I have found what seems to be the easiest to use frequency finder yet.
http://www.showmywhitespace.com
Type in the addresses for the places you play at most frequently and note the channels that show green check marks for those locations. Then find a wireless mic with a tuning range that covers those channels.
A few caveats:
* The site isn't perfect. It doesn't consider TV signal attenuation from terrain or buildings. In other words, it may indicate that you cannot use certain frequencies in a particular location when in actuality, you can. Better safe than sorry though.
* The data isn't perfectly current. New transmitters can pop up and create interference on frequencies that the site reports as clear.
If you need to convert the TV channels to actual frequencies in MHz, choose the "View Full Map" button and you will see frequencies on the left side
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