How Do You Find Unused FM Frequencies for iPod, Sirius, and XM FM Adapters?

A couple of weeks ago, I wrote about my satisfaction with the Kensington Digital FM Transmitter and Car Charger for the iPod. I got a question back from Ryan Kelly from Haplography:

Just curious - which Philly frequencies have you found work best?

My response was:

Sorry for the delay in getting back to you on this. I found unused FM frequencies in Philadelphia using the Sirius Satellite Radio Website. The URL you should look at is:

http://fmchannel.sirius.com/

It allows you to enter your zip code and it returns a set of frequencies you can try.

I've used 92.9 most of the time in the area from Princeton, NJ in the north, Exton, PA in the west, Chester, PA in the south, and Jackson, NJ in the east. I've been quite satisfied with the sound quality my car stereo receives from my iPod.

Sirius provides this valuable service because so many Sirius receivers use FM transmitter technology. It's important to note that FM frequencies identified by this website work with XM Radio receivers also.