Cobweb Antennas

Since I've done so many posts on the Cobweb antennas I thought it best to separate them on a different page so that potential Cobweb builders can see all the posts together. Hopefully this will make planning and building easier. 

I've tried to give as much information here as I can, but you'll notice I don't go through the actual precise build process, there are plenty of links within these posts or on the right hand side of the webpage that give the detailed measurements etc depending on what type of cobweb you wish to build. 

For starting out from scratch on the G3TPW I would recommend the original Cobweb Revealed Website and the YouTube video by Denmonkey  For the G3TQX version go to Steve Hunt's website 

As I've stated before the Cobweb antenna is fairly easy to build, I'm no great technical genius and yet I have managed to build both the G3TPW and G3TXQ antennas and if I can build this antenna, so can you! 

This is quite easily the best antenna I have used, it's small (8ft circumference) omnidirectional and produces very little TVI. Either type of Cobweb is ideal for small areas or where you are limited in raising an antenna.

If you decide to build a Cobweb do let me know how you get on and good luck!

Servicing the Cobweb G3TPW Antenna

Finally I've got round to begin the service of the home-brew G3TPW Cobweb. Taking it down from the mast wasn’t as bad as I thought, I decided to do it now as the weather forecast is not good with high winds expected over the next few days, so while there was a break in the weather I zipped up the ladder and began the removal process.

Once down the initial inspection was that it didn’t look too bad considering it’s been up on the mast for nearly two years and been subject to some pretty bad weather conditions. A bit of rust here and there on the metal tubes that hold the wooden poles, but perhaps through luck or my building capabilities it has managed to withstand all that has been thrown at it! 

The wooden poles and the box are covered in mildew and slightly bowed, but still sound, the cables are fairly loose which I suspect is the main reason my SWR has been out, especially on 20 meters which at the last reading was 2.5. Interestingly I thought the box would have been pretty rusty inside but surprisingly looks very clean apart from a good ½ “ of water slopping inside, first thought was hmm, I forgot to drill any draining plugs!

The wooden poles are going to be replaced with fibreglass, I managed to find some ex army surplus which should do the job nicely but I will have to wait as I’ve only just ordered them so it will take a few days to arrive. Since the main box is in such good condition I will do little except give the outside a good scrub and more importantly drill a couple of draining holes. 

The silicon grease has done its job and kept the contacts all clean and rust free, I'm amazed there is no rust on the copper wire or the steel screws. Originally I bought a Cobweb (back in the 90s) for the grand sum of £160 and when I sold it I remember inspecting the box and seeing how badly rusted everything was, so I was expecting the home-brew to be the same, just goes to show what a bit of silicone will do to protect.

Over the next few days I will tighten the cables and recheck the SWR and if needs be replace any that are suspect, luckily I have some spare cable that I got last summer from Maplins, (our version of Radio Shack). 

Looking inside the box it is a remarkable simple system that is very easy to build and yet gives great results. This is easily the best antenna I've ever built and quite honestly if I can build one anyone can. 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). I will include as many picture as I can during the service process so you can see just how easy it is put together.


Further work on Servicing the Cobweb Antenna

The fibreglass poles have arrived for the Cobweb antenna, these I suspect are fibreglass fishing poles but ideal for what I want and more to the point once I had removed the plastic ends with copious amounts of boiling water fit the metal sleeves perfectly. Their length is 7ft which provides the correct length for arranging the twin speaker cable. One thing to make sure when purchasing poles, you require fibreglass only, no carbon or metal!

I have replaced the 20 meter cable, although I could find no fault. I suspect the SWR was high either because of the element being too short or tapping point misaligned, so I have re-measured and added a few more cm for trimming, twisted and soldered the wire at the correct length for the tapping point. 

I’ve also smothered the internals of the main box with more silicone and added some draining holes to the bottom of the box. then lastly I attached all the poles together and slotted them in to the metal sleeves, ready for re-attaching the elements. 

