G3 is backwards compatible with G2 (and even 'G1') so long as the frequencies match. There were five frequency range variants in G2; A, B, C, D and E. There are currently six frequency range variants in G3; A, B, C, D, E and G.Oct 25, 2009 ll Sennheiser evolution transmitters are fully compatible - even back to the original G1 series. There are just a few things to watch:- 1) The newer ones have a much wider frequency window than the earlier ones - so you have to have the same frequency on both units. 2) If you use a G1 transmitter with a G2 or G3 receiver, then you have to switch "Pilot OFF" on the receiver as the G1 series did not have Pilot squelch. 3) If you use a mono radiomic. transmitter with an IEM receiver, you have to switch "Pilot OFF" on the receiver as the mono transmitters use a different pilot frequency than the stereo ones. All evolution series use HDX noise reduction, so they will all work with each other - G1, G2, G3 and 100 series, 300 series, 500 series and also including the new 2000 series - they will all work with each other. The extreme being a G1 unit with a 2000 series unit - it's all OK.

Licenses are required to use wireless microphones on vacant TV channels in the United States as they are a part of the Broadcast Auxiliary Service (BAS).
However, this requirement is often overlooked and rarely enforced by the FCC.
Licenses are available only to broadcasters, cable networks, television and film producers. However, the FCC has issued a Report and Order^[11] stating that they now no longer allow Broadcast Auxiliary Service (BAS) devices to operate in the 698–806 MHz portion of the spectrum due to their auction of the 700 MHz band.^[12] This change is unrelated to, but commonly confused with, the White Space device debate that is currently taking place in the U.S.

The same Report and Order, issued January 15, 2010, also permits most wireless microphones and other 'low power auxiliary stations' in the "core TV band" (TV channels 2 through 51, except 37) to operate with transmit power up to 50 mW without a license, under a special waiver of Part 15 rules. A rule change to make this permanent is proposed.

There are currently some wireless microphone manufacturers that are marketing wireless microphones for use in the United States that operate within the 944–952 MHz band reserved for studio-transmitter link communications. These microphones have the potential to interfere with studio-transmitter links, and their use must be coordinated by the Society of Broadcast Engineers. Licenses in this band are only available to licensees of radio and TV stations, and broadcasters are likely to report unauthorized use in this band due to the high potential for interference.

Changes beginning in 2017 concerning operation on 600 MHz frequencies. Beginning in 2017, the amount of TV band spectrum available for wireless microphone use is decreasing as a result of the incentive auction, which was completed on April 13, 2017.
 A significant portion of the TV band spectrum in the 600 MHz band, including most (but not all) of the spectrum on TV channels 38-51 (614-698 MHz), has been repurposed for the new 600 MHz service band for use by wireless services, and will not continue to be available for wireless microphone use. Specifically, wireless microphones that operate in the new 600 MHz service band (the 617-652 MHz and 663-698 MHz frequencies) will be required to cease operation no later than July 13, 2020, and may be required to cease operation sooner if they could cause interference to new wireless licensees that commence operations on their licensed spectrum in the 600 MHz service band. FCC 14-50, FCC 15-140, DA 17-314

Spectrum will continue to be available for wireless microphone use on the other TV channels 2-36 (
TV band frequencies that fall below 608 MHz), on portions of the 600 MHz guard band (the 614-616 MHz frequencies)
and the 600 MHz duplex gap (the 653-663 MHz frequencies), and in various other spectrum bands outside of the TV bands. FCC 15-100, FCC 15-99