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WiFi technical question

Tue Feb 17, 2015 10:18 am

I was wondering if any of the gerbils with RF engineering experience could explain something I've been curious about.

Over the past few years, a number of very small USB WiFi adapters have come on the market, where the entire adapter basically fits inside the USB connector. How the heck do you fit an effective antenna inside something that tiny? Compounding the matter, the antenna is located right next to a significant source of EMI!

Even cheap-ass ones (like this one) seem to work on par with -- if not better than -- the WiFi adapters built in to many laptops from just a few years ago. I've "upgraded" the WiFi on several laptops with these things (both the linked brand and "Edimax"), and I continue to be amazed by the bang for the buck you get. (They're also a cheap and easy fix for laptop WiFi adapters that don't play nice with Linux...)
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derFunkenstein
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Re: WiFi technical question

Tue Feb 17, 2015 10:24 am

I'm also curious. I basically have two of them right next to each other in ports stacked on top of one another. One is a wifi dongle like you mention, the other is a Logitech nubbin receiver for a keyboard/mouse combo. They'd don't interfere with each other, nothing else interferes with them, and they both work at substantial distances (I can stand across a 20-foot-deep conference room from the computer and still use the mouse, on the internet, running a web app over wifi with no slowdowns). They are plugged into the front of the computer, which faces where I am near the screen, and the computer is by the projector.
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The Egg
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Re: WiFi technical question

Tue Feb 17, 2015 11:59 am

The only information I can give you from my amateur radio days is that 2.4Ghz would calculate to a wavelength of 12.5cm. I imagine they're using a 1/2 length, or most likely a 1/4 length antenna coiled up inside. Obviously an uncoiled, full length antenna would perform better. That's about where my knowledge ends. :P
 
alloyD
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Re: WiFi technical question

Tue Feb 17, 2015 12:03 pm

As long as you can pick your frequency and stick to it, you can make an antenna pretty short by compensating with some inductors. This does come at the cost of some efficiency though. Also note that at 2.4GHz the wavelength is 4.9 inches which gives a quarter-wavelength of 1.225 inches. That's fairly small so shrinking it again isn't too unreasonable. At 5GHz that wavelength shrinks even further.

I'm an electrical engineer specializing in power and control systems, but I do dabble in RF. I'm sure someone could answer it more completely/correctly than I have.
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Re: WiFi technical question

Tue Feb 17, 2015 12:22 pm

Clearly it's all PFM.

Nice thing about these RF modules is that often you can use a little info to find more info thanks to the FCC licensing process.
If you can find the FCC ID number for the product, NHPWLN1513, you can find the FCC info.

https://apps.fcc.gov/oetcf/eas/reports/ ... NHPWLN1513

Including a peek inside the unit here: https://apps.fcc.gov/oetcf/eas/reports/ ... NHPWLN1513

You can see where they folded up the antenna and stuck it right on the edge of the casing.
Interesting to note the little 'hook' at the corner. I imagine that is a little extra for tuning....antennas at high frequencies do strange things.
 
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Re: WiFi technical question

Tue Feb 17, 2015 12:36 pm

Amazing use of limited space there. Looks like they even designed it with a gap in the pattern of folds, to allow just enough clearance for the WPS pushbutton to poke through the antenna! :lol:
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mako
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Re: WiFi technical question

Tue Feb 17, 2015 12:39 pm

Not an antenna guy, but I'll observe that full 3D electromagnetic simulation is a standard tool these days, so you can make whatever crazy antenna shape you want and analyze its performance. For a cheap consumer product, max efficiency/performance is not necessary, it just needs to be good enough. I would think maturity in the drivers and networking stack also improves the end-user experience.
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Re: WiFi technical question

Tue Feb 17, 2015 12:59 pm

Google "chip antenna", they are little ceramic packages that look like overgrown SMT capacitors. Here's a PDF comparing them to PCB trace antennas http://www.pulseelectronics.com/download/3721/g041/pdf

If you look at something like the PandaBoard photo
http://pandaboard.org/content/resources/references
The package at the top right with the white label on it is the Wifi/BT module and while there is a socket on it for an external antenna then that is not needed. There is a chip antenna on the top right edge of the board, you can just make out "WLAN/BT ANT 1".
 
ludi
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Re: WiFi technical question

Tue Feb 17, 2015 5:38 pm

Following lingo from the FCC disclosure documents, I found a fairly good explanation of the Planar Inverted-F Antenna. A GIS for same will turn up other examples of its construction.
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Re: WiFi technical question

Tue Feb 17, 2015 7:06 pm

ludi wrote:
Following lingo from the FCC disclosure documents, I found a fairly good explanation of the Planar Inverted-F Antenna. A GIS for same will turn up other examples of its construction.

All I can say after skimming that is, RF stuff is crazy! :o
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