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COBWEB TUNING ?
https://www.hamradiodeals.co.uk/forums/viewtopic.php?f=43&t=26119
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Author:  G0ASI [ Tue Dec 13, 2011 5:01 pm ]
Post subject:  COBWEB TUNING ?

I am having trouble tuning my cobweb on 20 mtrs. I can't seem to get it below about 5.1 .
the other bands are not too bad. its the single wire version as per Steve.
I have tried shortening it, making it longer, changing the co-ax length.
tuning has been done at 10 ft high. I did notice when trimming the other bands that
just nipping off 1 or 2 mm made a fair bit of difference. 20 on the other hand is being the
proverbial pig from hell. Anybody got any suggestions ???
thanks
Ken

Author:  G3TXQ [ Tue Dec 13, 2011 5:06 pm ]
Post subject:  Re: COBWEB TUNING ?

Ken,

First thing - are you sure the balun is constructed properly?
Secondly, can you check the SWR out of band with an analyser and see if it reaches a minimum above or below 20m?

5:1 is a very high SWR and indicative that something major is wrong!

Steve G3TXQ

Author:  G0ASI [ Tue Dec 13, 2011 6:00 pm ]
Post subject:  Re: COBWEB TUNING ?

Thanks for the reply Steve, that was quick.
I don't have an analyser unfortunately. My radio wont TX out of band.
I made the balun exactly as per your website, same coax same toroids.
same connections, same windings. it looked just like yours. and its wired
in the right way round. tuning has been done in the dry at about 10 ft in the clear
although there is a tin garage roof about 7 ft below it and to one side.

On 20 I start with an swr of a bit above 5-1 then with shortening ,(I originally cut the wires longer)
the swr drops to approx 4.5-1, then starts to rise again, its quite lively on RX. using your measurements,
what are the distances between the dipole ends on your antenna ?. would this make much of a difference ?.

the other 4 bands came down to less than 2-1 quite easily although the
bandwidths are not too wide, I only have about 18 ft of coax on it for tuning
and I expect it to get wider when I put it in situ with about 80ft of coax (RG8xx mini 8)
it will end up at about 28ft high.

Author:  G3TXQ [ Tue Dec 13, 2011 6:15 pm ]
Post subject:  Re: COBWEB TUNING ?

Can you try it further away from the garage roof, even though at that stage it might be lower?

Steve G3TXQ

Author:  G0ASI [ Tue Dec 13, 2011 7:08 pm ]
Post subject:  Re: COBWEB TUNING ?

I will give it a go a bit lower and further away. Tomorrow, its too dark now.
I just remembered I do have a GDO tucked away in the workshop. I just need to make up a bit of
a coupling coil then ill'e give it a dip, see what that shows, its not exactly accurate to
within a meg or two but should show me any harmonics, or indeed if I am too far off. I
was aiming at about 14.150 ish. I also have a whacking great big dummy load so I can
calibrate the gdo a bit. Makes you think this antenna stuff !!!.
Many thanks. will let you know results.
Ken

Author:  G0ASI [ Wed Dec 14, 2011 1:38 pm ]
Post subject:  Re: COBWEB TUNING ?

Hi Steve.
You are going love this.......
I tried everything. I even cut a new wire for 20, and made it exactly the same length
as yours. I dipped it with a GDO and it was just being daft, multiple dips everywhere.
I decided to remove the box and re-check the balun. Now then, when I built the box
I used a couple of big blobs of UNI-BOND silicone sealer just to stabilise the balun in
the box and take the strain off the connections. I noticed that it had not set, it was
pretty well just as it came out of the can. I did some further checks on it and discovered
that it is CONDUCTIVE, not just a little bit, a lot. I am going to build the box again needless to say.
it was a miracle I managed to tune the other bands, no wonder tuning was a bit sharp.

there's a lesson there somewhere.

Author:  m0jha [ Wed Dec 14, 2011 2:19 pm ]
Post subject:  Re: COBWEB TUNING ?

you can seal your bath and have an rx antenna :lol:

Author:  G3TXQ [ Wed Dec 14, 2011 2:55 pm ]
Post subject:  Re: COBWEB TUNING ?

G0ASI wrote:
there's a lesson there somewhere.


Useful to know - and something I bet will never catch you out again!
Let us know how it goes when "cleaned up.

73,
Steve G3TXQ

Author:  G0ASI [ Sun Feb 26, 2012 11:28 am ]
Post subject:  Re: COBWEB TUNING ?

OK.
Its now re-built with the up-rated balun for a bit more power out.
Tuning was tricky to say the least. 12-15-17 are all now at 1.1
10 is 1.2 and 20 is 1.9. that's as low as 20 will go without 10 going
right off tune so I have settled for the compromise.
Its now up at 28ft and the swr's are pretty much the same.
Its a very good antenna I have to say, much better than I thought it would be.
its on a par with my 40 and 80 dipoles on RX also, which is a bonus, it also tunes
on 40 and 80 with the internal ATU on my MP quite easily, although I doubt it
will work too well on those bands.
All in all I am well chuffed with it. Nice job Steve, many thanks.

Author:  G3TXQ [ Sun Feb 26, 2012 11:57 am ]
Post subject:  Re: COBWEB TUNING ?

I'm surprised the SWR is that high on 20m. You get similar interactions as on any fan dipole, so you need to start tuning on 20m and work your way through to 10m.

Sound as if you're getting good results, though!

73,
Steve G3TXQ

Author:  G0ASI [ Sun Feb 26, 2012 2:19 pm ]
Post subject:  Re: COBWEB TUNING ?

It is a little high on 20 I admit. I had about 4/5 attempts to improve it, going too far with the tuning in each case. I ended up scrapping the lead and fitting a new one each time. It seems that 10 and 20 affect each other considerably. I tried to split the difference and get them both as low as possible but it just wouldn't happen, I ended up opting for 10 quite low and 20 a bit too high. When I tried to get 20 to go low, 10 became quite silly, up over 3.1 in fact. I did wonder if it was down to the up rated balun having one less turn, ie; 7 instead of 8. As it happens its not a big problem. I don't have access to an analyser so I will have to live with it. In any event its a storming antenna, well worth the trouble of making it.

Author:  g7pan [ Mon Feb 27, 2012 10:23 pm ]
Post subject:  Re: COBWEB TUNING ?

I managed to get mine down to 1.1:1 in all bands ,not bragging at all ;) but it is possible, saying that though 12m is out as i cut it a little short.
I'll be bringing it down in April to have a look at it as it will have been up for a year and i want to make sure its ok and give me the chance to retune 12m.

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