Retrofitting a Senao Wi-Fi card in the Linksys WET11

Retrofitting the Senao Wireless 802.11B Card into the Linksys WET11 Ethernet Bridge

by Steven K. Roberts, N4RVE
Nomadic Research Labs

Part of the Shacktopus Hacks library

As this project began to take shape and I started casting about for the wireless tools of choice, it didn't take long to notice that two terms kept coming up over and over: the Linksys WET11 wireless ethernet bridge, and the Senao 2511CD PLUS EXT2 wireless PCMCIA card. As of this writing (mid-2005), both have something of a cult following among wireless geeks; the WET11 is small and allows any ethernet-enabled device to inexpensively acquire a Wi-Fi connection, and the Senao card has a hot receiver and can transmit up to 249mW... way more than the stock cards that are typically used in laptops.

It's also superior to the one in the WET11, which is what led to this retrofit.

First, it's important to note that there are two flavors of the Linksys WET11, and if you're going to do this retrofit, you must have the older version... and they are getting a bit scarce (I found mine on eBay). This model shipped with a PCMCIA Bromax card, which can simply be unplugged and replaced... and they also can be adapted simply to support power-over-Ethernet (see the excellent how-to here). The newer WET11 does not have a replaceable Wi-Fi card and I am not aware of any modifications that can bring it to the maximum legal power level.

To help prevent any more warranty-voiding than necessary, note that the newer one without the card is co-branded with Cisco, so it's easy to identify without prying off the rubber feet and breaking the label.

Here are the internals of the two WET11 versions:

The NEW version of the Linksys WET11, not hackable

WET11 old version

The original version of the WET11, with replaceable Wi-Fi card

Assuming you have the original WET11 and a Senao card, the swap is pretty trivial... but there is one little mechanical nuisance that should be dealt with. See the little end cap on the PCMCIA card in the photo above, with the screws going down into stand-offs? Well, the Senao doesn't have that. For a physically static application, that won't much matter, and a cable tie around the whole thing can (crudely) hold it in place, but in some applications (like Shacktopus), a bit more retention is necessary to prevent vibration or shock from shaking the card loose from the connector pins.

In my case, I decided to provide a little step below the flat plastic end of the Senao card; this allows a retention clip for maximum serviceability, or an adhesive such as 3M double-sided tape for bomb-proof reliability. I shortened the original brass stand-offs to .330" tall, and added a thin aluminum shelf with countersunk holes for the metric screws:

New aluminum nest for Senao card on original WET11 standoffs

As you can see in the photo, I kept the pigtail from the MMCX card connector to the RP-SMA antenna connector on the board edge; I'll use the original WET11 rubber duck on the secondary Senao antenna position to provide diversity receive performance. The primary antenna (the one closer to the top in the photo below, marked with a star) is carried to a pivoting 5.5 dBi rubber duck on the case exterior, which can be unscrewed to allow connection of a beautiful 7-element, 12 dB yagi that is small enough (16" long) to fit in the Shacktopus carrying case. Both of those antennas are from RadioLabs.

Here's what it looks like, all assembled in the polycarbonate enclosure:

Senao 2511CD PLUS EXT2 card mounted on original version of Linksys WET11

This installation is still serviceable, as I have not yet added double-sticky tape or other adhesive between the Senao card and the aluminum nest. The gray clip in the photo was hacked from an old Panduit cable-retainer, and snaps nicely into place. As my use of the system stabilizes, I'll add goo if necessary to make it more solid.

Incidentally, I'd suggest a couple of additional minor hacks if you are building one of these into a portable system. First, the toroidal coil with the blob of hot-glue was not actually stabilized to the board in my case, and I saw another photo on the net that looked about the same. This may be an assembly error, or it may be intended solely to prevent microphonics, but in any case that's a very massy object dangling off two spindly pieces of solid wire. I added a couple of dabs of hot glue to keep it from flopping around and failing.

Second, there are only two mounting holes in the WET11 PC board, largely because in the original packaging that is quite sufficient. At the moment I am only using those with a couple of thick fiber washers, and depend on a slight preload against the bottom stand-off screws to provide stability. It would be a bit better to use the stand-off close to the front edge (just visible under the nearest MMCX connector in the photo above) to receive a third mounting screw.

A future article in this series will deal with the more networking-related issues in this installation... so far I have only performed basic tests in the lab, a 3,000 square foot metal building full of Wi-Fi (hardly a challenging environment for all this high-performance gear!)

The Senao 2611-CB3

It has been pointed out since this article was posted (thanks, Dan!) that there is now a product available from Senao that eliminates the need to retrofit an obsolete model of the Linksys WET11. This unit incorporates the 2511 card and provides similar functionality: the 2611-CB3. The board has four mounting holes, is 3.75 x 4.75 x 0.75 inches, and is already configured for POE without an extractor necessary. The 200 mW card is the same as shown in the photos above; here's a look at the innards of a slightly earlier version (almost identical):

Senao 2611-CB3

So it looks like Shacktopus II will use one of these, instead of requiring me to prowl eBay for a discontinued model to retrofit!

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