I am utterly amazed at the lengths that some people will go to to get on the air. Where there is a will, there is a way. Part 15 refers to FCC Part 15 rules, which cover unlicensed operation. Such things as wireless microphones, cordless phones, garage door openers, WIFI, other intentional and unintentional RF generators like computers. Subpart C deals with low power, unlicensed broadcasting.
There are several rules regarding unlicensed Part 15 broadcasting, the most often cited rules are 15.209, 15.219 and 15.239, which sets the signal strengths allowed for various frequencies. For the FM band (88-108 MHz), the signal strength requirements are very straight forward; it is
150 250 µV/m measured at 3 meters from the antenna. This results in about 200 foot radius from the radiator or antenna reception distance. Slightly more can be gained by broadcasting in mono.
For the AM band (530-1,700 KHz) there are several different measurement criteria. First, FCC 15.209 states that the signal strength for an unlicensed medium wave station shall not exceed the value of (24,000/F(KHz) which varies from a maximum 45 µV/m at 530 KHz to a maximum 14 µV/m at 1,700 KHz measured 30 meters from the antenna.
Medium Wave broadcasting lends itself well to power line transmission, also known as carrier current. There used to be many carrier current college radio stations in the country as it was a very easy way to broadcast to a limited area without the expense of a license and large transmitter. There are still some carrier current stations out there, but many have gone dark. For carrier current stations, the signal strength requirements can be found in FCC 15.221, which states:
(a) Carrier current systems and transmitters employing a leaky coaxial cable as the radiating antenna may operate in the band 525–1705 KHz provided the field strength levels of the radiated emissions do not exceed 15 µV/m, as measured at a distance of 47,715/ (frequency in kHz) meters (equivalent to Λ/2Π) from the electric power line or the coaxial cable, respectively.
(b) As an alternative to the provisions in paragraph (a) of this section, intentional radiators used for the operation of an AM broadcast station on a college or university campus or on the campus of any other education institution may comply with the following:
(1) On the campus, the field strength of emissions appearing outside of this frequency band shall not exceed the general radiated emission limits shown in § 15.209 as measured from the radiating source. There is no limit on the field strength of emissions appearing within this frequency band, except that the provisions of § 15.5 continue to comply.
(2) At the perimeter of the campus, the field strength of any emissions, including those within the frequency band 525–1705 KHz, shall not exceed the general radiated emission in § 15.209.
Finally, there is Part 15.219, which states:
(a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed 100 milliwatts.
(b) The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters.
Thus, there are several different ways to look at Low Power AM (LPAM) broadcasting. In all cases, LPAM stations are not to be employed on the same frequency of an licensed AM station within its protected contour. Part 15.219 appears at first to be contradictory to 15.209 which sets a specific signal strength value. On reading the FCC’s recent NOUO and NOVs it appears the 15.219 is an exception and is left deliberately ambiguous, somewhat cryptically noting:
Another exception for some transmitters operating in the 510 kHz to 1705 kHz band is found in 47 C.F.R. S: 15.219. Specifically, Section 15.219(b) of the Rules states “the total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters” (see 47 C.F.R. S: 15.219(b)).
From FCC EB-FIELDWR-12-00001143
Thus, from a technical and legal standpoint, which criteria will an FCC inspector use if they are looking at a possible violation with an unlicensed LPAM station? In at least one case, it appears to be up to the inspecting officer.
With a well designed 3 meter (9.84 feet or 118 inches) vertical antenna and good ground system, it is very likely that a 100 mW station, particularly on the upper part of the AM band, could carry up to a mile or so, depending on the local ground conductivity. There are several cases where multiple LPAM transmitters have been chained together, creating a SFN (Same Frequency Network) which covers a significant geographical area. This is a video showing two LPAM transmitters synchronized in Sioux Falls, SD.
Pretty amazing considering all the power lines and such. Then of course, there is this, which shows that they might not be operating at 100 mW after all. I don’t know at which point they began operating above the legal threshold, perhaps that video was taken during legal operation.
Even so, it is a very interesting concept, when one considers using a battery, solar panel, wireless LAN bridge and an AOIP device such as the Barix Extreamer to connect transmitters. There is one particular FCC certified transmitter that allows external synchronizing from a GPS source or by chaining the units together on a RS-485 buss. I have spent several days driving around and listening to static on 1,700 KHz, much to the annoyance of my wife and children.
There are several sources of information regarding LPAM broadcasting:
- Master List of Part 15 Radio Stations, North America
Those are just a few, if you know of others, leave them in the comments. One thing to note: If you are going to broadcast LPAM, make sure that you can demonstrate compliance with either 15.209 or 15.219. Any type of unlicensed station that broadcasts with a regular schedule over significant coverage area will be noticed.
If you are a licensed broadcaster and are concerned that a legally operating Part 15 station is going to cut into your market share, you are simply doing it wrong.