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 Cobwebb Spreader advice please. 
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Post Cobwebb Spreader advice please.
Hi All, As the title for this post says, i`m looking for advice on spreaders for a cobwebb antenna which is going to be my next antenna project.
On that famous auction site i have found two lots of fibreglass poles from the same seller.

One set`s diameter measures 29mm tapering to 21mm
The other set 19.7mm tapering to 8mm

I always seem to build too big and straight away thought the bigger set would be best, But thinking how it would look up in the air, the smaller set looks promising.

However, and the real reason for my question is, would the smaller diameter (8mm) at the end support the elements without drooping to much. And do we think they would have the strength required to last a few years. Or, should i go big and strong.

I should also add i`m going to try the twin wire folded loop with tapping points so you will have an idea of the weight of wire on the spreaders. (42 strand twin 0.2mm)

Any advice or thoughts will be well received.
Thanks in advance,
73 2e0joc
Bob

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Sun Jun 20, 2010 12:10 pm
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 Cobwebb Spreader advice please. 
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Post Re: Cobwebb Spreader advice please.
The 29mm/21mm option will look pretty "chunky" in the air. I've used 20mm/8mm with 1mm wall thickness, and they look fine - some "droop" but not enough to be objectionable.

Make sure whatever you get is fibreglass, not carbon fibre.

If you really want many years use with no maintenance I'd probably go for a fibreglass tubing option rather than fishing poles.

73,
Steve G3TXQ

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Sun Jun 20, 2010 1:55 pm
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 Cobwebb Spreader advice please. 
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Post Re: Cobwebb Spreader advice please.
Cheers Steve,
Like i said i do tend to build big, its not something i do on purpose, just sort of happens.

So the smaller diameter ones will be OK, thanks for that Steve. It is hard to judge without having them in my hands, hence the question.

I did look at the Sandpiper website for a selection of tubes. Still thinking

73 de 2e0joc,
Bob

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Sun Jun 20, 2010 2:03 pm
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 Cobwebb Spreader advice please. 
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Post Re: Cobwebb Spreader advice please.
I can echo what Steve says regarding that, fibreglass tubing is much better than fishing poles.
The tubing on my Cobwebb is very thick walled. Ending in solid bar at the ends.
Where as fishing poles are very thin walled, but will certainly do the job!
No so much a strength issue, moreso when joining the 2 poles together.
With thick walled fibreglass, you've got something substantial to screw in to.

Best of luck :)

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Sun Jun 20, 2010 7:41 pm
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 Cobwebb Spreader advice please. 
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Post Re: Cobwebb Spreader advice please.
Hi, thanks for the replies.
I think i forgot to mention that i am rather tight and like to go for the Ham/cheap option if i can and do the best i can with it.
So i have went with the fishing rod option for the moment which leaves me a few sheckles to buy the wire.
The Aluminium hub plate i have now made at work will last forever so can be reused in the future if required.
I thought, from past experience, that binding the rods at the joint with strong thread then applying super glue might be a good idea. Finish that off with some self amalgamating tape, should be ok for a while.
Thing that has me stumped at the moment though is where to find 25mm U-bolts to go over the pipe i`m using on the centre/hub plate. Still looking for these little blighters, any help here would be fantastic.
73, Bob

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Mon Jun 21, 2010 6:16 pm
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 Cobwebb Spreader advice please. 
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Post Re: Cobwebb Spreader advice please.
Bob,
Just googled 25mm u bolts and came up with this one:-
http://uk.farnell.com/duratool/7237686/ ... dp/7237686
plenty of other suppliers there to choose from.

73 de Geoff


Mon Jun 21, 2010 6:46 pm
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 Cobwebb Spreader advice please. 
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Post Re: Cobwebb Spreader advice please.
If I was picky I'd substitute the superglue for epoxy. Something with a little bit of give in it.
Superglue is obviously mega strong, but also pretty brittle when its got movement in it.
Also, take time to get it to resonance on the bands mate.

I know its a pain in the arse with the sweat pouring out of ya,
one leg balanced on the 20m loop while you fanny around with the knot on the 12m loop etc,
It ends up being like a unfunny game of twister :lol:
But it pays off big time when you do. Mine creeps up on the 15m and 10m band edges.
Otherwise it never goes over 1.5:1 anywhere else.

Keep us posted how you get on.

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Mon Jun 21, 2010 7:32 pm
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 Cobwebb Spreader advice please. 
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Post Re: Cobwebb Spreader advice please.
When I use fishing poles I don't use "U" bolts - I find it's too easy to crush the pole. I bolt a short length of thick-wall aluminium tubing to the centre-plate, and then slot the fishing poles into the tube. This is how I did it on one of my Hexbeams:
http://www.karinya.net/g3txq/hexbeam/ga ... ?cat=2&n=4

Good luck!

