Cobwebb antenna advice please.

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Re: Cobwebb antenna advice please.

Postby mark1 » 02 Jan 2017, 14:24

kilimax wrote:
bc73 wrote:Well New Cobwebb arrived today very prompt delivery. will try to get it built in the next few days and keep you all posted.


Mine is out of its box and my mast is now down for some winter maintenance which should be completed tomorrow. I'm hoping to get the cobwebb up either late on tomorrow or Tuesday all going well.

I hadn't realised at the time but I've bought the 7 band version! :o


Be interesting to see what you think of it once its been up a bit. What have you be using up to now for those bands to compare it with?

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Re: Cobwebb antenna advice please.

Postby kilimax » 02 Jan 2017, 16:16

I'm using a combination of a Hustler vertical and a couple of single band dipoles. The vertical as you would expect, is a bit noisy so I'm hoping this antenna will be a bit quieter being horizontal.

Anyway, it's on the mast now but I've still got the mast tilted over for some other work I need to do. It was relatively easy to build. You will need a 10mm & a 17mm (for mast to boom u-bolts) spanner to assemble it plus a good strong philips screwdriver. The fibreglass tubing is pre-drilled but the screws are very difficult to tighten. You will need a bit of force!
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Re: Cobwebb antenna advice please.

Postby mark1 » 05 Jan 2017, 19:09

Forgot to say, i drilled a few holes in the underside of the box to allow any water that might get in have an exit point.
Have you tried it out yet?

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Re: Cobwebb antenna advice please.

Postby kilimax » 05 Jan 2017, 21:14

No, still got mast down. The winch cable that tilts the mast is knackered and has jammed. I can't get it off the winch. Going to carry out some heavy duty cutting on Saturday and then fit the new cable.

Weather permitting, and everything being equal, it will be back up on Sunday I think. I've drilled some holes also as per his instructions (if that's what you can call them!)

One thing that slightly concerns me is its close proximity to my dual band 6M/4M yagi. I have it mounted above this but don't want to put it too close to the top of the stub mast. I'll need to see if there is any unwanted interaction and/or detuning when I test it compared to it being on its own!

Will post when it's up in the air....
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Re: Cobwebb antenna advice please.

Postby mark1 » 08 Jan 2017, 11:25

Good luck with getting the cable off your winch, nightmare when they jam. Ive never used my cobweb with anything else on the same pole so trail and error there as are most things amateur radio. Saying that i have been tempted to move my x50 white stick above it so i can put my 10m vertical back up where the x50 is now as the 10m vertical is not receiving much lying down in the garage.

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Re: Cobwebb antenna advice please.

Postby kilimax » 08 Jan 2017, 13:02

Well, ended up having to cut the cable off the winch one piece at a time with a hacksaw. Took me over 2 hours just to get this bit done. Took the opportunity to clean everything up before fitting the new cable.

I've started to tune the Cobwebb. Despite the makers claim that it has already been tuned, it was miles out on all of the bands. With exception of 10m, minimum swr was between 700-850khz low. On 10m, minimum swr was at the top end of 29Mhz indicating dipole length too short. 12m seemed to be too long although minimum swr on this band seemed to be higher than desirable at over 2.0:1

It was easy to tune 20-15m and I now have minimum swr at 14.200, 18.150 & 21.200. All now 1.1:0
10m & 12m however are giving me a bit of a headache. I am unable to find a point where 12m will come under 2.0:1 regardless of element length and 10m won't come down in frequency either & this is despite lengthening the element size as per the instructions. So something not right on the 2 upper HF bands. I am able to use my ATU but I'd prefer not to.

Weather has hampered me somewhat and I've had to abandon the work for now.

About your x50. I also have a Colinear at the top of the mast which is a V2000 but I'm swapping that out for something a bit better on 2 & 70. So long as it's not too close to your Cobwebb, it should be fine.

In an earlier post, I did mention my concern that other antennas on my mast may affect the Cobwebb tuning so my next bit of work is to take everything off and just have the Cobwebb to see if there's a change.

Will keep you posted.
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Re: Cobwebb antenna advice please.

Postby paulears » 08 Jan 2017, 14:40

Thanks for that - very interesting.

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Re: Cobwebb antenna advice please.

Postby mark1 » 10 Jan 2017, 19:57

kilimax wrote:Well, ended up having to cut the cable off the winch one piece at a time with a hacksaw. Took me over 2 hours just to get this bit done. Took the opportunity to clean everything up before fitting the new cable.

I've started to tune the Cobwebb. Despite the makers claim that it has already been tuned, it was miles out on all of the bands. With exception of 10m, minimum swr was between 700-850khz low. On 10m, minimum swr was at the top end of 29Mhz indicating dipole length too short. 12m seemed to be too long although minimum swr on this band seemed to be higher than desirable at over 2.0:1

It was easy to tune 20-15m and I now have minimum swr at 14.200, 18.150 & 21.200. All now 1.1:0
10m & 12m however are giving me a bit of a headache. I am unable to find a point where 12m will come under 2.0:1 regardless of element length and 10m won't come down in frequency either & this is despite lengthening the element size as per the instructions. So something not right on the 2 upper HF bands. I am able to use my ATU but I'd prefer not to.