 With the poles in place I started fitting the elements

Something that I learned whilst originally building the antenna was that the Cobweb plans do not seem to allow for extra lengths when tuning/trimming, so it's worth adding some extra length when measuring up. Also when tuning for a good SWR, if the antenna can be raised to a minimum height of 6ft, tuning may well be easier. Over time building these antennas I've come to learn that although the tuning can be completed at ground level, sometimes raising the antennas will help.The PDF is a very useful guide but there are other very good and possibly simpler instructions besides the G3TPW design. It's worth searching on the web to see what else is about, G3TXQ does a similar antenna but uses a 1:4 balun and single wire which probably makes life a lot easier. 

 Starting to take reading off the MFJ on 28 and 24MHz, wire, bits of string and loops everywhere!

Now comes the usual tricky bit of tuning up the elements for each band, sometimes you get lucky and other times it becomes a real test of patience.

 The ties for holding each element is some simple plastic tie straps with a screw to stop it slipping inwards, Ella one of my labs takes a mild interest!

Generally I start at with 28Mhz and work downwards, (but it doesn't really matter) naturally with new poles and slightly different lengths I knew I was going to have to re-tune. So far I’ve roughly got all 5 bands in to the ball park figure of around 1.8/2.0 SWR but I need to do some final adjustments which consists of individually checking each twin cable with the MFJ and adjusting the loop at each end of the antenna to get a good reading. 

 All elements fitted and just ready for the final trimming.

The only issue I came across was that I had to add some extra twin lead to the 18Mhz band as it was giving me a 1.5 at 18.170, herein lies the mystery because with the wooden poles it was spot on, typical!

For the rest of the weekend I will continue to trim up and hopefully by tomorrow afternoon I should be ready to think about mounting the antenna back up on the mast although there's no real hurry and I may just end up waiting till next weekend.

If It Ain't Broke Don't Fix It!

As the old saying goes "if it ain’t broke don’t fix it", I think I went a bit too far in servicing the G3TPW Cobweb. I tried tweaking the elements to the absolute minimum SWR and got in to a real mess which eventually got me readings of 5 on pretty well all of the bands! 
Just to explain, I'm not an antenna theorist, so I'm not going to go in to the ins and outs of the Cobweb design, my explanations are based on layman's terms, so to all purists out there apologies in advance!

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. The box was fine, all connections were good and everything was in order.

I removed all the elements and started measuring each one and to my surprise found that some were uneven and out by a few inches, this probably happened from the tuning and trimming when I originally built the Cobweb with the wooden supports, no wonder my SWR was out. It just goes to show if you make alterations such as changing the poles, measurements will go out of the window and you need to recheck them! These things happen and you just have to except your mistakes, learn from them and move on.

So I re-measured all the elements and added wire where necessary, once completed I re-attached them to the box and laid them back on the poles and took some readings, everything looked much better and in some cases the SWR was lower than the original build.

I haven’t done my final tune/trim yet as the weather has turned and there is snow on the ground, the forecast for the week is very cold with more snow predicted. What’s noticeable is how the twin speaker wire which is quite thick (42 strands) gets very hard to manipulate in cold weather so I shall leave things until it gets a bit warmer. These are my SWR readings so far:

28 MHz                   28.782          1:2

24 MHz                   24.248          1.2

21 MHz                   20.633          1:2

18MHz                    17.664          1:2

14 MHz                   14.079          1:1

The only one which is pretty well spot on is the 20 meter band so obviously I now have to adjust the loops on each band element by lengthening or shortening to get the SWR readings to exactly where I want them. 

Bear in mind I use CW most of the time so I want the lowest SWR in those particular segments of each band. I don't find that the gap between each elements makes any real difference, generally its about 6 inches apart and seems to work fine, although theorists may have other ideas.

 Each loop is held by plastic ties so that it can be easily adjusted

For instance on 28MHz I want to lengthen the loop on each end of the element by a small amount to bring the SWR down to around 28.010MHz, whereas on 24 MHz I have to shorten the ends of the element to raise the SWR up to approximately 24.890 MHz. 

