I've seen a number of designs on the Internet that claim to be Cobwebbs.

Am I right in thinking that the correct design is that of G0MTD which involves shorting the two conductors 'midway' down the sides of the square to deliver the correct impedance?

I have seen several designs which merely connect the furthermost ends of the loop to create a kind of square folded dipole but I suspect these aren't entirely accurate?

Comments....

Dave

G0MTD ? as far as i am aware the original design was by Steve G3TPW http://www.g3tpw.co.uk/

Most that have copied steve`s design seem to always get the dimentions wrong.

Ah I was referring to here - http://test.g0mtd.co.uk/CB1.pdf - which gives a detailed description of a practical design...

Dave

Ahh that pdf wich is steve G3TPW cobwebb. Last time i spoke to steve he laughed about that circulating,
He mentioned its nearlly right but not quite

If your going to build one you could try steves G3TXQ design http://www.karinya.net/g3txq/cobweb/ ,i made one last year and works a treat lighter weight as well due to the single wire.
I have heard it can be a bit hit and miss with the placing of the shorting link due to different types of wire used on the G3TPW cobwebb as well.

Dave,

Maybe this will help:

When you bend a half-wave dipole into the shape of a square, its impedance drops from around 70 Ohms to about 12 Ohms. One way of getting a better match to 50 Ohm coax is to make the dipole a folded dipole - that has the effect of raising the feedpoint impedance by a factor of 4, so the 12 Ohms becomes 48 Ohms.

But it turns out that when you form a folded dipole you inadvertently create two short-circuit stubs that appear across the feedpoint. If you build the folded dipole with wide-spaced bare wires, those stubs have almost no effect on the feedpoint impedance; but if you build with close-spaced "figure-8" wire with high dielectric constant insulation, the stubs most certainly cause a problem. The solution is to place a shorting link inboard of the dipole ends at a point where you force the stubs to be an exact quarter-wave long.

The exact shorting position depens on the wire characteristics: copper diameter, conductor spacing, insulation thickness and insulation type; which is why folk end up with differeing shorting positions depending on what wire they use, and is why you can't fully trust dimensions published on the Net unless you use exactly the same wire.

I came up with an alternative solution which is to use single-wire dipoles and a 4:1 balun at the feedpoint.

By the way, G3TPW is bound to say the web article is "nearly right but not quite", isn't he

If ever you can't get to sleep one night, I have a web page dedicated to the "stub issue":
http://www.karinya.net/g3txq/folded_dipole/
If you take a look at the first graph, you'll see how much the SWR could be out if you don't get the shorting link in the right place.

73,
Steve G3TXQ

Thank you all for your assistance.

Interestingly in view of the final comment I did have a sleepless night last night and reread all the stuff I had printed out on the construction of this antenna.

I now have a better understanding of the impedance matching issues and so in the absence of access to an antenna analyser I think I will go with the balun for impedance matching.

I'm just trying to get this right as it will be my first antenna after receiving my callsign. I want make the cobwebb folding or otherwise demountable as I haven't yet convinced the home management that I can have anything fixed in the garden.

Dave

Hiya Dave..

Welcome to the hobby.

Don't get too hung up on things, the Cobwebb is a pretty straight forward antenna, it just needs a little thought before you start doing anything.

It will work well, even at low heights, but the demountable bit you mention could make it a pain in the arriss. The Cobwebb is roughly 8 foot a side, made from 4 elements on a central plate which hold the wires in a tensile state. There is a 5th eleemnt that holds the feed arrangement.

The .PDF you mention is a good starter point, but it has been updated my someone else, PM me should you want the mark 2 version.

Any piece of wire will radiate a signal, how close it is to your desired frequency in resonance/and or physical size will depend on how well your power is radiated.
As we (In the UK) tend to live in smaller properties than say our American counterparts, size of an antenna is an issue.

