investigate the design in a little more detail. After downloading a copy on MMANA-GAL I started to get to grips with it. Well I say get to grips with it, it was more like a wrestling match where the result was pretty much pre-determined. Guess who won?
Modelling the Cobwebb antenna is a little more complicated than you may imagine. The construction is based around twin cables and basically the effect of having 2 wires close together gives MMANA-GAL a bit of a headache.
I had to laugh as even I can get the SWR a little lower than that!
Seriously though the modelling package is similar to the EZNEC flavour which I have had a play with in the past and its something I would like to get my hands a bit more dirty with in the future. My Sotabeams MFD is looking a little tatty after some heavy use last year and I’d quite like to make up a rucksack antenna that fits my pack and that doesn’t get blown over in the strong winds we get here on the fell tops in Cumbria.
What is good about all these mistakes and mishaps is that they are free in terms of money, they only cost a bit of time and without this learning I doubt I would still be that interested in ham radio. There are of course frustrations but these are far outweighed by the successes. I can’t thank the dedicated and talented individuals enough who put out the various pieces of software that allow us to tinker effectively for nothing.
Santa very kindly left me with a new addition to the radio set up yesterday. A 5 element LFA antenna from InnovAntenna’s. I must admit I gave him a little steer in the right direction but even so it was delivered on time.
Its figures suggest its a good yagi for my QTH for use with the local net and SOTA / WOTA on FM and into the NI repeater GB3NI at a push.It’ll also be useful for any back packer work as well as club special event stations if needed. The website throws the usual information and as always it should be read in conjunction with your own experience of your site. Never less the numbers are:
Gain: 11.16dBi @ 145MHz
F/B: 19.59dB @ 145MHz
Peak Gain: 11.18dBi
Gain 10m above ground: 16.93dBi
Peak F/B: 20.32dB
Power Rating: 5kw
SWR: Below 1.4.1 from 144MHz to 146MHz
Boom Length: 1.789m
The antenna came in a large cardboard tube but with nothing else in it, no clues as to assembly, which is odd as you get pages of helpful advice from people like Sotabeams who sell cheap out and about antennas. It also states that the elements are solid 1/4 inch (6.35mm). Although mine were not and the use of high quality insulating blocks is let down by the choice of threaded inserts. Which in my case spun one the box section holes that were drilled and made the insulators sit slightly proud of the section and hence area point which over time will allow fretting. One of the elements isn’t quite as secure as it could be and when I looked inside the box to see the type of insert used I was met by quite a few bits of swarf. Nice!
Still, the new toy is up in the loft (until I can sort out the wobbling elements. Each block needs a deeper countersink in order to make sure the raised insert sits fully inside the insulator and not on the taper of the countersink, if you follow that. The spinning insert will be dealt with by a bit of Loctite 603 or other bearing retainer before the holes get too big.
Whilst I am cheesed off that the manufacturing 
quality is a bit below par for my liking I’m sure once I’ve fixed the issues it will be a very useful antenna. Here is is in the loft aiming at GB3NI and getting in (just) and you can just about see a bit of the non perpedicularity / wonkiness of the loose elements. Once the go ahead for an exterior installation has been approved by the in house planning department I’ll be fixing it outside to make the most of the beam. Until that occurs I’ll be manually rotating it by going up into the loft and moving it about, There’s plenty of room up there so there is no problem and means that it’ll give me the kick I need to board out the rest of the central part of the loft.
During the summer I finished off my Cobwebb antenna. It was an antenna build that wasn’t exactly testing the idea for me was that it was portable and easy to assemble and take down when I was away on travels for holidays and such like. I used some fairly heavy gauge figure of 8 wire (speaker cable) and found that the resonance was slightly off where I wanted it to be and consequently after a lot of tuning managed a compromise.
Two things have caused a bit of heartache with this antenna (I think heartache may be a bit strong but they were annoying enough to make me want to look at it again) were; firstly, the weight of the thing hardly made me want to hump it up a fell and when out and about on SOTA and WOTA activations I reached for the lighter weight (but poorer performing) miracle whip whenever I was planning on an HF activation. The other bee in my bonnet was that it was a bit of a hassle to assemble as there was wire everywhere that didn’t form a nice shape round the antenna. Not a performance issue but it took a while to ‘organise’ it.
I came across an article on G3TXQ’s website which moved the chokes to the centre feed point and did away with the need for figure of 8 cable and a tapping point. So I’ve ordered a couple of ft140-61 toroid’s from AMTools on eBay and time permitting will be putting together a single wire Cobwebb to make a comparison over the Christmas holidays.
With an armful of redundant wire from a previous job (Most of it either 12 or 16AWG) I set about creating a 5/8 50Mhz vertical antenna. The idea was to make a wire version of the G3JVL antenna as featured on the UKSMG website. All the parts came out of the scrap bins at work one lunchtime with the exception of the bolts. Only the hole saw and drill had to be borrowed.
The instructions are fairly self explanatory and it shouldn’t be hard to follow, or so I thought. As with most of the designs on the internet I found that there wasn’t much to back up the picture should anything not give the results you were looking for. Would you believe it but mine just doesn’t want to be resonant at 50Mhz, 40 yes or maybe 41Mhz but 50Mhz? Not today, or tomorrow for that matter. So its to the scrap bin again to find out the problem. I suspect its down to the coil.
I knocked together a wound coil which I can tap at any of the whole coils, wound round a piece of ~20mm OD PVC pipe. I will use this to see if I can get the beasty to be resonant on 50Mhz or thereabouts by changing the tapping point. If it all went to plan then it wouldn’t be any fun right?
WOTA Spots
