Part 15 Broadcasters

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:

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.

7 thoughts on “Part 15 Broadcasters”

  1. Nice writeup. Two comments about it…

    The maximum field for unlicensed operators in the FM broadcast band is 250 uV/m measured 3 meters from the transmit antenna [Part 15.239(b)].

    In your link to the FCC citation to a Sioux Falls AM operator, the FCC stated that “The field strength of the signal on frequency 1640 kHz was measured at 14,500 microvolts per meter (uV/m) at 62 meters,…” and went on to state that “the approximate length of the antenna, transmission line and the ground lead was 34 feet (10 meters).”

    It is likely that much of the field strength measured by the FCC resulted by radiation from the long “ground” conductor, rather than using a transmitter operating above 100 mW DC input to the final r-f amplifier.

    The link below leads to a NEC analysis illustrating this.

  2. Richard, thanks for the info, it is very interesting. It seems that LPAM’s operating with the 15.219 exception need to be ground mounted to be compliant with those rules. Any elevation above ground that includes a ground lead would be non-compliant. I also wonder if the audio input and power inputs are RF decoupled, as those can also act as RF radiators.

    I corrected the FM signal strength.

  3. Nice article, Paul. Just as an FYI, Part 15.221 also applies to free radiating AM on the grounds of an educational institution. I maintain one such station on the grounds of a high school and found I was able to hit the maximum field strength with the transmitter set for its minimum output. It provides sufficient coverage for the parking areas as well as the athletic fields allowing play-by-play to be broadcast for those who may wish to hear more than the public address announcer.

    Part 15 FM is pretty much limited to small areas and often times used for sound reinforcement private listening applications in fitness clubs. Utilizing information provided by the now-defunct LPB Inc., I was able to install a radiating cable FM station in a high school which provides good coverage within the building. The LPB spec called for measuring the 15.239 field strength limit at three meters from the building’s exterior walls. At this level the signal can’t be detected outside of campus grounds with either a Potomac FIM-71 or a sensitive FM radio so this solution provides the student population with a real-world broadcast experience without creating interference to other stations or nearby residents.

    It was the high school station project that inspired me to start, (later merging it with, to provide a clearinghouse of resources for the Part 15 low power radio enthusiast.

  4. What about just an ordinary individual such as myself who would like to set up a small neighborhood station,..would I be legal at 100 mW with a Part 15 transmitter on LPAM or even LPFM? Just curious!

  5. @Dewey, as long as you comply with all of the Part 15 rules, you can do just that without applying for a license. If you want to do anything more than 100mW, then a license is required.

  6. While Mr. Fry pointed out the long ground lead as being suspect, I would add that since the Rangemaster can be improperly tuned in excess of 100 mW input power, this would in turn cause the the 2nd and 3rd harmonic to be out of compliance with the certification standards.

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