FCC Rule 15.239 specifically addresses operation in the 88__108 MHz band. This rule by itself does not tell you everything you need to know about using a device of this kind. Therefore we are noting a series of Part 15 regulations which should be observed.
The bandwidth allowed is 200 KHz. This is more than you need for a standard FM stereo transmission.
FCC rule 15.215a says: "Unless otherwise stated, there are no restrictions as to the types of operations permitted under these sections." This general provision appears to leave you free to use your FM stereo transmitter in a manner similar to operations of an FM broadcasting station, or to use it for any other non-interfering, practical application.
FCC Rule 15.5: General conditions of operation: "Operation is subject to the conditions that no harmful interference is caused and that interference must be accepted that may be caused by the operation of an authorized radio station, by another intentional or unintentonal radiator, by industrial, scientific and medical epuipment, or by an incedental radiator. The operator of a radio frequency device shall be required to cease operating the device upon notification by a Commission representative that the device is causing harmful interference."
FCC Rule 15.239: The field strength of the signal must not exceed 250 microvolts/meter at a distance of 3 meters from the transmitter. The field strength is determined by the antenna, as well as transmitter power. In this instance, the antenna is a simple 3 meter element.
In all cases the field strength of a signal decreases in direct proportion to the distance away from the antenna. Power decreases by the square of the distance: for every doubling in distance, the signal power is quartered, but the field strength voltage is only halved. Using this theory, we can construct a simple chart to show the maximum permitted performance of a non licensed FM band transmitter. The theoretical figures assume a simple 1 meter recieving antenna in all cases, and do not take into consideration that reception can be greatly enhanced with larger, multi-element antennas and preanplifiers. In the following chart, the field strength gets even stronger as you move from the edge of these circular boundaries toward the antenna.
Distance from transmitter (feet) field strength (uV) 10 250 20 125 39 63 78 31 157 15 315 7.5 630 3.8 1260 1.9 2520 .95 5036 .5The most significant distance in the above chart is the 1.9 uV signal stregth permissable at 1260 feet. (about 1/4 mile). A look at stereo FM receiver specifications shows typical sensitivity of 1.7 uV.