 A closer view of one of the loops

When I was working with the SWR readings I did a lot of browsing on the internet to see if I could find any further information. I came across the G3TXQ website which I have seen before but as it was a different design I'd not really had a good read. He uses single wire (as apposed to twin) and a 4:1 balun which by all accounts seems a much simpler and easier design to build. 

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. It will be interesting to see the difference and more importantly how each one compares on the bands. The toroids and other bits are on order so more in later posts!


G3TXQ Cobweb Build

As I said in my previous post I’m going to have a go at building the G3TXQ Cobweb and see just how easy it is to build compared to the G3TPW original. To be honest the only difference will be the terminal box and the single wire elements as opposed to the twin wire elements on the G3TPW design, the frame will be exactly the same, but I suspect with the single wires and smaller terminal box the antenna will be a lot lighter.

Whilst I have the G3TPW Cobweb down from the mast I am going to change the pole support to the proper aluminium set up (before I used an old plastic bread board together with a thin slice of aluminium plate to make it sturdier). The elements have all now been re measured and the antenna is sitting in my shed almost ready to go back on the mast. But I will now wait till I’ve completed the G3TXQ design, compare notes on both and then put up the winner!

So to the G3TXQ build, firstly the terminal box; thankfully as the Cobweb has become more popular there some small companies dedicated to Amateur radio you can find on Ebay that supply bits and pieces ready made for this antenna, one of them being Spratreader which sells the 4:1 Guanella Balun in a kit form which is extremely easy to put together. 

This balun is slightly different to the G3TXQ version, in that it uses the ferrite core, but has 6 winds (as opposed to 8) and uses thick copper wire instead of the RG-58 coax. I have a small enclosure for the terminal box and will use some old copper roof slate tags as conductors for the elements. 

I will be connecting one end of the balun to an SO239 connector (red to center, black to ground) and the other connections to the two copper plates which then connect to each of the 5 element on each side of the enclosure via small bolts. I’ve still got some spare twin 42 stranded wire so I will peel single strips and use these for the elements.

The pictures above show the mock up, I've drilled all ten holes for the element connections and fitted the copper plates I've also fitted the SO239 and its just a matter of trimming the wires from the balun to fit neatly in to place and solder up.

I'm still not sure whether to use the frame I already have or to to purchase some more fibreglass poles and build a separate frame for this version, we'll see. More to follow in later posts.

G3TXQ Cobweb Build Update

The terminal box has now been soldered up and completed and I must admit I’m quite pleased as it is far smaller than the one I had on the original cobweb. 

 All soldered up and ready to go.

I’ve now made up all the single wire elements and soldered connectors on each one so that they can just slip on each of the 10 bolts quickly and easily, I just have to cut the elements to the correct length for each band but that will be done at a later stage. 

Connectors added to each element which then fit neatly on to each bolt.

I decided in the end to go for a completely separate frame and have ordered some more fibreglass poles and will attach these to the aluminium plate that has now arrived, I still have to cut the plate in to two and then drill the holes for the clamps. The 100mm x 150mm plates as suggested by the "cobweb revealed" are quite small so I will increase the size to 150mm square (e.g. just cut it in half) to give me a bit more room to play with. 

 The new aluminium plate is 300 x 150mm

On my first Cobweb the plates were built with a mixture of an old plastic bread board and some thin aluminium sheeting which actually works quite well, but the size is considerably larger than the 100x 150mm, it’s more like 400 x 200mm and therefore a lot heavier. 

 The original frame made out of breadboards and aluminium plating

As the whole idea is to reduce weight on this new antenna I have to come up with better solutions than home brew breadboards etc! The weather here is still miserable, there is snow on the ground and it is very cold, so getting outside to play antennas is a bit of a no no at the moment. Hopefully next week temperatures will pick up and I can get on with building the frame.


Carrying On With the G3TXQ Cobweb

Unfortunately I didn’t get much done this weekend so I’ve only progressed very slowly on the cobweb. But I have managed to measure out the elements and build the plate for holding the fibreglass rods. The aluminium was fairly easy to cut and with some accurate measuring the U bolts holes were drilled and everything fitted smoothly.

