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Portable 2m beam construction


#1

Morning All
I am having an interesting engineering challenge whilst building a portable beam for SOTA use. I am building to a slightly modified DK7ZB design for 2m and 70cm using 22mm plastic tube. The elements in the modified version pass through the centre of the boom (making them easier to carry as they don’t have the plastic clips on them). The problem I have is how to accurately drill the holes through (across) the round tube for the elements, so that all the elements are in line. I have had a couple of attempts but the view “down the boom” shows discrepancies in the alignment of the element ends. Not sure electrically if it matters, but it knocks your eye out when looking at it!!
I have attempted clamped V blocks to stop the tube rotating – this worked quite well, but when passing across the table on the pillar drill some lateral movement was evident resulting in the drill not passing through the boom directly across the centre line. I made a tool to mark the centre line along the length of the boom, but due to the four v clamps supporting the length of the boom (and stopping rotation), it is necessary to remove the tool to get past each one, resulting in a slight miss alignment when continuing the line. Square section 22m plastic conduit was also tried but due to wall thickness was not an option without additional bracing. My mill and lathe are also too small to take the length of the boom, and I don’t have the room to make a 6 foot cross slide to fit the mill.
I have made an all aluminium version (Square section, easy to work with…) for 2m activity contests, but it is quite heavy by comparison and not suitable for lightweight SOTA.

My own “spirit of SOTA” is to use home brew kit where possible - with the exception at present of my trusty 817. Any help or guidelines in getting around problem gratefully received.
Thanks
Tim


#2

In reply to G4YTD:

Hi Tim.

I have encountered the same issue with beam construction and have yet to make a 23cms beam that I wish to take ownership of. Even modifying my SOTABeam SB5 to take additional elements for 70cms to the DK7ZB design required a bit of careful work with minifiles and some of the elements are not as tight a fit as I would like. I think this is inevitable with home construction, particularly since I am working with hand tools and don’t have the facilities that you have to hand.

I just wonder whether you might be better considering splitting the boom into two parts and using a coupling sleeve to align the two halves. The SB5 is split into two sections and the longer boom length is similar to the length of the standard SOTAPole, so is protected when the sections are tie-wrapped together and strapped to the side of the backpack.

73, Gerald


#3

In reply to G4YTD:
Hi Tim,
The elements I use are 2mm stainless steel welding rod through a 12mm od aluminium boom. All the elements (4) fit inside for transit. I usually mark the holes as well as is practical, then drill the hole for the director. Drill the hole for the next element with a very small drill. Fit the director and use a thin straight wire (thinner welding rod) to check alignment by eye. Open up the holes gently in the appropriate direction with a needle file till the element fits and lines up. Repeat for the rest. Not engineering at its best but with care it works.
Good luck and hope to work you soon
73
Rob
G4RQJ


#4

In reply to G4OIG:Hi Gerald
I have split the boom in two parts, and turned up a coupling on the lathe from very lightweight engineering plastic. The coupling fits inside the boom and is solvent fixed with a backup grub screw into the short section, with a mounting hole that is secured with a modified nylon bolt through the boom of the long section (and through the coupling). I still have the problem of aligning the element tips to make a true horizontal plane. Its one of those simple problems that will have an equally simple answer - I just don’t have it at present!!
I am working from home today, and the temptation to go into the garage to make a substantial drilling jig is quite strong…hey-ho.
I have been sketching a drilling jig using two lengths of L section back to back, with a travelling (and adjustable) drill guide that sits on the top. If I can’t find an easier way this may be the answer, correctly designed it would allow drilling of various boom diameters, and be usable for all bands. Watch this space.
Hmm, contract boom drilling? Now that’s a thought.
Tim


#5

In reply to G4RQJ:
Hi Rob
Thanks for the advice. The version I am using uses 3.2mm Tig welding rods (£15 per KG from a farm supplies specialist, 1m long each, there are loads in a box!). The master plan is to build an ultra lightweight box of four for 2m and the same for 70cm and 23cm – Not for SOTA use. They will be used on a hill near to where I live for activity night contests through summer, unfortunately access for the last 250m is on foot, hence plastic tube featuring heavily in a –one-trip-from-the-car kind of way. I also need to build a version for SOTA use, so plenty of holes to drill. I see from your post you are using Aluminium tube for the boom, what design and attachment method are you using for your driven element? For the big system, I have built a bending jig for folded dipoles (from the GM3SEK web site), and have also experimented with machined nylon bolts to hold the element together and in place – this is quite bulky though, and connection of the coax is critical for matching.

Cheers
Tim


#6

My two metre portable beam is very similar to what you described. To get all the elements in line I clamped a long length of 1/4 inch batten to my bench and used it to draw a fine pencil line down the boom. I then marked both ends of the tube and turned it over and matching the marks and drew a line down the other side of the tube. I then put a tie wrap around the tube and by sliding it along I drew lines around the tube where the elements and bracket holes where to be placed. Rather than use a bench drill I used a modellers drill and drilled each side separately. In each case I drilled a pilot hole first with a fine modelling bit as I found an accurate start could be made by pushing a dint into the PVC tube with the bit before drilling. Once all the pilot holes where done I opened them up with a drill bit slightly smaller than the elements for a nice tight fit.

I would not bother with building bending jigs for making folded dipoles. I bought various sizes of pipe benders off eBay and never paid more than a tenner. I even saw one of the small central heating pipe size benders in our local local pound shop a week after I paid a fiver for one.

