Check this out.... https://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet63/oet63rev.pdf
Go down to the charts for the rules for unlicensed use and get to part 15.231.
If your transmission is not all the time but temporary or intermittent you have double the field strength allowed.
The title of FCC §15.231 seems appealing, but reading further into its following text brings disappointment (see below):
Wouldn't the frequency range of 88-108 MHz be above 70 MHz?
Restricted to control signals such as alarm systems, door openers, remote switches, etc. Continuous transmissions, such as voice, etc are prohibited. So you can double the range of your garage door opener if it happens to be in those frequencies ranges. Whoo-Boy!
TIB