"n" and "g" networks

Hello
I have an old domed airport extreme that I'm guessing is a "g" network. I just got an airport express ("n") to try to the extend the range of my network. The extreme isn't picking up the signal when I move it a couple of rooms away from the AEBS. I've set the express to "join my wireless network". I'm wondering if it's the difference in the "g" and "n" that maybe is causing the problem?
Thanks in advance for anyone's help.

Yes, that could be the issue. Make sure your "N" setup is "b, g and n compatible" (ie uses the 2.4GHz frequencies).

I would also recommend setting up the AEBS as its own, standalone wireless router before you try extending. Most wireless routers are designed to cover a typical household all by themselves. So, unless your placement is less than ideal, a range "extender" probably isn't necessary.