I've fitted some more metal tubes (as with the original cobweb build) to hold and strengthen the fibreglass rods when they are fitted. The extra rods I had ordered finally arrived over the weekend so I need to measure them out and mark up where the corners of each element will sit, hopefully that shouldn’t take me too long.

Finally I fitted the plate and the terminal box to the one rod that comes off the inner X, so hopefully the majority of the work is now done and it's a matter of waiting for a reasonable day so that I can fit the four rods and elements and start to begin the tuning process.

However, as the rain continues and I am stuck indoors I had a thought that might prove interesting and that is to try setting up this new antenna in my attic. I had a quick check and it will fit with about a couple of feet to spare on each side. Not sure how it would behave, but the height would be approximately 25ft which should be OK. 

The G3TPW Cobweb website promotes how good the antenna behaves in an enclosed area and does not cause much interference, it maybe worth having a play especially if this horrid weather continues and I'd love to see how much of a difference when compared to my original up on its mast!

Antennas in the Attic

Well I decided to give my “antenna in the attic” plan a try and have installed the new G3TXQ Cobweb up in the roof. I managed to squeeze it in, it was a bit nightmare to get all the elements threaded and in place. Resting on old boxes for support it is approximately around 25 feet above the ground, so it will be interesting to see how well it works.

The trimming proved interesting as surrounded by wood, slate and bricks I knew it might prove difficult, but in the end after much swearing I eventually got the SWR readings pretty reasonable but I am sure I can improve with some further delicate trimming.

14.030 - 1:2

18.070 - 1:2

21.030 - 1:2

24.900 - 1:3

28.030 - 1:1

 On receive it seemed a bit noisy (that could be present conditions) especially when compared to my 30 meter vertical, but a quick QSO on twenty at 5 watts to an SM station proved that I'm getting out and I got 449 but I will give it a proper try tomorrow morning and let you know the results.

Both Cobwebs Now Up and Running

Well after much playing about with tuning and trimming, I finally got the G3TPW Cobweb back up on the mast yesterday evening.

After my initial tuning the other day I had to dismantle the antenna as the weather was proving very bad, so yesterday by the time I had re-assembled and started up the tuning process the morning had passed quickly by. Finally with a bit more trimming completed, I climbed the ladder with antenna in hand and clamped it in to place.

I switched on the MFJ 259 and did some checks, the SWR was all over the place, I had a problem, something was definitely not right and after a quick check of all the coax cables, I had to make the decision to bring the antenna back down, and so I clambered back up the ladder and removed the antenna.

I spent most of the afternoon with the terminal box open trying to figure out what the problem was, I knew it was shorting somewhere but in the end I had to go through each connection to find the short. Eventually I found the problem, it turned out to be one of the 12 meter elements had split at the join to the choc block causing one of the U connections to short and all it needed was re-stripping. Then it was just a case of re waterproofing the box, sealing up and mounting back on the mast.

Dusk was fast approaching when I clambered back up the ladder and attached the Cobweb back on the mast, I quickly said a prayer and then did my checks on the MFJ, thankfully all proved OK. 

Finally back on the mast!

The SWR measurements are now pretty good, they are all set for CW except for 10 meters which I set for 28.500 as I like an occasional rag chew on 10.

14.000 - 1:2

18.070 - 1:0

21.050 - 1:0

24890 - 1:2

28.500 - 1:1

With the G3TXQ Cobweb now set up in the attic I am at last ready to do a bit of testing with both cobwebs. Obviously as this antenna is in an enclosed space there is bound to be a difference. The most noticeable being the background noise but that cannot be helped as the attic naturally has bricks and mortar and electrical wiring.

Here is the final SWR reading for the G3TXQ:

14.075 1:2

18.080 1:2

21.073 1:3

24.890 1:1                        

28.015 1:1    

As suspected there is not a lot of difference, but the SWR is slightly higher on the G3TXQ on a couple of bands (most likely because of the attic positioning). I could trim to match the frequency of the G3TPW, but to be honest I couldn't be bothered to get it exactly the same. 

