Topic: Oh no...not another WMR200 Antenna Modification !!!

[aweatherguy]

I was poking around with my WMR200 and came up with this antenna modification that seems to work pretty well. It is self-contained and does not make the WMR200 look too much worse aesthetically.

First, refer to this post for instructions to open up the WMR200:

        http://www.wxforum.net/index.php?topic=9096.0

The first photo shows the antenna mounting. Un-solder the antenna wire (purple in this photo, but probably black in your unit) and then remove the four screws and two brackets holding the antenna in place. Once the antenna is removed, use a sharp knife to pry off the two end caps on the plastic antenna rod. Using a small screwdriver or other tool, the wire/coil antenna element can be removed from the plastic housing by pushing and pulling. The second photograph shows the wire and antenna element that was removed from the plastic antenna housing.

The replacement antenna is fabricated from a 7-inch (18cm) piece of 14-gauge copper wire (insulated or bare -- insulated wire is shown in this example). Smaller or larger diameter wires will work too.

Cut a 6-inch (15cm) (or longer) piece of stranded, insulated hookup wire. Using a small screwdriver (for example) thread the hookup wire through the antenna tube so that one end comes out the top and one end comes out the side. It may take a bit of pushing, pulling and coercing to accomplish this. Strip some insulation from the end that comes out the top of the plastic tube and solder it to one end of the 14-gauge wire. Push the 14-gauge wire down the plastic tube until the end is even with the side exit hole. Pull on the stranded wire at the side hole in the tube at the same time to keep it tight.

Replace the end cap at the bottom of the antenna. Optionally, drill a hole in the top cap and slide it over the new antenna wire.

Now the antenna tube can be re-installed in the WMR200. Cut the hookup wire coming out of the side of the antenna tube to length. Strip some insulation from the end of the wire and solder it to the antenna terminal on the PC board as shown in the first and third photos.

Finally, trim the 14-gauge wire so the length from the antenna pivot point to the tip of the wire is 6 inches (15.2 cm).

The last two photos show the completed installation.

Another option is to cut the 14-guage wire to 21-inches at first and then trim the installed length to 19.5 inches.

Both of these designs have been simulated using the NEC antenna simulation program and should result in resonant antennas at 433.92MHz. The antenna impedance is close to 50-ohms including the lower piece of wire around the edge of the case.

For those RF engineers out there -- you can see a matching network at the antenna interface on the PC board and who knows what impedance OS has designed this for? If it is not 50 ohms then this modification could be improved but it certainly works better than the factory antenna in my case.

[DanS]

Looks as if you've basically matched the factory coiled antenna just stretched out straight.? Be interesting to see what length the coiled antenna (inc. coil length) is for a comparison.

[aweatherguy]

Hmmm...well not intentionally. I didn't specifically design the modification to match the length of the coil if it was stretched out straight. Instead, I used the NEC program iteratively to find a length of straight wire that was resonant at 433.92MHz. I suspect (but could easily be wrong) that if the stretched out coil matched the new length it might be just be a coincidence.

As for the nitty-gritty details, the NEC simulation of the original coiled antenna has an impedance of 27-j140 ohms. The NEC simulation of the new (6-inch) design has an impedance of 54+j0 ohms. I'd be glad to post the NEC simulation input on request.

I suppose that another caveat worth mentioning is that I did not simulate all the copper on the WMR200 main PC board (NEC cannot do that). This could also make the simulation results less accurate. On the other hand, this design certainly works better for me and where antennas are involved reality trumps theory every time    (witness the success of metal paper clip modifications)