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Author: Subject: Forming a spiral from tube
ChrisW

posted on 14/3/15 at 04:48 PM Reply With Quote
Forming a spiral from tube

Hi everyone

I want to build a quadrifilar helix antenna for weather satellite reception. Something like the ones described here: http://perso.wanadoo.es/dimoni/ant_qha.htm

To do this I need to form some 'spiral' shapes from metal tube. All the DIY efforts I have seen online look rather clumsy due to the difficulty of forming the shape. Either people use thin wire which obviously can be bent fairly easily into the shape but can also deform quite easily. Other suggestions have been micro-bore copper pipe, brake pipe, etc. The same issue comes up - if it's pliable enough to be bent by hand it's not going to stand up to life outdoors in all weathers. I also need to consider that if it looks to the casual observer like there are a load of mangled plumbing bits on the roof I'm opening myself up for a lot of earache from SWMBO!

What would be much better would be to use a stiffer material eg:

Stainless: eBay Item
Aluminium: eBay Item

...but here lies the problem, how do you form the shape accurately from a material like that? I guess some kind of jig will be needed,

I know we have loads of excellent engineers on here. Can anyone suggest a way of building something that can accurately form the curved pieces?

Perhaps it could be done on a lathe?

Cheers, Chris





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theconrodkid

posted on 14/3/15 at 05:40 PM Reply With Quote
have a word on here http://www.mig-welding.co.uk/ they are the masters of all things metalic





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owelly

posted on 14/3/15 at 06:28 PM Reply With Quote
Have a look at stainless Swagelok pipe (http://www.swagelok.com/products/tubing-tube-accessories/tubing.aspx). It comes in different sizes and can be bent without too much hassle and doesn't kink easily. You can weld, braze or even silver solder it or just use the Swagelok fittings. To bend the sort of helixeseses you want, draw the helix on a cardboard tube (wrap string/elastic round it for a guide). Carpet shops should have tubes. If the tube is not strong enough, fill it with sand and pack it down hard.
Swagelok is a brand so there may be other stainless pipe that can be bent by hand but I've used a lot of Swagelok so I know it works!





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adithorp

posted on 14/3/15 at 08:04 PM Reply With Quote
I know nothing about this stuff so might make no sense but...

Could you not form it from something thin/easy to bend, around a former (made from 200mm plastic drain pipe) but leave the former in place (hot glue them on) so that the aerial is a cylinder rather than an open frame? You'd have to work out a way to mount the centre piece but it'd be more substantial and less subject to damage.





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HowardB

posted on 14/3/15 at 08:43 PM Reply With Quote
I've seen the same thing made round the body of a pop bottle,.. also with a series of sticks in a pole,. more complex no doubt, but it would allow for more gain.

hth





Howard

Fisher Fury was 2000 Zetec - now a 1600 (it Lives again and goes zoom)

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MikeRJ

posted on 14/3/15 at 08:52 PM Reply With Quote
quote:
Originally posted by adithorp
I know nothing about this stuff so might make no sense but...

Could you not form it from something thin/easy to bend, around a former (made from 200mm plastic drain pipe) but leave the former in place (hot glue them on) so that the aerial is a cylinder rather than an open frame? You'd have to work out a way to mount the centre piece but it'd be more substantial and less subject to damage.


PVC (and some other plastics) can be quite lossy at high frequencies.

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AdrianH

posted on 15/3/15 at 01:40 PM Reply With Quote
I have an old APT receiver and crossed circular dipole, seems to work well.

Don't use it much as the satellites are dying out, down to 3 the last I used mine. Still here and plugged in but!

Most seem to be going for the high res stuf one can get from geo stationary satellites.

Adrian





Why do I have to make the tools to finish the job? More time then money.

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Dave Ashurst

posted on 15/3/15 at 04:03 PM Reply With Quote
Doesn't brake pipe work harden when you bend it?

If so then would it be possible to use a pipe bender to make a continuous, overlapping circular bend (like a spring) then pull it out lengthwise to get the shape right?

Would that both achieve the shape and harden it for enough durability?

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hizzi

posted on 17/3/15 at 08:47 PM Reply With Quote
i run a ham radio club we make antennas from alloy tube from B &Q, 8mm can be bent very accurately with a plumbers pipe bender. long sweeping curves can be made by marking the curve on a large bit of timber then cutting the line with a jig saw, then clamp the two bits together again with the tube in the middle, this will give consistent results
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bart

posted on 17/3/15 at 10:45 PM Reply With Quote
copper tube

looking at the dimension's shirley ( stop calling me shirley ) 15 mm copper tube would do the trick with 90 deg elbows at each end.
you can bend "new soft" copper tube over your knee the amount you need in the picks.
you might mess a few up but its not that expensive.
if you want to get really tech cap off , fill with sand ram down till hard cap the other end off then bend away no crimping
use a piece of drain as a mandrel. you could always run a blow torch over before to anneal the tube.
with sand bet you could do larger as well.





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hizzi

posted on 18/3/15 at 07:03 AM Reply With Quote
sand is for steel pipe wont work on copper its too soft and collapses anyway, plumbers bending springs are used inside copper for 15 and 22mm external springs ar used for 8 and 10 mm pipe
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40inches

posted on 18/3/15 at 09:37 AM Reply With Quote
You can buy copper spirals, possibly stainless. LINK
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HowardB

posted on 18/3/15 at 10:01 AM Reply With Quote
how about a slinky???





Howard

Fisher Fury was 2000 Zetec - now a 1600 (it Lives again and goes zoom)

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Angel Acevedo

posted on 18/3/15 at 03:45 PM Reply With Quote
Wireline Spring Steel

Chris.
Perusing a junk yard here in my hometown, I found a coil of very bright Stainless Steel Wire about 3 or 4 mm in diameter. I was told that it was used in Wireline Operations in oil wells.
I wanted to take home a few turns to fabricate a grille, but turned out to be VERY stiff.
Maybe you could use it cutting to length and extending on a frame. As it is very stiff, I don´t see it losing its shape once it has been welded and fixed in place. Of course this would work if the curvature is larger than the coil. Coil diameter of the wire I found was about 80 cm.
HTH.
AA





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