Step 1: Installing an Antenna in the Screen Casing
This was the first step. Opening the screen casing is really not much fun as without the right tools, it is almost impossible to do this without damaging it. You can get tools for opening the screen casing off eBay. Do it.
I opted to remove the webcam and install this large cellular antenna in its place. This is OK for me. I never use the webcam however in future I will seek out a smaller antenna and re-instate the webcam just for completeness sake.
The 3G enabled models are advertised as having a diversity antenna system. I assume that a small antenna goes either side of the webcam. The only strange thing is that I could not see how two extra antenna cables could be routed down and out of the screen casing for this without cutting plastic. There is only room for one. Oh well. No problem for me-- My 3G card doesn't have a second antenna connector anyway.
The antenna I have used here is extracted from a windshield stick on cellular antenna and is optimised for 900-1800MHz usage. It's probably a bit long for 2100MHz (HSPA) but for the record, it performs very well at this frequency. At some stage I'll design up a couple of meander-line antennas to slip into the side of the screen casing, using the LCD foil as a groud plane.
Some may be tempted to use one of the built-in WIFI antennas. Sure thing, but, beware: Depending on what you have fitted, these antennas are likely optimised for for 2400-5100MHz operation and are far too short for 900MHz usage which your HSDPA modem may use at some point if falling back to 2.5G. It is my opinion that these antennas are not adequate, and may even damage your modem; even if they do work to some extent.
If you are going to do exactly as I did, pay close attention to how your donor antenna is wired. On mine the active element of the dipole has a small link half way down whereas the ground does not. Antenna connection and placement is not something you want to get wrong. If you can get a proper 3G netbook antenna (perhaps even the Genuine Acer part)-- Use it.
Step 2: Source Components for 3G Modem Installation
Personally, I did the antenna installation first as it took a long time for the needed parts to arrive.
Now I have an impressive collection of SMD components from my general stash (none of which were needed) and the SIM card holder, the mPCIe connector, the modem and an Intel 5300 wireless adapter.
The mPCIe connector must be a more-difficult-to-obtain 4.0mm high version. I repeat: 4.0mm high. Any higher and the door on the bottom of the netbook will not close with the 3G modem installed.
Step 3: Prepare for Solder
Now we need to remove the excess solder that was blasted onto the pads through the paste stencil at the factory. Use solder wick to carefully remove it. Note that I have placed a thin strip of high-temperature Kapton tape over pad 20. Explanation later on in this howto.
If you're thinking standard eletrical tape might do the trick here: Forget it. It'll melt and pin 20 will fuse with the pad below and you'll never be able to separate it again witout desoldering (and likely destroying) the mPCIe slot.
After this you can solder the connector on farily easily using plenty of liquid or paste flux. Clean it off with flux cleaner afterwards. Leaving it on there is just messy.
Step 4: Solder on SIM Card Slot and Install Wire Links
Finding a SIM card slot which fits straight onto the Aspire one is hard. The most commonly available is pinned out for the Eee PC. You have to reverse every pair of pins to make this compatible with the Aspire One.
Maybe one day the correct type of slot will turn up on eBay once there's enough of these in the market. I've kind of done this a bit messy with stranded wire because I couldn't find my prototyping wire at the time.
While you here, You're going to need to install a couple of links (circled) so that the 3G slot gets its 1v8 and 3v3 power rails. I had all these ideas about how I might put proper zero ohm links, SMD fuses, or even ferrites in here but in the end I caved and put in wire links as I didn't really have anything suitable on hand.
Step 5: What to Do With That Pin 20
Well this is really up to you. There's several things you can do with it.
What is pin 20? I had previously read that this pin was something to do with radio control but there is little information about how it works. The pessimist in me was expecting it to be something ridiculous like Dallas's 1-wire bus with some diabolical handshaking process and all the rest, but, I was pleasantly surprised to find that it's a TTL logic signal generated by the host computer which is asserted (+3.3v) when the PCIe card is allowed to enable its radio. Perhaps you could also say it's a negative logic signal which is asserted to disable the radio on the card. Whatever. This is fantastic because it's a full hardware solution for enabling and disabling the radios.
There's some logic on the mainboard which asserts/de-asserts this signal when the user toggles the radio disable switch. The switch one the Aspire One actually goes both ways: Slide it left to enable/disable 3G. Slide it right for WLAN. Of course you'll need to hack out some of the plastic to make this work, assuming that enough components are fitted for this to also work on the 3G slot (I did not test this mechanism).
Me? I've just joined pin 20 on each slot so the one switch enables and disabled both radios. I prefer this.
You could also just cover it up and have the 3G radio permanently enabled.
NOTE: Observe earlier when I placed Kapton tape over the pad for pin 20, to stop the pin on the slot from being soldered to the mainboard.
Step 6: Putting It All Back Together
Now we can put this whole thing back together and try it out.
Note that I also installed an Intel 5300 abgn in here. Not because I wanted MIMO (That would be a bit silly on a netbook now wouldn't it?) but because I wanted to be able connect to 5GHz networks which I have one of at home.
Many may freak out at the sight of a third antenna connector and go and hack in a third antenna in fear of what may happen if they don't. Unless you *really* just want to put in a third antenna, don't do it. Normally these cards work just fine with two or one, but, this card doesn't seem to like having just antennas 1 & 2 connected so I had to connect 1 & 3 instead.
UPDATE: The Intel 5300 is a bad choice. Buy an Intel 5100 adapter instead. The difference is that the 5100 is designed for use with two antennas (with slightly, negligible, lower performance) whereas the 5300 requires three. The 5300 does work with two antennas but get slightly confused in certain situations. The 5100 is your card here.
If you are hoping for PEX on the second slot. You're out of luck. I repeat: No PEX. The card that goes in here must be USB. I would not stress about this as I'd challenge anyone to find a mPCIe format HSDPA modem that connects via PEX.
There is a spare PEX root port which may be wired to this slot (but some series terminating resistors will likely need to be fitted to make it work)
Step 7: Installing Software
There aren't really any quirks here. Everything worked first go for me.
Here's my token Device Manager screenshot: