G4ZFQ'S VERSION OF THE COBWEBB Antenna. Read the PDF file. Link available on G0MTD'S site, link below, and a Google should find it elsewhere. (Cobwebb CB1.pdf)
Before I finished my construction I looked at the web and found Steve's site. Look at http://domain1809176.sites.streamlinedns.co.uk [[January 2011 this is Steve's new site, I don't know how long it has been there. The PDF is still here:- http://test.g0mtd.co.uk/CB1.pdf ]] The pdf shows the 17m element as 4400mm, I think 4040 was intended. I do not think the pdf gives the element support dimensions very clearly. I measured one, from the centre along each spreader 900, 1072, 1262, 1482, 1862mm. The feed point of the 10m element is 915mm from the centre. I think Steve's new site has got a lot of information about dimensions. G3TXQ says dimensions may depend, to a certain amount, on the dielectric properties of the wire insulation.
I have adjusted tuning by altering the amount of end folded back. G3TPW, http://www.g3tpw.co.uk/ the designer, seems to infer the amount of foldback is critical. To me it just seems to alter the effective length of the elements, just as when adjusting any other wire to resonance. I suggest using slightly longer elements. G0MTD shortened his but mine needed lengthening, the 15m element was not long enough. I suggest 50-100mm more on the ends each element. So far I have got 1:1 on all except 20m which is 1.6:1. this may need adjustment of the T match tap to get closer. Maybe altering the gap between the ends may make a difference, but I used fixed points on the spreaders. 10m gives about +/- 200KHz before the SWR rises to 1.5:1. Adjustment on the four higher bands did not alter much with height but I needed to raise it to 20ft to get a reasonable result on 20m.Final tweaks were made after measuring at the full height of 32ft. I just used a SWR meter. An MFJ or similar might give an idea of how to get a better match on 20m. I suggest making the elements of one piece, it is not necessary to cut and join both wires at the centre of each element.
The overall view. Each side is around 2.7m, 8ft 10in. The element ends are rather crudely held by cord and hot melt glue. The elements are located on the spreaders by aluminium clout nails (holes drilled for them) and hot melt glue. A different method could be used to make a portable version, it would not take long to assemble.
My construction method, a second clamp is on the other piece of aluminum angle. This technique is best suited for a single mast diameter. The centre of the antenna is offset, this makes opposite spreaders a slightly different length. I do not expect the pine spreaders to survive a severe storm, but just in case they do I have used preservative. [[[Update 5 March:- I did not notice the 2m pieces of pine dowelling were all made of THREE pieces joined. Spreaders now replaced with some old 21mm hardwood.]]] My next one will have fibreglass spreaders, I wonder if fishing rod blanks are good enough?. The N chassis socket has a sealed cable connector, wrapped with self amalgamating tape, insulating tape and then hot melt glue. My foldover mast is located behind the Summerhouse, as far from civilisation as possible!
I did not like the Cobwebb feed I saw, the connector block had seriously corroded. Mine is minimal, uses single sided copper clad board. The copper is removed by scoring and then using a soldering iron to heat the unwanted section. Tin this piece, then lift it with a blade, gradually pull away with pliers while heating. The second conductor does not need to be cut, it passes underneath the board, is secured by hot melt glue. The holes are used to help position the wires, a match or the like is temporally inserted. This is now all covered in hot melt like the cable connection. |The coax braid and centre conductor was tinned, self amalgamating tape wound over part of the the soldered ends to stop any possible moisture ingress. After wrapping with insulating tape it was stuck to the board and covered in hot melt. This does not look too good but I expect it to outlast a method that requires you to seal a box with a lid and over a dozen holes. I use N connectors outside when needed, they are weatherproof before the application of self amalgamating tape.
November 2011 Another way of making and matching the dipoles http://www.karinya.net/g3txq/cobweb/ maybe easier to match but not so easy too keep water out.
I think hot melt glue is wonderful! Cheap, sticks to most things but avoid poking it with your fingers! When my gun gets hot the glue sticks to fingers and makes unpleasant burns! If you cover soldered connections that later need removing just melt the glue and solder with an iron. A quick wipe on a wet sponge cleans the iron and you can resolder without needing to clean all the glue from the joint.
Performance:- Early days yet! On 20 and 17m it compares with dipoles at about the same height. I tend to feel that general purpose antennas like long wires and G5RVs do not often work well at the higher frequencies. We tried a quick check at the Club when the portable 3 element beam was erected there, it vastly outperformed the 270ft long wire on 20m. I always recommend a resonant dipole as the easiest way to get good results from a single band, if this antenna works similarly to a dipole on all bands then it should be worth having. But it will be a few years before HF starts working well!
I have realised that resonant dipoles may appear to perform better than multiband antennas that require (ATU) matching units because of the losses in the matching components. It has been suggested that these losses can be significant in some circumstances.
November 2008 A second look by Peter, G3LDO in the December edition of "Radcom" shows the Cobwebb can actually be better than a fixed dipole at the same height. He did EZNEC plots of a Cobwebb and dipole at 10 metres high. The dipole was 1db better broadside on but the Cobwebb, being omni directional was 4db better than the dipole when the dipole was end-on to the direction of the signal. Note Peter's plot of the dipole takes account of the effect of ground reflections which which "fill in" the theoretical nulls off the end of a dipole. He says that under certain conditions EZNEC shows a 10db advantage over a dipole! I do not know how often that would be achieved in practice.
