The DLINK DWL-G650 wireless card was a popular choice a few years ago due to support in Linux and resulted in a number of websites describing the hardware mods to this card, however most of the images were lost in abandoned online image hosting accounts with the result that just the "tell " is left over from the "show and tell ".
There's basically 2 ways to add on an external antenna, solder a pigtail ( the thin coax and SMA connector in the pic below ) directly to the circuit board or solder a connector onto the circuit board, which the pigtail of your choice will plug in to.
Im using a Hirose U.FL to RP-SMA pigtail with its corresponding U.FL connector that I harvested off an old mini pci card.
Be aware that its a SMT (surface mount technology) component and thus very small, so a fine tip soldering iron and magnifying glass are needed.
Step 1: Disassembly
First take off the back plastic cover, then the back metal cover and finally the front metal cover.
I find it easier to work on a corner for the plastic cover, there are 4 tabs holding it in, and on the side at either the top or bottom of the metal covers, bear in mind they are soft aluminium and distort easily.
Once this is done remove the board from the plastic frame so as to make it easier to work on.
Step 2: Soldering on the Board
There was an empty spot for a U.FL connector on the top antenna, this would also be a good place to solder a pigtail directly to the board. I then cut the track of the top onboard antenna to prevent problems arising from having 2 antennae in parallel at that point.
Step 3: Cutting the Case for the Pigtail
Step 4: Re-Assembly
Step 5: And Finally...
I also tried covering the internal antenna area with tinfoil to see if it made a difference forcing the card to read the external antenna only.
The difference was small enough not to really go to all the trouble.
Im now able to hook up a homemade helical pvc antenna to my desktop pc which results in my wireless link to the router going from a 24% signal quality to 98%.
I used the opportunity to drop the Tx power on the router to 50% and my radio card still connects at 90% signal quality with a TX rate of 54Mbs.
This results in a more secure network because Im not broadcasting as far beyond my property like I used to.