Cobweb antennas

[VK4WDM]

Hi Dave

I will go and have a look at those in the morning! They are a bit short but I am thinking that I could use three together and clamp another one between them thus strengthening the inner portion and extending the length. Thank you for the heads up!

73

Wayne VK4WDM

[VK3AV]

In the Hex Beam discussion a little while ago the availability of fibreglass reinforcing rods for concrete was highlighted.

As I am building a hex I ordered six 3.5m rods to use as spreaders. If you order any, ask for non-coated rods as they also come with a epoxy/sand coating for use in concrete.

The company is V.ROD Australia based in Adelaide. http://www.vrodaustralia.com.au

You will find some specs in the PDF docs on their web site and whilst I opted for 12.7mm diameter there are other diameters available.

Regards
Bernard

[VK4WDM]

Hi Bernard

Sounds like exactly the right stuff. I am still going through the Hex vs Cobweb debate in my mind so very useful info.

73

Wayne VK4WDM

[VK2FAK]

Hi all...

Found this interesting idea on a South African radio forum....called the Silver Spider...

no need for the fibreglass spreaders, can use ali

[VK4WDM]

Hi John

Interesting read! This idea has been around for a while. The X beam" was quite popular in the 1970's which evolved from a single element quad laid horizontally. Some guys also made them out of metal using a rotary clothes line - the story was you could change the the efficiency by adding or removing clothes, especially underwear

We don't have a rural supplier in our area ATM but I am going to take a country drive and check out the electric fence spreaders (which are apparently the same as the ones used on power lines). As well as the cobweb they could be used as elements for super-lightweight yagis by taping wire along them.

73

Wayne VK4WDM

[VK3AV]

There are some electric fence spreaders here -

http://www.thefarmstore.com.au. Look under --> fencing >> electric fencing >> fibreglass equipment.

The max length is 2m.

Regards
Bernard

[VK4WDM]

Hi Bernard

They look really good, two different diameters too. Should be good for cobwebs, hexbeams and moxons.

73

Wayne VK4WDM

[VK3AV]

Wayne,

The folk who run that business are very friendly and have good follow up in my experience. I have dropped into their shop/warehouse to pick up some clips for the hex build.

No involvement with the business, just a happy customer.

Regards
Bernard

[VK2AOH]

I am getting the parts together for a hex beam, the fibre glass rods/clips am pretty certain can get these from a local ag supplier.

The hub is proving a problem. Does anyone know of a local Aus supplier that sells the hub similar or same as the one K4KIO uses for his hex beams ?

Thanks for any help with this.

[VK4TI]

I sourced some 5mm alloy plate from a welding company in Beaudesert for a fair price , I should expect most towns will some access to metal supplies suitable , add some stainless clamps and your beam will be done , if all else fails weld some angle together as needed as long as it holds the arms

[VK2AOH]

Thanks, have seen some remarks on home made hubs and don't have a welder at hand. A pre made hub would save a lot of bother, the rest I can do myself.

Pretty certain I saw some one comment on hubs but can't find the post despite a search.

[VK5PL]

Thanks for the link to the farm store. I was looking at getting some 2m rods from a fibreglass supplier.
The Farmstore is supplying them for half the price of the fibreglass place.

[VK3BGS]

Check out NBS Antennas, I am sure I have seen a hub on his web site for a hex.

Denis
VK3BGS

[VK4TI]

Not having a welder is not an excuse , simply driving into an industrial area in any country town will find you some one who can do the work for you at a nominal sum
or go buy one if you can foresee some future use for $99.00 in many hardware stores

[VK5TM]

Or if there is a local Mens Shed, they would probably do it for a small donation.

(Shameless plug for the Tintinara Community Mens Shed)

[VK2AOH]

Thanks for the replies, I came across NBS antennas, apart from getting the hub ex stock Brian gave some good tips on construction and sources for the other parts.

[VK3AV]

Here are a few items that might be of use...

For the spreaders go to - http://www.vrodaustralia.com.au You want the V-Rod Vinyl Ester Plain 12mm #4 round rod. 6 of them 3500mm long. The price is around $72 plus freight. Ensure that it is plain rod that you ask for as they are normally coated in a sand and epoxy material for use as reinforcing in concrete construction. They are very friendly and helpful plus getting used to amateurs calling. You will also need to paint them to stop the fibreglass from splintering and as the business owner Ken said to me "that stuff is nasty, always wear gloves"

To clamp the rods to the base plate you can use Stauff clamps. Go to the Stauff Australia web site http://www.stauff.com.au and look for a local distributor. You only need the clamp shells not the complete clamp unit. The 12mm size is part number 212,7PP. I purchased 12 x 12mm, 4 x 10mm and 8 x 6mm shells and it totalled $33. Pick up so no freight cost. As you can see they are quite cost effective. I purchased stainless steel machine screws in a suitable length to mount the clamps to the base plate form a local industrial supplier though you can get them at from Stauff as well.

I was going to make the hex base myself and purchased some 5mm plate from Capral but in the end I purchased an NBS base as it is swaged to fit into aluminium scaffold tube making the mounting easy. It removed the need to sort out the clamp arrangements for the base. If you make your own centre post then you can get a PVC adaptor from Bunnings etc that will fit over the tube on the upper side of the base and allow the use of PVC 32mm pressure pipe for the centre post.

NBS also has antenna wire in green that has a high strand count they should work well for the element wires. That is what I am using.

The above is based upon a lot of research, some discussions on this forum and info from the Yahoo hex antenna group.

I have fabricated my centre post with RG-213 feed on the inside of the tube (frankly a messy job as it meant cutting the tube into smaller lengths and using tube joiners) but the final assembly needs me to get organised to get the mast mounted (also an NBS base). If the weather would hold I could get the hole drilled and the concrete delivered.

Hope this helps.

Regards
Bernard

[VK2AOH]

Thanks Bernard for the really good hands on details. As for rods I came across this company in Minto, Sydney http://permex.com.au/

They supply rods of all sorts and cater for sail boat owners whom need spreaders, battens or whatever. Am waiting for my base plate to come in from NBS and then get the 12mm rods.

Noted the remarks on clips and will get those. As for wire I have a drum of 16/0.20 ex Telecom insulated wire which should do. Has a tough teflon insulation.

Saw the green stuff at NBS and was nearly going to get that. Brian has a lot of useful stuff there, pity he is in Victoria and so far away.

Thanks again for the info, when I finish the beam will put a couple of photos up as there may be others interested in a lightweight beam.

73's Nick

[VK4WDM]

Finally the cobwebb is up.

I used fiberglass electric fence droppers for the speaders, and twin lead speaker wire as in the original design, tied to the spreaders with hose clamps and cable ties. I made the elements lengths about 10cm longer than the supplied dimensions and tuning was easy with my antenna analyser.

SWR is around 1:2 on 20, 17, 15, and 24M. On 10m it is 1:8 and I cannot get it down. This is easy dealt with my outboard tuner. The SWR was unstable and changed when the tuner was touched - a classic sign of common mode current on the line, despite the inclusion of the recommended 5 turn coaxial choke and I have increased the number of turns to 7. Still not completely satisfied and I will experiment with some ferrite beads or may be a 1:1 toroid balun in addition to the choke.

I have it mounted on the roof with a total height of only 6m ATM.

How does it work? Obviously early days but it is certainly quieter than the OCF dipole and received signals on 20 and 15M are about the same as the OCFD. More to come.

73

Wayne VK4WDM