speed of light / 5 GHz / 4 ~= 1.5 cm.

If you can keep the size of the largest crease/feature/defect in your foil smaller than this, you should be okay.

I've seen microwave focusing mirrors for high-power applications that were rough CNC milled surfaces. The overall shape had been optimized fairly precisely by simulation, but because the intended wavelength was so long, the tool marks were irrelevant.

To get some intuition of why quarter wavelengths seem to come up a lot here, it's helpful to imagine a long transmission line carrying your signal. What would happen if you were to add a quarter wavelength stub to this transmission line?

So something like:

  Source ----------------------------------- Dest
                         |
                         | lambda/4
                         |   stub

The transmission line carries an electrical current proportional to the signal from "Source" to "Dest." When the current gets to the stub, you can think of it as going down two paths -- continuing to Dest, as well as down the stub. When the current component going down the stub gets to the end of the stub, it reflects and travels back up the stub (reflection coefficient of the "open circuit" at the end of the stub is 1).

By the time the reflection gets back to the transmission line, you have a phase offset of half a wavelength relative to original signal on the transmission line. If you add two signals that are half a wavelength apart, they combine destructively (cancel each other out). So the quarter wave stub acts like a very basic filter, and you actually won't see much of your signal at "Dest" when your transmission line has a quarter-wave stub like this.

Microwave engineers use all sorts of tricks involving transmission line segments that are a quarter wavelength long. So when you're trying to build an antenna (e.g. out of foil, or if you're adding metal support structures to a large antenna), it's helpful to use the heuristic: "if you need to add features that are not part of the original design, try to keep them smaller than a quarter wavelength."

blackguardx is right that restricting the maximum feature size to 1/10 of the wavelength is probably a better rule of thumb if you want good performance. If you can get it down to say, 1/2 of a quarter wavelength (1/8 wavelength), there's a reasonable chance it'll still work.

Wow, thanks for this — I did HS-physics but much of my knowledge has since left me... :)

Would love to dig into the science behind this a bit more — I don't really know where to start — i.e. why you're dividing by four.

So as not to waste your time, is there a named phenomenon that you can link me to to learn more about this?

Thanks again!

As a quick off-the-top-of-my-head calculation, I take 300,000,000, the speed of light in meters/second (approximate), divide by cycles per second, you get wavelength in meters. In this case, parent calculates a quarter wavelength. A full wavelength is about 5.9cm. It’s easy to get off by an order magnitude in your head, but if through sheer repetition you know 28Mhz is about 10m, you’ll know you’re off if your 30Mhz calculation gives you 0.98m. And you can do the math from there to figure out that 14Mhz is about 20m, etc. Do it enough and you can mentally move some decimal points in your head to get a pretty good approximation for about any frequency. I learned it at some point fiddling with amateur radio, but here’s a page that I think explains it well: http://www.digitalairwireless.com/wireless-blog/t-5ghz/calcu...

300 000 000 m/s

Yeesh, it’s right there in the article I linked, innit? Fixed, thanks.

It has to do with quarter wavelength features making good antennas.

I would say that you want to be less than 1/10 of the wavelength to minimize the effect though.

Antennas are usually built to 1/2 or 1/4th the size of the wavelength. I'd imagine that's why - but am curious myself if it is something else. It's the only 1/4th fraction that comes to mind.

I don't know if there's a specific name for it, but a quarter wavelength shows up as an important feature size in a bunch of RF (and acoustic) formulas. The exact physical reasons seem to vary by context.

I would also like to know this. I don't know if there's a save function in h.news or not so leaving a comment

If you click through to the comment, you can "favorite" it and then it shows up in https://news.ycombinator.com/favorites?id=dukes_haven&commen... which you can also find on your profile page under "favorite comments".

Wow. I'm one of today's lucky 10,000. Thanks!

It will dissapear more slowly here: http://handlr.sapico.me/Home/ByTag?Name=diy :p ( unfinished webapp)

Could you tell me where the /4 comes from, please?

Quarter wavelength. Waves will flow around something smaller than 1/4 of their wavelength almost without interruption.

Thanks. Could you point me in the direction of some reading material?

That has the look of a rule of thumb, which as a civil engineer that ended up in IT, sounds a bit familiar.

I've heard good stuff about the ARRL antenna book, although it's pretty large and spendy.

It’s always fun when you get to use the speed of light in a commonplace application!

it's really speed of radio.

creases could help scatter the signal into more directions and maybe change the signal properties so it's easier to go though some materials. I know nothing about radio but creases makes sound more even (lookup Diffusion) if it's random because it doesn't strengthen any particular wavelength, and sound bounces in different directions instead of straight.

Most of the DIY's I've seen on this use aluminum cans. You could try that.

Pringles cans were really effective for me