FAQ: An FCC field agent told me that the operation of a micro-powered AM or FM transmitter under Part 15 requires an experimental license. Is that true?
It is not true.
Part 15 covers many low powered devices that are authorized by rule and not by license (Part 95 covers some license-free devices such as CB, FRS and MURS).
The operation of micro-power AM stations are covered in two different rules:
§15.219 is the rule that applies to most of those who use AM for microbroadcast as it calls for the maximum input power at the final radio frequency stage to be at or less than 100 milliwatts (0.1 watts) and the length of the antenna system (transmission line, radiator and ground) to not exceed 3 meters (10 feet).
§15.221 specifically refers to the use of Part 15 devices for broadcasting in the case of carrier current (over power lines) in paragraph (a) and in paragraph (b) into an "intentional radiator" (e.g. an antenna) on the campus of an educational institution. The rule specifies that at the perimeter of the campus, the field strength of the signal can not exceed the general radiated emission in §15.209 of the rules. (field strength of 24,000 divided by the frequency in kHz. 1700 kHz = 14 microvolts per meter at 30 meters where 540 kHz = 44.4 microvolts per meter at 30 meters).
It should be known that both rules put the upper cut-off at 1705 kHz. This is to accommodate a 1700 kHz center frequency. With that said operation on 1710 kHz does not fall under the rules above that apply to the AM band. 1710 kHz with an AM broadcast carrier would fall under the general radiated emission limits of 100 microvolts per meter at 30 meters. Unless you have a field strength meter and can take measurements, it's best to stay off of 1710 kHz.
The operation of micro-power FM stations are covered in the rule §15.239 which is where you have the language that the field strength is limited to 250 microvolts at 3 meters. Despite common belief, there is no "power limit" (such as 100 milliwatts, etc.) for FM. Also, it is important to point out that the rule refers to 88-108 MHz which covers the center carrier frequencies for the 100 FM channels 88.1 to 107.9 only but not 87.9. 87.9, being in the spectrum of television channel 6 is subject to §15.209(a) which specifically forbids "fundemental emissions" in the TV broadcast spectrum. The only exception is for devices that only key up for a few seconds at a time. While §15.209 prohibits the use of 87.9 for microradio devices, I have seen field agents apply the 100 microvolt per meter at 3 meters general requirement when doing enforcement.
Yeah yeah.. but do I need a license?
The simple answer is no. The field agent must have been confusing your part 15 device with the part 15 certification standards that all devices must have before they are mass produced for sale in the United States. The rules allow you to build your own Part 15 compliant device for your own use without certification. But if you start building Part 15 transmitters for the purpose of selling them, you must go through the FCC's Part 15 certification process. An experimental license is normally provided to manufacturers and industrial users who wish to use devices in a manner that is not consistent. The major makers of Part 15 devices such as the Hamilton Rangemaster and FM devices such as the C. Crane units in use here at REC all have been through the FCC certification process and have registration numbers. We do warn though that some of the Chinese imported pirate FM transmitters may have certification numbers, a search of the FCC's certification database shows that these registration numbers are for different devices. In other words, if they claim to have an FCC certification number, it does not always mean that it is legal.
With the exception of the campus broadcasting language, Part 15 does not otherwise distinguish using a legal Part 15 transmitter in the broadcast band for broadcasting as opposed to using a "toy" such as the old Mr. Microphone in the band. It should be noted that in the Part 73 broadcast rules, §73.3550(l) states that "users of nonlicensed, low-power devices operating under part 15 of this chapter may use whatever identification is currently desired, so long as propriety is observed and no confusion results with a station for which the FCC issues a license."
Bottom line, if you have certified gear from a reputable manufactuer and it has not been modified inside and you are using the appropraite antenna (less than 3 meters for AM, the included attached antenna for FM), then the unit is legal and if you get a visit, you can show compliance with the rules.
For more information on this subject and to see some the rules for unlicensed devices in other countries, visit http://recnet.com/unlicensed