Fred Lass, director of engineering at WRGB in Schenectady, N.Y. has posed an interesting question: Can the popular low power FM transmitters being used with iPods or after market XM-Radio receivers legally transmit on 87.9 MHz? He noticed articles recommending the use of 87.5, 87.7 or 87.9 MHz to avoid interference from (and to) FM radio stations. Channel 6 TV stations, with an 87.75 MHz audio subcarrier, also fall in this part of the spectrum. Car radios and nearby home receivers can receive these mini-transmitters.
Monster sells a lower power FM transmitter for use with portable audio devices. The Brisbane, Calif.-based maker of high-end audio and video cables is very careful to point out that the transmitter tuning excludes 87.7 and 87.9 MHz.
Delphi doesn't seem concerned, as its Delphi FM Modulator is capable of transmitting on 87.9, 88.1 or 88.5 MHz.
NewerTechnology Inc. says its RoadTrip broadcaster transmits on only one frequency, 87.9 MHz, plus or minus 0.5 MHz, meaning that it could transmit as low at 87.4 MHz. This could cause interference to the Channel 6 chroma subcarrier at much lower signal levels than required to affect Channel 6 audio.
These devices are considered intentional radiators. The GPO's eCFR covers Part 15, Subpart C, which contains rules for unlicensed devices. According to the GPO, intentional radiators are allowed on Channel 6. However, under Section 15.231 "Periodic operation in the band 40.66-40.70 MHz and above 70 MHz," paragraph (a) states: "Except as shown in paragraph (e) of this section, the intentional radiator is restricted to the transmission of a control signal such as those used with alarm systems, door openers, remote switches, etc. Continuous transmissions, voice, video and the radio control of toys are not permitted."
Paragraph (e) states, "Intentional radiators may operate at a periodic rate exceeding that specified in paragraph (a) of this section and may be employed for any type of operation, including operation prohibited in paragraph (a) of this section, provided the intentional radiator complies with the provisions of paragraphs (b) through (d) of this section. ..."
The allowed field strength in the 70-130 MHz band is 500 microvolts per meter. Below 900 MHz, paragraph (c) limits bandwidth to 0.25 percent of the frequency used. At 87 MHz, this is over 200 kHz, plenty of bandwidth for an FM transmitter. Paragraph (d) doesn't apply to frequencies in this range. However, at the end of paragraph (e) is this restriction: "In addition, devices operated under the provisions of this paragraph shall be provided with a means for automatically limiting operation so that the duration of each transmission shall not be greater than one second and the silent period between transmissions shall be at least 30 times the duration of the transmission but in no case less than 10 seconds."
This would appear to make low-power FM transmitters operating below 88.0 MHz illegal. Section 15.239 "Operation in the band 88-108 MHz" paragraph (a) is very clear. It states, "Emissions from the intentional radiator shall be confined within a band 200 kHz wide centered on the operating frequency. The 200 kHz band shall lie wholly within the frequency range of 88-108 MHz."
Lass also cited FM transmitters sold by Griffen and Transpod, as well as a unit sold by Dension requiring a special two button trick to hit Channel 6 frequencies.
Lass has notified the FCC Office of Technology about these transmitters. Currently, the commission is aggressively enforcing the rules relating to illegal FM transmitters (see next story). Companies selling transmitters capable of operating at TV Channel 6 frequencies may be targeted. Monster is aware of the risk, but others are clearly listing the illegal frequencies on their Web sites. All that is required to find them is to do a Google search of "87.9 MHz" to get a list of manufacturers and retailers selling these devices in the U.S.