Weather has hampered me somewhat and I've had to abandon the work for now.

About your x50. I also have a Colinear at the top of the mast which is a V2000 but I'm swapping that out for something a bit better on 2 & 70. So long as it's not too close to your Cobwebb, it should be fine.

In an earlier post, I did mention my concern that other antennas on my mast may affect the Cobwebb tuning so my next bit of work is to take everything off and just have the Cobwebb to see if there's a change.

Will keep you posted.


Oh dear not good, i bet that took some doing with a hacksaw but at least you got it off and are moving forward in the right direction with the antenna. Funny you should say about not being able to get the SWR down on 2 of the bands as when i put mine on the antenna analyser i cant get the SWR down below 2.0:1 on 20m? I was hoping that playing around altering the element lengths would resolve this when i get it down but now you have said that i'm wondering if i'm going to hit the same snag as you have {bnghd}

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Re: Cobwebb antenna advice please.

Postby kilimax » 10 Jan 2017, 20:46

mark1 wrote:
Oh dear not good, i bet that took some doing with a hacksaw but at least you got it off and are moving forward in the right direction with the antenna. Funny you should say about not being able to get the SWR down on 2 of the bands as when i put mine on the antenna analyser i cant get the SWR down below 2.0:1 on 20m? I was hoping that playing around altering the element lengths would resolve this when i get it down but now you have said that i'm wondering if i'm going to hit the same snag as you have {bnghd}


I had to try and lever out the parts of the winch cable that moved and painstakingly cut them with my hacksaw. In the end I think it came off in about 6 lengths. The cable had managed to become kinked and folded back on itself. Then with the weight on the cable when raising the mast, it has slipped through and jammed. There was no way I was getting it off intact but at one point, I thought I was going to be replacing the winch!

Been reading up on the Cobwebb in terms of tuning and it's certainly a strange one. To begin with, I found that it needed to be at least 6 feet off the ground otherwise 20m would not tune. It sat at over 3.0:1 until it was well off the ground. Thereafter, it settled down and as previously mentioned, 20-15m was easily tuned. It took no time to bring it to 1.1
I was really surprised though with 12-10m as I expected these bands to be straightforward but they stuck their fingers up at me! They both behave differently too. According to my MFJ analyser, 10m is resonant with best match at 30.125 so lengthening the dipole elements should bring this down in frequency but it doesn't budge. I've actually extended the dipole to its longest but it makes no difference. 12m just sits at 2.0:1 at around 24.400 then climbs to in excess of 3.0:1 either side. I've even checked it by conventional means by squirting good old rf down the feeder and measuring with a bridge just in case my MFJ wasn't right but it's the same.

Weather had been rubbish since Saturday and forecast for next few days is looking right wintry so I don't think I'll be spending much time outside.
When the weather does improve, I'll spend some more time on it and see if I can figure out what's happening but I'm beginning to think that there's something causing these strange readings.

Hope you manage to get your antenna tuned ok. As I said, ensure your antenna is a minimum of 6 feet off the ground.
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Re: Cobwebb antenna advice please.

Postby MH14 » 11 Jan 2017, 14:14

kilimax wrote:It was easy to tune 20-15m and I now have minimum swr at 14.200, 18.150 & 21.200. All now 1.1:0
10m & 12m however are giving me a bit of a headache.


You should be able to achieve resonance (good SWR) on all five bands.

On 10m you may find that bandwidth is a little short to cover the entire band (28 - 29.6) with a low SWR throughout, as it will rise towards the edges. But still within acceptable limits.

Also remember that internal ATU's were in fact initially designed to trim in near resonant antennas. Most commonly beams were the culprits. For example, you may have a 20m beam with a 150-180hz bandwidth. Not quite the full 350hz required for the full band. The internal ATU solved that problem and is why they usually can only handle a couple of thousand or so impedance wise. They were never built to fool transceivers into using non resonant antennas (unlike external ATUs).

Point being? You might find that useful on the 10m element? Because you can centre the resonant point around the 28 Mhz SSB portion of the band, then use an ATU to trim in the higher end of the 10m band, or, of course the lower 26-27 Mhz frequencies ;)

Another point worth noting, is that (assuming your over free space), you may find the resonant dips on each band are a little higher than expected when tuning the antenna at ground level. But once adding a bit of height into the mix then all is well. That is quite the norm. The only exception is when using an antenna analyser. The super low power input into the antenna from an analyser helps removes any coupling or reactance issues that may be present due to near by objects etc.

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Re: Cobwebb antenna advice please.

Postby kilimax » 11 Jan 2017, 15:12

MH14 wrote:
kilimax wrote:It was easy to tune 20-15m and I now have minimum swr at 14.200, 18.150 & 21.200. All now 1.1:0
10m & 12m however are giving me a bit of a headache.


You should be able to achieve resonance (good SWR) on all five bands.

On 10m you may find that bandwidth is a little short to cover the entire band (28 - 29.6) with a low SWR throughout, as it will rise towards the edges. But still within acceptable limits.

Also remember that internal ATU's were in fact initially designed to trim in near resonant antennas. Most commonly beams were the culprits. For example, you may have a 20m beam with a 150-180hz bandwidth. Not quite the full 350hz required for the full band. The internal ATU solved that problem and is why they usually can only handle a couple of thousand or so impedance wise. They were never built to fool transceivers into using non resonant antennas (unlike external ATUs).

Point being? You might find that useful on the 10m element? Because you can centre the resonant point around the 28 Mhz SSB portion of the band, then use an ATU to trim in the higher end of the 10m band, or, of course the lower 26-27 Mhz frequencies ;)

Another point worth noting, is that (assuming your over free space), you may find the resonant dips on each band are a little higher than expected when tuning the antenna at ground level. But once adding a bit of height into the mix then all is well. That is quite the norm. The only exception is when using an antenna analyser. The super low power input into the antenna from an analyser helps removes any coupling or reactance issues that may be present due to near by objects etc.


Thanks for the info but there's definitely something not right as I'm unable to find a point within these bands where there is an acceptable swr. As I've stated, my analyser sees best match & resonance on 10m at 30Mhz and no amount of tuning makes that change. This is being done in free space so height is not an issue. On 12m, swr shows 2.0:1 as best match and I cannot improve on this.

There's either an issue with the Cobwebb or as I've hinted at, possibly some detuning is occurring due to my dual band 6/4m yagi being "close". It may explain why there's little effect on the lower bands.

Won't know for sure until I get this work done if it ever stops blowing a gale, raining or snowing!

The bandwidth is far too narrow to expect to cover the larger bands without the use of an atu but I'm trying to tune all 5 bands to the ssb portion of their respective bands to minimise the need. As you've said, internal atu's are not designed for tunjng out non resonant antennas but I would be happy if I could even find a point where there's some acceptable readings.

Anyway, perhaps this weekend if weather breaks, I'll make some progress.
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Re: Cobwebb antenna advice please.

Postby MH14 » 11 Jan 2017, 15:58

It might be worth checking the tapping points (gamma matching) on the elements which your having issues with?

I once seen a home brew cobwebb which was struggling to find resonance on one band in particular. Long story short, the person who had made it, had cut the elements to the correct size for sure. They had also placed the gamma tap in exactly the right place. But then promptly (in error) fitted the elements back to front when actually assembling the antenna. That very effectively shunted the gamma match points to completely the wrong place. Reversing the elements solved the problem he was having.

People think a Cobwebb is just a load of dipoles bent in a square. Which to a degree it is. But there is much more to them because of the gamma matching setup which is used on each of the elements. If you are using an antenna analyser, note the impedance on the bands that are giving you hassle, if there not 50 ohm, or close to it, then something is wrong.

Those tapping points help create the 50 ohm match. Instead of the normal 75 ohm expected with a dipole. If it was possible to "slide" those tap points up and down the elements, you would witness the impedance changing on your analyser as you did so. Even without changing the overall length of the element itself. Just think what happens when you have something like an Imax CB antenna (dial-a-match mentality), which uses metal rings on a thread at the bottom of the antenna to tune it. When you do that, at no point is the overall length of the antenna itself changed. But depending on where you have those rings, is where you will find the best 50 ohm (low SWR) match.

Now of course, it is highly likely that your elements were already attached when you got your antenna? So there is no way you could have screwed up. But even if so, my first port of call would be a quick check. Just in case the numpty who assembled it made such a simple mistake. You know, off home early for the weekend and all that. :wtf:

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Re: Cobwebb antenna advice please.

Postby kilimax » 11 Jan 2017, 20:26

MH14 wrote:Now of course, it is highly likely that your elements were already attached when you got your antenna? So there is no way you could have screwed up. But even if so, my first port of call would be a quick check. Just in case the numpty who assembled it made such a simple mistake. You know, off home early for the weekend and all that. :wtf:


Perfectly plausable! Yes, already attached. So a few things to check then once it improves outside. March I reckon..... :(
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Re: Cobwebb antenna advice please.

Postby MH14 » 13 Jan 2017, 18:05

kilimax wrote:
MH14 wrote: March I reckon..... :(


Lightweight :D

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Re: Cobwebb antenna advice please.

Postby mark1 » 29 Jan 2017, 17:02

bc73 wrote:Well New Cobwebb arrived today very prompt delivery. will try to get it built in the next few days and keep you all posted.


How did you get on?


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