On Wed, 03 May 2006 16:43:18 +0100 (BST), Dave Liquorice wrote:

What to look for in a radio mic is true diversity reception, decent
battery life, and power output (but watch for marketing hype). I'm not
sure if any of the "licence free" ones will have more than one frequency
available. Make it a condition of sale that if it turns out that the
church down next door also uses the same frequency that you can return it
for a full refund.

Alternatively, use Bluetooth, get 100m range, and you don't have to worry.