Based on several other postings, here's another mod that is working pretty well for me. It has the advantage of not requiring connectors, coaxial cables or separately mounted antenna elements. It only requires drilling one small hole in the housing and can be removed with little aesthetic damage to the WMR100.
This modification is pretty painless, too. All you need are the tools to disassemble the WMR100 as posted elsewhere, a 1/16th inch drill bit and about 18 inches or so of 14-gauge copper wire. Here are the steps I followed in making this modification:
1) Disassemble the WMR100.
2) Cut the upper antenna element off about 2 inches above the point where it is soldered into the WMR100's circuit board. Remove the upper antenna segment (with the coiled end) and discard.
3) Drill a 1/16th inch hole in the top of the front half of the plastic housing.
4) Strip enough insulation from the copper wire so it can fit through the hole and overlap with the remaining antenna element for soldering. You could also use bare (non-insulated) copper wire and skip this step.
5) Insert the copper wire through the hole in the top of the plastic housing and solder to the remaining antenna element. Two notes here -- the antenna element seems to be steel and does not accept solder very well so you could replace that short piece with some 18 or 20-gauge copper wire (just be careful to bend it like the original wire so it does not interfere with the plastic case during re-assembly. I tried soldering the 14-gauge wire directly to the antenna terminal on the circuit board, but it is too large and interfered with re-assembly of the housing. Also, you can see where I accidentally touched the plastic case with the soldering iron and melted it a bit so watch out for that.
6) Cut the copper wire to length -- the distance from the connection at the circuit board to the tip of the antenna should be about 18 inches (give or take 1/4 inch).
7) Re-assemble the WMR100. Done.
Optionally, you could skip step (2) and simply solder the copper wire to the existing antenna element. I don't know how well this would work but might be worth trying because it leaves the original antenna intact. If you find the reception less than ideal, you could try trimming as much as 1-1/2 to 2 inches from the top of the copper wire in 1/4 inch increments. Theoretically, the "ideal" length is around 17 inches but your mileage may vary (see hand-waving below).
Here is the idea behind this. The upper section of the antenna is now about 5/8 wavelengths long at 433.92MHz. A common antenna design is a 5/8 wavelength vertical wire mounted on top of a large ground plane. This design has the 5/8 wavelength vertical wire, but substitutes the existing lower dipole section for the ground plane. Okay, so this design is just plain odd in terms of commonly used antenna designs.
<hand-waving>The presence of the large WMR100 circuit board in such close proximity to the antenna makes the analysis/simulation of this antenna (before OR after modification) extremely difficult. As is often the case in the world of antennas, sometimes an antenna just works and it is not worth the trouble to figure out why.</hand-waving> As a result, it is reasonable to conclude that there may be other wire lengths that work even better so don't hesitate to experiment.