Hobby Beacons with Low Power Radio

Lowfers, hifers, beacons and baby monitors in low power radio experimentation.

In this article, hobby broadcasting in the USA means to experiment with radio while complying with US law. Experimenters who make a good faith effort to follow the guidelines below (and who make a commitment to keep up with major changes) will be able to enjoy hobby broadcasting across the bands.

The body of law to follow, called Part 15, also contains useful technical guidelines for operators.

01. Part 15 applies to:
a. Intentional radiators - low-power transmitters intended to transmit a modulated radio signal through the air - AND
b. Unintentional radiators - devices designed to generate radio frequency energy but not transmit it through the air, including radio receivers and "digital devices" such as computers and peripherals - AND
c. Incidental radiators - devices not designed to generate radio signals but that do so as a by-product of their operation, such as motors and light dimmers and switches.

02. Interference and Use of Part 15 Devices
a. Part 15 devices must not cause harmful interference - that disrupts licensed radio services - even if the devices conform to the Part 15 technical standards.
b. Users of Part 15 devices must accept any interference caused by licensed radio services.
c. Any user of any Part 15 device must stop operating the device if notified by the FCC that the device is causing harmful interference, and may not use the device until the interference problem is corrected.
d. Users of Part 15 devices are not protected against each other's interference.
e. Users of Part 15 devices do not have and can not obtain any right to use any device, frequency, or emission type.

03. Except for authorized law enforcement operations, Part 15 devices may not be used to listen to or record private conversations without the consent of all parties concerned.

04. Part 15 devices must be constructed per good design and manufacturing practices to not emit any signal that is not in accordance with FCC Regulations.
a. Part 15 devices must suppress extraneous signals as much as practicable. In general:
(1) No extraneous signal can be stronger than the desired or intended signal.
(2) Extraneous signals must usually be at least 20 dB below the level of the desired signal.
(3) All signals less than 20 dB below the level of the desired signal must be contained within an authorized band, or a bandwidth within that band.
b. No user-accessible controls can allow the device to operate in violation of the FCC Regulations.
c. No radiator should emit any stronger signal than what is needed for reliable operation.
d. A commercially-made Part 15 transmitter that comes with an antenna permanently fixed to the transmitter enclosure may not be used with another antenna except a substantially identical replacement.
e. NOTE: If antenna length is limited, Part 15 does not mention how it is measured (except 49.82-49.90 MHz, only single-element antenna under alternate standards). This is a special concern for the 160-190 kHz and 510-1705 kHz bands, where a "capacity hat" or "top hat" is often used at the top of the antenna. However, in several inquiries, the FCC has not objected to measuring antenna length from the base of the antenna to the end of one of the "hat" radials (the longest if all are not equal). The rim of the "hat" is not considered, nor are loading coils.

05. This notice must be affixed to the enclosure of a Part 15 device that is home-built or is not allowed to carry a different notice or identifier: This device complies with Part 15 of the FCC Rules. Operation is subject to the following two conditions:
(1) This device may not cause harmful interference, and
(2) this device must accept any interference received, including interference that may cause undesired operation.
If the device is in two or more parts, the notice must be affixed to the enclosure of the main control unit.
If any of the enclosures is too small to affix the notice to it, the notice must be placed on the packaging or in the documentation for the device.

06. Home-built devices that are not made from a kit (any packaged collection of electronic parts, whether complete or not) and that are built in quantities of five or less for personal use, do not have to be authorized by the FCC (see Section 15.23).
Home builders who do not have test and measurement equipment to determine precise compliance with technical standards must use good engineering practice to meet those standards to the greatest extent practicable.

07. Makers and users of Part 15 devices must make those devices and any required documentation available for FCC inspection "upon reasonable request." Any information or documentation requested by the FCC must be provided promptly to the greatest practical extent. The FCC may require that any Part 15 device be shipped (at the maker's or user's expense, both ways) to the FCC Laboratory in Columbia, Maryland, to be tested for compliance with applicable technical standards in force on the date of manufacture.

08. Power measurements:
a. The maximum radiated field strength, or the input power to the device or the final RF amplifier if appropriate, must be measured:
(1) With a new battery for a battery-powered device, OR
(2) With the supplied voltage varied between 85% and 115% of normal for an AC-powered device.
b. Field strength, or input power if appropriate, must be measured with:
(1) Any user-accessible controls adjusted for maximum emitted signal, AND
(2) Any specified or appropriate leads or cables attached, each one meter (just under 39-3/8 inches) long, or as long as normally needed, generally whichever is greater.
c. Multiple-component devices or systems must be tested with all components (including available accessories):
(1) Connected and operating ("operating" including "emitting" for multiple emitters), AND
(2) Arranged for maximum total emissions within the expected range of configurations.
d. Field strength is not usually measured by home builders.However, if measured:
(1) It must be measured at the prescribed distance(s) unless impractical, in which case the circumstances must be recorded and a correction factor must be determined and applied.
(2) The prescribed distance(s) apply from the point of measurement to the device, support equipment, or interconnecting cable(s), whichever is closest.