Physically the G3TXQ is far lighter and easier to handle also the tuning is much quicker because you only have one wire for each element. But that said, I do like my G3TPW, it is heavier and chunkier with my larger home made plates and it definitely feels more solid especially with the windy condition we have round here.

It was very apparent when lifting back on to the mast how much heavier it was compared to the G3TXQ, you feel the lighter cobweb would literally blow away in the wind, although that said it's exactly the same build just lighter materials.

The main difference on receiving is of course the background noise but if the G3TXQ was raised outside on my mast I doubt you would know the difference when listening. On transmit I've now used both Cobwebs on 20 meters and reports from both are good with the G3TPW showing  slightly stronger RSTs, again no doubt due to due location.

So all in all as suspected there is very little difference, for me I'd go for the G3TPW simply because it feels stronger, but that personal choice from QTH circumstances. It's been fun servicing the old cobweb and building a new one and I hope you have enjoyed the entertainment as I've struggled along. I'm not a technical antenna geek so all this is very basic but hopefully gives you a rough guide to both types of the antenna.

Not sure what I'm going to do with the G3TXQ cobweb now, so for the moment it's sitting snugly up in the attic out the way. Currently I'm saving up for either a K2 or a TS590 so I'm tempted to keep the G3TXQ and connect to the new radio, then I'll be spoilt for choice with two cobwebs!


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. I’d wanted to re test it against the present G3TPW cobweb I'm using, just to go over the pros and cons and possibly replace the G3TPW. 

With the G3TXQ quickly set up on a piece of 6ft scaffold pole and after doing an approximate tune I couldn't believe my luck when wondering over the 21Mhz band that I managed to have a qso with a Russian station who gave me a 599 which made me think maybe it was time to swap the cobwebs over. 

Saturday morning I set to work and I did the full tuning with the MFJ 259 which took an age as the G3TXQ definitely has much finer tuning points, it dips very quickly and the freq range is literally a few hundred kilohertz, a noticeable difference to the much wider freq of the G3TPW. 

Because I was tuning/trimming the antenna with the MFJ 259 at only 6ft above the ground I had to take the risk of setting each of the elements to tune a little lower than the freq I wanted to operate on, the theory being that once raised the SWR would rise slightly from the tuning at ground level.

By Saturday evening I had adjusted all the tuning, secured the ties and finally waterproofed the junction box, I hadn't needed to do this before because this antenna had only been set up in my attic, So by Sunday morning the G3TXQ was ready for mounting

I grabbed the ladder, lowered the 40ft mast and removed the old G3TPW cobweb, I’d forgotten just how heavy the antenna is and it took a while to gently bring it down the ladder back to mother earth.

Placing the two antennas side by side it was interesting to see the physical difference. The G3TXQ is so much lighter; certainly the single wire and smaller aluminium central clamps make all the difference. 

The G3TPW has a much heavier clamp and is beginning to look very worn!

The G3TXQ is a much neater and lighter clamp, a definite improvement.

I also needed to do a few adjustments to the mast, add some more kevlar cord guides and do some general maintenance. Once that had been completed I grabbed the G3TXQ and hoisted it up the ladder and clamped it to the top of the mast, I have to admit it was so much easier than with the G3TPW and just looked a lot tidier! 

The final result and there was a definite improvement in performance.

By the afternoon it was all finished and after a quick SWR check I called a CQ and got another Russian Station (Alex R3ZK in Belgorod) who gave me a another 599, I was very pleased with the final results. 

There’s a few little bits I need to do, (like add the top mast clamp cover which I forgot.....doh!) But everything went pretty well according to plan and I shall now dismantled the G3TPW and store it away in the attic until I have another mad moment!

Final SWR results for the G3TXQ:

20 meters   1:2
17 meters   1:1
15 meters   1:1
12 meters   1:1
10 meters   1:1