73 Steve GW7AAV


#7

In reply to G4YTD:
Driven element is a straight dipole, I’ve put a picture on Flika of the centre section. It’s basically a cable gland (B and Q)that clamps on the boom with a suitable small panel to carry the terminals. Would have liked two of the large terminals but they’re hard to come by. Easier than screw fittings in the cold. In the most recent version the gland is reversed so the main body is between the terminals. This allows a couple of blind holes to be drilled to support the ends of the elements. Cuts down on droop. Had an article on the system in PW in about 2003, all folds down into my walking pole. Got a copy somewhere I could Email you if of interest.
73
Rob


#8

In reply to GW7AAV:
Hi Steve
That is a neat solution for marking the centre line. I have just had a go (over lunch break of course!) and it worked well on my “practice” boom. I have a small modellers drill too which should prove useful as described. Without a load of work, I don’t think there is a simple solution to automating the process (drill jigs and sliding guides etc). Ok on the pipe benders too, the material I have for the driven elements is 12x2mm flat bar – recovered and rescued from a work project so not sure if it would work with one. I could of course buy (shudder) some tube and do it that way.
Thanks for the tips.
Tim


#9

In reply to G4RQJ:
Afternoon Rob
Thanks for the offer of a copy of the article, that would be good if you can find it.
my e-mail address is tim AT g4ytd.co.uk (get ride of the spaces and use @ instead of AT).
Do you have a link to the picture on Flika too?
There are some very good solutions to problems out in Amateur World, thanks for the info.
:slight_smile:
Tim


#10

In reply to G4YTD:

No the pipe bender would not work with flat bar. When I have used flat bar I heated it up and turned the ends over a wooden dowel. It never seemed easy to get a nice even curve both ends but with practice it get easier.

I tend to buy aluminium tube but recovered/recycled copper tube is a good choice not least because you can solder directly to it and it can always be weather proofed with paint.

I once made a tv antenna from a broom handle, block connectors and some welding rods because my mates wanted to watch the world cup and their bought aerial was rubbish. They were amazed my broom beam worked much better, but 16 elements beats 5 any day. I can’t stand soccer but I enjoyed making the antenna.

Steve GW7AAV


#11

In reply to G4YTD:

The method I use is to tape the boom to the open side of a 2m length of aluminium U section, then mark along one edge. The U section is smaller in width than the boom diameter. L section also works, but not quite as well…

Regards,
Nigel. G6SFP.


#12

In reply to G4YTD:
http://www.flickr.com/photos/92134728@N00/3273544877/
This should get you to the photo Tim. I’ll mail you the article later, the attachments are quite large so I’ll send a couple of mails in case you have a limit on size
Rob


#13

In reply to G4YTD:
Hi Tim.One method of getting the holes in line would be to get a short piece of wood with a large enough section to be able to drill a hole through the centre the same size as the boom.If the boom is not a good tight fit pack with tape.Then mark out the position of the holes.Before drilling a hole set the wood level with a spirit level.And repeat this operation for each individual hole.Hope this makes sense.ATB Geoff G6MZX


#14

In reply to G4YTD:
Hi Tim,

It appears you actually have a number of problems. Possible solutions (from my experience)

a) Marking the tube: hold two lengths of tube together (side by side) on the bench and draw a line with felt tip along the intersection where they touch - marks both tubes and gives you a straight line along the length

b) When drilling the holes creep up on the correct size, don’t bang through with the finished drill size in one go (2mm is rather small and wobbly though so perhaps you should aim for finished element diameter of 4mm)

c) Drilling in-line on drill press: I make two (or more - depending on the length of tube to be drilled) rectangular clamp blocks bored to take the tube and split in from one side to the centre of the hole so the hole can be clamped down by inserting a wood screw across the saw cut and the blocks then clamped to the tube. Build a false drill press table as long as possible and screw a fence to it (essentially a long piece of ply/MDF with a perpendicular fence) the rectangular blocks register against this fence and allow the assembly to slide beneath the drill chuck. Line everything up with the marked centre line and you should then be able to drill on the marked line by sliding against the fence. Removing the work to change the drill size should mean you can return back to where you were without losing alignment.

Sorry if I am stating the obvious or have missed the point

regards

Barry GM4TOE


#15

In reply to G6SFP:
Thanks Nigel. I have had mixed results with the U section, unfortunately the stuff I have is either too wide, or too narrow, an our local supplier only sells it in 5m lengths now, with a minimum order value of £50! Thanks for the comments.
Tim


#16

In reply to GM4TOE:
Hi Barry, thanks for the info.
The tube marking has been ok, once they were stopped from rotating, your solution is good though, very simple and no need for loads of clamps etc.
My finished size is 3.2mm tight, so yes, agreed on the multiple passes. The problem I had was with passing directly through the centre of the tube, and guaranteeing it for the holes on either side. The solution you have in C) should help with this no end, and I will give it a go in the morning and report back (hopefully on 2m with my new beam…)
Thanks for all the comments. Much appreciated.
Have a good weekend.
Tim


#17

In reply to G4RQJ:
Hi Rob
Thanks for the e-mails, they were waiting for me tonight. I will have a browse through and hopefully have all the answers to my problems now. See you from a SOTA summit soon.
Have a good weekend.
Tim