09. Part 15 home-built transmitters are usually built to operate on a particular frequency, and otherwise virtually always to operate over a frequency range of 1 MHz or less. If a home builder can obtain the use of test and measurement equipment normally considered to be commercial-grade or better:
a. Signal measurements must be made when the transmitter is operating on the intended frequency or the center frequency of the range.
b. Checks for extraneous signals must be made up to a frequency that is ten times the intended frequency or the upper frequency of the range (but not higher than 40 GHz).
c. Unless specified otherwise, field strength must be measured using:
(1) An average detection method for frequencies above 1 GHz (1000 MHz).
(2) An adjusted-peak or "CISPR quasi-peak" detection method for:
(a) Frequencies at or below 1 GHz (except 9-90 kHz, 110-490 kHz, 26.96-27.28 MHz, 49.82-49.90 MHz, and 88-108 MHz, where average detection is used), AND
(b) Signals of any frequency that are conducted onto the AC line, for any device that can be or must be connected to AC power (see Section 15.207).

10. Part 15 permitted field strengths for each frequency band are shown below. Unless stated otherwise, any intentional radiator emissions shall not exceed the field strength indicatd in the table below:

Above 960
Field strength
distance (meters)

a. Table above from Paragraph (a) of Section 15.209
b. Frequency is that of measured emission, not operating frequency of device
c. Extraneous emission strength may not exceed that of desired emission regardless of table above
d. Tighter limit applies at band edges, i.e. limit at 88 MHz is 100 uV/m, not 150 uV/m
e. "Microvolts/meter" refers to the reading on a tuned and calibrated field strength meter with an antenna that is one meter long

11. Low-power transmitters may be used in the following bands, approximately ranked from first to last according to ease of construction, procurement, and/or use, combined with permitted signal strength (permitted frequencies above 108 MHz are not discussed below):

a. 160-190 kHz
(1) 1 watt input power to final RF amplifier
(2) Total length of antenna, transmission line, and ground lead (optional) not over 15 meters (50 feet)
(3) No emissions below 160 kHz or above 190 kHz that are less than 20 dB below the level of the unmodulated carrier wave at the operating frequency

b. 510-1705 kHz
(1) 100 milliwatts (0.1 watt) input power to final RF amplifier
(2) Total length of antenna, transmission line, and ground lead (optional) not over 3 meters (10 feet)
(3) No emissions below 510 kHz or above 1705 kHz that are less than 20 dB below the level of the unmodulated carrier wave at the operating frequency

c. 13.553-13.567 MHz
(1) Field strength limits:
(a) Desired signal not over 15,848 microvolts/meter at 30 meters (approximately equivalent to 4.9 milliwatts into either a half-wavelength dipole antenna in free space or a quarter-wavelength vertical antenna over an ideal ground plane)
(b) Extraneous signals:
1) Not over desired signal within 13.553-13.567 MHz
2) Not over 334 microvolts/meter (about 16.8 dB below desired signal) in 13.410-13.553 MHz and 13.567-13.710 MHz
3) Not over 106 microvolts/meter (about 21.7 dB below desired signal) in 13.110-13.410 MHz and 13.710-14.010 MHz
4) Not over limits stated in Item 10 below 13.110 MHz and above 14.010 MHz
(2) RF carrier frequency maintained within +/- 0.01% (+/- 1.3553 kHz at lower band edge):
(a) Over -20 to +50 degrees C at normal power supply voltage, AND
(b) Over 85% to 115% of normal voltage at 20 degrees C

d. 49.82-49.90 MHz
(1) Field strength limits for non-homebuilt devices (or home-built devices constructed per Section 15.23 [Item 06 above] but not using alternate standards):
(a) Desired signal not over 10,000 microvolts/meter at 3 meters
(b) Extraneous emissions in-band and up to 10 kHz above and below band edges at least 26 dB below unmodulated carrier level or not over 100 microvolts/meter at 3 meters (whichever is stronger)
(c) Any emission more than 10 kHz above or below band limits not over limits stated in Item 10
(2) Alternate standards for home-built devices made according to Section 15.23 (Item 06 above):
(a) No more than 100 milliwatts total power input to transmitter (not just final RF amplifier)
(b) RF carrier and all modulation products contained within band limits
(c) Single element antenna not over 1 meter long permanently attached to transmitter enclosure
(d) Out-of-band emissions at least 20 dB below unmodulated carrier level

e. 26.96-27.28 MHz (only limits are on field strength)
(1) Any in-band emission not over 10,000 microvolts/meter at 3 meters
(2) Any out-of-band emission not over 30 microvolts/meter at 30 meters (approximately 14.7 dB below in-band)

f. 88-108 MHz
(1) All intended emissions confined within band limits and within bandwidth of 200 kHz centered on operating frequency
(2) Field strength of any emission:
(a) Within 200-kHz bandwidth not over 250 microvolts/meter at 3 meters
(b) Out of bandwidth not over limits stated in Item 10
(c) Monitored and enforced aggressively, significant monetary penalties for violations

g. 40.66-40.70 MHz
(1) Field strength limits:
(a) In-band emissions not over 1000 microvolts/meter at 3 meters
(b) Out-of-band emissions not over Item 10 limits
(2) RF carrier frequency maintained within +/- 0.01% (+/- 4.066 kHz at lower band edge):
(a) Over -20 to +50 degrees C at normal power supply voltage, AND
(b) Over 85% to 115% of normal voltage at 20 degrees Centigrade.
NOTE: This is an unofficial document. No warranty of accuracy is made. This is a digest and thus not complete, and only reflects two out of six subparts.

A more exhaustive review is available at the HF Underground Part 15 Wiki