Steve G3TXQ


Mon Jun 21, 2010 8:16 pm
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 Cobwebb Spreader advice please. 
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Post Re: Cobwebb Spreader advice please.
I love this site, nowhere else do you get such quick helpful replies.
Steve, the centre i have made is 1/4 inch aluminium and i was going to use some fibreglass thick walled tube i found at work to bolt with u bolts to this plate and then fit the spreaders into this. Ali tube would be better but i have found this fibreglass bit and it was just the right diameter and length and best of all, free, hi.
The spreaders i have just bought are about 3/4" short for the job and this fibreglass tube is my idea for making up this shortfall and giving the poles some strength and support.

Heres a quick one for you though, trawling the internet for element sizes and spacings give`s good consistent advice so far, BUT, how critical is the spacing between the different band elements??
I mean, can i rely on the element spacings i have found and fit the wire permantly to the spreaders or do i have to do it in a manner that they can be moved on the spreaders till i get it right. Trimming and folding of the ends not withstanding.

And here for the curious are the size`s that i have found so far. Still a bit of research to do.....
Band Element Length Shorting point
10M 5080 mm 1560 mm
12M 5860 mm 1790 mm
15M 6900 mm 2080 mm
17M 8120 mm 2220 mm
20M 10300 mm 2720 mm

73 Bob

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Mon Jun 21, 2010 9:02 pm
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 Cobwebb Spreader advice please. 
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Post Re: Cobwebb Spreader advice please.
Changing the end spacing, and "folding back" the ends of the elements, gives you some degree of adjustment; but I don't think I would ever dare make the wire positions on the spreaders permanent until I'd got the tuning pretty close.

Of course you might get lucky .......

Steve G3TXQ


Mon Jun 21, 2010 9:09 pm
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 Cobwebb Spreader advice please. 
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Post Re: Cobwebb Spreader advice please.
Thanks again steve,
Thought as much. Need lots of cable ties i think.
I`ll update this post once i get all the stuff together and start the build. Letting folks know how its going, i think this is going to be a fun challenge but from what i`ve read about the Cobweb it should be worth it.
Thanks again to all.
73 Bob

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Mon Jun 21, 2010 9:15 pm
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 Cobwebb Spreader advice please. 
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Post Re: Cobwebb Spreader advice please.
Forgot to say that wire lengths and shorting points will depend on the wire gauge, the insulation thickness, the insulation dielectric constant, and the spacing of the twin lead, so your dimensions wont necessarily be exactly the same as published ones.

Steve G3TXQ


Mon Jun 21, 2010 9:16 pm
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 Cobwebb Spreader advice please. 
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Post Re: Cobwebb Spreader advice please.
I've just dug out the instructions Bob though it gives no spacings.
The only bit of info I can find that may be of any use is...


The approximate amount of frequency drift, per centimetre, on each band
(adjustment made to each end of each dipole to keep it all balanced) is as follows:-

14mhz 50khz/cm
18mhz 65khz/cm
21mhz 75khz/cm
24mhz 100khz/cm
28mhz 120khz/cm

It should be noted that when the antenna wires get wet in the rain, the resonant
frequencies will reduce anyway, due the very high di-electric constant of water.
This effect is unavoidable with a high efficiency small and therefore high Q antenna.
It is probably best to leave the antenna with the resonances in the middle of each band
and use the auto ATU to trim them up if you operate at the band edges.
If the resonances are adjusted for the CW end of the band, it will be found that the SWR
at the top of the band will be about 5:1 and a simple auto ATU might no be able to match it


Though these adjustment dimensions could be a touch out for yours.
There's that many variables involved, but could be a rough guide if need be.

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Mon Jun 21, 2010 9:21 pm
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 Cobwebb Spreader advice please. 
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Post Re: Cobwebb Spreader advice please.
Hi Steve, yes i realise that bit and with that in mind was going to use the 42 strand 0.2mm twin that a couple of the other builders have used to try to get in the same ballpark to start with. Of course i wont know how the insulation will affect it till i try. It`s all part of the fun.

Geoff, Looked at farnel site, and am still confused by these u-bolt sizings. The 25mm you rightly pointed me to is really 32mm between the legs. I dont know how they size these things but will keep looking, thanks.

73 bob

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Mon Jun 21, 2010 9:26 pm
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 Cobwebb Spreader advice please. 
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Post Re: Cobwebb Spreader advice please.
Thanks Scruffyonions, I have a PDF CB1 from the net with a lot of info and this just adds more detail to it. Is that a real Steve Webb antenna in picture or one you made.....
Should be an adventure this one.
Thanks again
73 bob

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Mon Jun 21, 2010 9:33 pm
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