CL and SP antenna question

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CL and SP antenna question

Postby francoishaha » Tue Apr 10, 2012 11:34 pm

Hi, I recently made my first cloverleaf and skew planar antenna and they work ok, I think there is room for better.
Most of the people i find on the net use MIG wire to make the antenna but I wonder if there is a better choice?
Copper is soft material but it's a good conductor, would it be better to make it with this material?
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Re: CL and SP antenna question

Postby RCModelReviews » Wed Apr 11, 2012 12:19 am

If you're making 5.8GHz antennas then copper wire is a much better option (IMHO) than MIG wire.

The problem with MIG wire is that it only has a very thin "flash" of copper on it and that copper will corrode (oxidize) very quickly when exposed to the air.

Unfortunately, it is the nature of high-frequency RF that it only flows on the outside of a conductor. This is known as the "skin effect" and means that once your think layer of copper turns to copper oxide, the effective resistance of your antennas will rise significantly -- thus reducing their performance.

I use 1mm solid copper wire for my 5.8GHz antennas because it is strong enough to maintain its shape and the slight increase in thickness also makes the dimensions (therefore the tuning) a little less critical.

However, once you go to larger antennas (such as 2.4 and 1.3/900) then copper will probably not be strong enough (although you might get away with it on 2.4GHz if you're careful.

In that case, you'll probably have to use MIG wire -- and perhaps try a very light coat of *clear* polyurethene lacquer over the top -- to try and stop the oxidation.
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Re: CL and SP antenna question

Postby kaptain_zero » Wed Apr 11, 2012 1:46 am

I wonder if a copper sulphate solution would be sufficient to *restore* the thin copper coating on mig wire?!?! I remember using it for layout work, way back in the day.... a clean steel object could be given a thin copper coat by simply wiping it with a copper sulphate solution and I recall it took a fair bit of elbow grease to get the dang stuff off after doing the work.

As for using "*clear* polyurethene lacquer"..... uhh, I think you *might* be getting your coatings mixed up. Lacquer and polyurethane are 2 different beasts... I suspect you were referring to polyurethane coatings, and that still leaves us with the question of waterborne or oil-based... My preference would be for the latter as I think it would have a better chance of a complete seal, but multiple layers would be required and spray or wipe applications would need to occur about once an hour... Wait too long and the second coat would not adhere to the first, too soon and you wash the first coat off.

Lacquer would be great for antennas. No need to worry about the solvent and what it might dissolve, in which case I would just buy a small bottle of clear acrylic nail polish, it even comes with a brush! :mrgreen: Lacquers and nail polishes which are in the same category can be re-dissolved with lacquer thinner, acetone and the like (sure, why not.... nail polish remover too). The only problem with brushing lacquers is that the solvent in your second coat begins to melt the first coat, so you have to be quick and yet careful.

Polyurethanes are a one way deal, once the cross-linking is complete, it cannot be dissolved by it's original thinner and must be mechanically removed. And it is for this reason that additional coats must be applied within a short window of time... You need to wait for the cross-linking to start and firm up the first coat, but you need to apply the next coat before the first coat finishes cross-linking so the two coats can link together. Yes you can mechanically abrade a coat that has fully cross linked so the next coat can *key* into it, but it is not as strong a link and imperfect prep would lead to failure of the protection one was seeking.

Regards

Christian
"I'd like to live as a poor man with lots of money."
-- Pablo Picasso
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Re: CL and SP antenna question

Postby RCModelReviews » Wed Apr 11, 2012 3:24 am

Just grab a spray can of clear polyurethene and give it a *really* light coat -- trying to get a little puff from all angles to ensure total coverage.

That's what I use on my home-etched PC boards to stop them oxidising and it works brilliantly -- without affecting the electrical characteristics at all.
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Re: CL and SP antenna question

Postby francoishaha » Wed Apr 11, 2012 9:26 pm

thanks, i will try the polyurethene spray.
would it be a good idea to sand the copper wire before bend and solder?
if yes, then, is there a difference between sanded around the axis, like sanding a shaft on a lathe or sanding on the length?
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Re: CL and SP antenna question

Postby RCModelReviews » Wed Apr 11, 2012 9:44 pm

I wouldn't sand the wire -- just make sure it's clean and free of grease. Try not to get too much spray on the center area where the coaxial cable exits. A very fine mist of spray will be enough.
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Re: CL and SP antenna question

Postby francoishaha » Wed Apr 11, 2012 10:30 pm

Thank you very much!
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Re: CL and SP antenna question

Postby Shotglass » Wed Apr 11, 2012 11:12 pm

francoishaha wrote:would it be a good idea to sand the copper wire before bend and solder?


terrible idea since surface roughness plays a huge role in rf losses
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