I live in a tiny lot, alright I have a long garden, but, the width is only 12 foot Max. A cobby looks positively huge on the ground here, but acceptable in the air. However, my antenna, all built here at home with advice from members of this site and others, works very, very, well.. I will never be a big gun, but every now and again, this little antenna will throw up a real surprise.

I would advise you to take your time making the elements, use heavy duty sewing needles to short the elements first, the solder them when happy. Read the .PDF's ( watch the 17M ele though, the mk2 version is closer ) measure several times, and cut once is the old, true, adage. Never rush it.

I built my antenna from a piece of aluminium plate, cut, then bent at 90 deg. I braced it with a piece of old element, flattened the ends with "Uggg" my trusty lump hammer.. it then held the plate at spot on 90 deg with the elements and wire on, without being too heavy.. It has survived 50 MPH plus winds. The spreaders I have used are from "spratreader" on ebay, Les jackson.. Essentially, 5M fibreglass poles, the thick ends filled with foam and the plastic bungs removed by soaking them in boiling water.. Scribe the centre of the plate and hold the poles in place with hydraulic pipe fasteners.

Even if you don't get it right first time, there is always a lesson to be learned, just don't get too hung up on the precision. If you need any help, please PM me.

Have fun with your project
Kev

go with the 4.1 balun design . the "hardest" bit is making the balun if you don't have bits to hand but it can be done in the warmth of the house single wire no messing design , i could never be bothered messing around with shorting points and twin wires .

End of the day you will still have a 4.1 transformer for other projects too.

BTW the centre plates can be had for £24 i think off e bay AMTOOLS , i think the guys username is spratreader .. The poles are a bit steep but you can get them cheap anywhere

I got mine from hamtools ,i think its cheaper to go through his site than ebay ,i doubted at first that they would be up to the job but mines been up nearly a year now without any problems in some very strong winds.

Hi Dave.....I have been playing with my cobweb for a while now... my two pence worth, for what its worth....go with Steve's
(G3TXQ) design....Its single wire, so is cheaper. You don't have to experiment with shorting points to get the SWR right.
Its lighter. The only tricky bit is constructing the balun, but to be honest its not too bad. I have built several now and have ended up with the up-rated one with larger toroids so I can run some power if need be. One word of advice I would give about the balun is do not use silicone to stabilize the balun in the box. I did and it caused no end of problems. If need be, find something non-conductive. Tuning is tricky but as Steve says, build over-length and cut very small pieces at a time. It does take a while. An analyzer would be a distinct advantage. One dipole affects the others a fair bit, especially 20 and 10 I found.
The finished antenna is amazing for its size. It beats the pants off a flat top dipole, especially with stations that would be off the ends. Its easy to get in the air, mine is on the top of a 21ft scaffold pole at a total height of about 25/26ft ish. It hardly moves at all in the wind either. It works very well on receive on 40 and 80 as well (as a bonus). It does load up on those bands quite easily but is inefficient on transmit. Well worth building.
All the best with it
Ken

I went with steve's design works well and is a lot easier to tune, although I would recommend an antenna analyser, cut my first one too short. Not sure if this is correct but on the second attempt I cut too approximately the correct size then started folding the wire back on itself until I got the correct frequency this allows for fine tuning and you can never go too short

Ian

Can any one help me please Iam looking for a clear picture of page 25 of the pdf test.g0mtd.co.uk/CB1.pdf
as Iam going to try and make a cobweb this summer.

Its probably easier to give the dimensions. The only ones that count are the ones on the spreaders where the tags go for holding the cables as the dimensions for the cable and the short point change with the cable used as Steve said.

On my genuine one taking the measurement from the center where the spreaders cross, working out from the center, the tags for holding the wires in place need to be:

10m - 890mm
12m - 960mm
15m - 1050mm
17m - 1270mm
20m - 1650mm

And that is on all four arms.

The fifth spreader holding the junction box is 1000mm long and the junction box is a double wall socket size with the end flush with the end of the spreader.

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