4. Is it Legal?
Since transmission takes place by radio and the worlds airwaves are pretty crowded, the allocated radio frequencies are legally regulated in order to prevent uncontrollable interference from other traffic. It is the job of the regulatory body to ensure that all types of broadcasting devices (TV, radio, walkie-talkies, mobile phones, WiFi, Bluetooth, GPS etc) can live alongside one another in as trouble-free a manner as possible. The regulator has therefore allocated specific frequency and power ranges to different applications, which are enforced by law (see http://www.ofcom.org.uk for more information).
VHF and UHF ranges both have groups of frequencies that require a license to use, and (fortunately for musicians!) some that are free to use without a license. There are global regional variations in the licensed and unlicensed bands, so the job of regulation is delegated to different organisations in different parts of the world. If you are in doubt as to whether your transmissions are legal or not (especially if playing abroad), then consult the vendor of your apparatus, your production manager or the venue management. Sennheisers Frequency Finder is also an excellent resource ( http://www.sennheiser.co.uk/uk/icm.nsf/root/products_sennheiser_wireless-systems_frequency-check ). In a worst case scenario, temporary licenses are usually available at modest Prices.