Copper tube Antenna Bending

Hi All,

I have 1 inch diameter hollow copper tube, i need to bend the tube into circular coil of diameter 30 cm, i am wondering in calculating circumference using formula C=2*pi*r, what should be the value of radius, should i calculate r from centre of circle to inner diameter/outside diameter/centre of inner and outer diameter?

Thanks

Generally, you calculate such things using the center line of the tube's cross section. This way, some portion of the structure will be slightly larger/longer than nominal, and some part will be smaller/shorter than nominal. In the grand scheme of the design, things balance and give you acceptable performance around your desired center frequency.

Why are you using such thick tubing? Are you approaching a significant percentage of a wavelength using 1" tube?

Thanks for the reply, i have designed the system for wireless power transmission in hfss and we took 1 inch diameter copper tube in calculations, so now need to stick to it.

There is no significance amount of wavelength required in my case, but we already have purchased all material, just the final task of bending pipes is left, so before cutting pipe, was wondering what radius to be taken into account, so that we end up with desired or nearby results.

One thing to be aware of is the losses in the tuning capacitor. Another thing is that when the capacitor heats, its capacitance changes and changes the tuning.

Thanks flatulent, but its just raw structure we are building right now, and considering your suggestion, we have already taken 10-15% tolerance to be acceptable from simulation results.

The length is neither the outer nor inner circumference, as the tube will stretch and compress. It will probably stretch more than compress, so the length you need will be closer to the inner circumference. You may want to just roll a spiral, then cut to length afterwards.

Do you have the specialized equipment that will be needed to bend such a tight circle without kinking the inner wall on such large tubing? Also, the copper will work harden significantly, so you may want to ave a way to anneal it periodically.

John

Yes, working copper tube into specific shapes can be very tricky. To avoid kinking in the tube, many people will fill the tube with find sand or some kind of liquid (like brake fluid, very messy though), seal the ends, then bend it. Even then you will probably have trouble unless you have something to form it to a specific diameter.

Freeze water into ICE!

Thanks mtwieg, I have a bended copper tube that i purchased from one company, as you can see in image, now moulding that to specific diameter of 30 cm, i am bit confused with that, will adding sand as you said will work?

Thanks jpanhalt, the problem is that i don't have any specific tool to bend tube, so what one of the guy at university suggested is to create a form of circular structure and bend it around it but even that seems to be tricky or like experiment.

If you are trying to bend a constant radius, the easiest ways are to bend around a form or use three offset rollers. Both will require some experimentation.

Remember, you must overcome the yield point of the metal and consider the relaxation/return/spring back. If you've ever bent a piece of metal, you know that it will bow/flex before it gives way (yields) and actually has a permanent bend. Also, recall that when you quit applying force, the metal springs back slightly (you have to over-bend, then let it spring back to the desired size). So, find some rigid cans/tubes that are smaller in diameter than your target inner diameter. Try bending some scrap and see what the right size will need to be. If you feel up to crunching the numbers, your friendly neighborhood mechanical engineering student can probably help you figure it out using the math, but I'm a trial-and-error kind of fabricator .

If you want to do a more controlled/adjustable bend, here are some example 3-roller tubing benders that show the general concept (outer two rollers are fixed, the center roller is adjusted to vary the bend angle.
http://www.youtube.com/watch?v=oDhgOlH7y-g
http://www.youtube.com/watch?v=J09E_luT1pM