A couple of my wooden spreaders broke in the gales at the end of last year. I replaced them this month with fishing rod blanks, £1 each at a rally some years back. Let's see how they last. This break reminded me that it is important to secure the wire elements to these spreaders. For several days before the winds subsided one was hanging on just one wire, threatening to spear anyone below.
G4ZFQ MARCH 2007 Not Cobweb, it is made by Steve Webb!
http://ei7im.webs.com/construction.htm Kevin details his construction including a simpler feedpoint arrangement. and his antenna's measurements.
I've had a few emailed questions, I copy them here in case I have not made these things clear above.
>he gives distances for the short points on all bands, and on your
construction tips you say its better to make out of one length and short then out. Can you tell me if the two antennas differ on performance ? and
what is the point of the shorts.
My comment about making the elements from one length is to avoid having to rejoin one of the pairs in the middle. I do not like the connector block method. Apart from this the antennas are the same.
The short part way along the elements is for matching, I may have been able to get a better SWR on 20m by experimenting before making the short permanent. "PUSH PINS" AS USED ON MAPS MAKE AN EASY TEMPORARY SHORT FOR TESTING.
I can't imagine any significant difference in performance of anything built to the same general design. The only thing is if the SWR is a bit too high you will not get full power if it is fed directly from a modern rig.
>As you have worked with the Cobweb, do you think it will outperform a
I have a vertical, Butternut HF9V, nearly every time the Cobwebb is better. But it might be different for other people with different setups.
>Following your length corrections, is it really easy to tune this antenna?
I found it easy but on 20m the SWR was 1.6. If you want to get it better than that it might be best to test the tapping points that need shorting by pushing pins through the cable before you make it permanent. I do not worry too much about SWR, I either use lower power or run a valve amplifier.
>You said that the 17m element was shortened to about 4040. Did you have to
move the tapping point as well or is that measurement ok?
No the tap point was OK. I think the original length was typed wrongly.
Note the tap point on my 20m element is not ideal. I have not tried to adjust it but the SWR on mine is 1.6. I've no idea which way the tap needs to go.
>Can you confirm that its best to lengthen all elements by about 50-100mm or is it just the 15m element?
Yes, my elements were all a bit longer than the ones in the pdf. Someone else may find they need to be shorter because construction methods may vary slightly.
>Did you use the 42 strand speaker wire and where did you buy yours from?
Yes, an internet sale from TLC in Portsmouth. I found them cheap and reasonable carriage. I bought 100m, enough for two.
I have recently added a few notes on the web page. Cobwebb.htm
The selection of a suitable material for the spreaders is possibly the main problem. The commercial one uses thick fibreglass, if you are keeping costs low anything else may need replacing after a hard storm.
It sounds as if you live close to others so best to make sure they will not break!
Yes, I still use it, it works as well as anything I have tried.
73 Alan March 2008
Original Message ----- From: "Fred" Subject: Cobwebb June 2009
Your comment about the HF9V shows how different situations suit different antennas. Mine, (Ten years old when I got it for £10) I would not describe as useless, although the Cobwebb DOES work better. About a year ago one of my spreaders did break, (I just stuck it together with Araldite and a hose clip). Lasted until December 2011.
This reminds me of when I had a VK2ABQ break (A two element, 5 band, built with 12ft wooden spreaders - similar but twice the size of a Cobwebb) This was in the Jan 1990 storm, three of the spreaders broke but as the element wires were reasonably secure on them nothing actually fell down. It all just hung there! The same with the broken Cobwebb spreader. Amazingly two Americans were coming in on 20m 59+20 with the VK2ABQ like this, I did not call them but have always wished I did!
This was rebuilt on a secondhand commercial frame with fibreglass spreaders. It weighed 12 pounds but VK2ABQs are difficult to handle on my own.
I was looking on the net for where I purchased some fishing rod blanks, I thought it was Moonraker. I did see they sell fibreglass "masts" http://www.moonrakerukltd.com/Poles-and-Masts/Fibreglass-Masts which might do the job. They seem a little expensive to my frugal nature but the result would be much less than the commercial product. A search on www.Google.co.uk might help. http://www.google.co.uk/search?hl=en&q=fibreglass+rods+tube&btnG=Search&meta=cr%3DcountryUK%7CcountryGB looks interesting. Also http://www.sandpiper.entadsl.com/connectors%20and%20tubes.html
Presumably Google (and other search engines?) enable searches within pages from many individual countries. I don't recommend carbon fibre because it's conductivity MAY affect the antenna .
73 Alan G4ZFQ
> I have seen an article using 1/2" white polyurethane water pipe> whereby all spreaders are supported by the extension of the mast by a foot with nylon fishing line from each tip to the central point of the 1 foot extension on the mast as support. [Note this is the way I supported the VK2ABQ spreaders, with a 3 foot high extension. On the supporting cords I tried elements for 4 & 6 metres but they did not work too well.]
Click link to email If you see no link or it does not work Click HERE Please include my Callsign in the Subject Line. Make the subject VERY relevant - with the amount of spam I get an email from an unknown source can easily be discarded!
G4ZFQ Main index