Part 15 is one of the sections of FCC regulations (47 C.F.R.) dealing with unlicensed low-power radio operation. The FCC defines these low-power transmitters as intentional radiators. See also: Part 15 beacons. Operation in several bands is allowed, including low frequency 160-190 kHz, medium frequency/medium wave (AM broadcast band), HF frequencies and various VHF/UHF bands. The list below is not exhaustive list and does not include of all bands/frequencies available under Part 15.
Part 15 should not be confused with ISM (Part 18).
The sections (usually) of most interest to radio hobbyists include Part 15 Subpart C - Intentional Radiators and Part 15 Subpart D - Unlicensed Personal Communications Service Devices. Part 15 Subpart C Intentional Radiators includes sections 15.201 – through 15.258 (47 CFR 15.201 through 47 CFR 15.258).
15.217 Operation in the band 160-190 kHz.
(a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed one watt.
(b) The total length of the transmission line, antenna, and ground lead (if used) shall not exceed 15 meters.
(c) All emissions below 160 kHz or above 190 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification may be based on measure- ments at the intentional radiator's antenna output terminal unless the in- tentional radiator uses a permanently attached antenna, in which case com- pliance shall be demonstrated by measuring the radiated emissions.
15.218 Operation in the band 510-1705 kHz.
(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.
(c) All emissions below 510 kHz or above 1705 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification may be based on measure- ments at the intentional radiator's antenna output terminal unless the in- tentional radiator uses a permanently attached antenna, in which case com- pliance shall be demonstrated by measuring the radiated emissions.
15.221 Operation in the band 525-1705 kHz.
(a) Carrier current systems and transmitters employing a leaky co- axial cable as the radiating antenna . . . .
15.225 Operation within the band 13.110-14.010 MHz (includes the 13.553 MHz - 13.567 MHz 22 meter portion)
The 22 meter band or the 13.56 MHz 13560 kHz 13553 kHz - 13567 kHz band is very popular with hobbyists due the relatively higher power allowed compared to other Part 15 bands. See here for a list of 22-meter band beacons.
15.225(a) is of the most interest to hobbyists - as it relates to the 22 meter band or the 13.56 MHz band - 13.553 MHz to 13.567 MHz or 13553 kHz to 13567 kHz. The field strength limit of 15,848 microvolts/meter at 30 meters or 84 dBµV/m at a distance of 30 meters is not easy to convert to dBm or better yet, mW or milliwatts, and will be different depending on the antenna used.
Roughly, these equate to 4.6 mW into a 1/2 wave dipole or 2.3 mW into a 1/4 ground plane. Since those are the most commonly used antenna types, they are listed. For folks interested in building a beacon transmitter for this band, there are kits available, you will have to build your own antenna.
(a) The field strength of any emissions within the band 13.553-13.567 MHz shall not exceed 15,848 microvolts/meter at 30 meters.
(c) Within the bands 13.110-13.410 MHz and 13.710-14.010 MHz the field strength of any emissions shall not exceed 106 microvolts/meter at 30 meters.
(d) The field strength of any emissions appearing outside of the 13.110-14.010 MHz band shall not exceed the general radiated emission limits in § 15.209.
(e) The frequency tolerance of the carrier signal shall be maintained within ±0.01% of the operating frequency over a temperature variation of −20 degrees to + 50 degrees C at normal supply voltage, and for a variation in the primary supply voltage from 85% to 115% of the rated supply voltage at a temperature of 20 degrees C. For battery operated equipment, the equipment tests shall be performed using a new battery.
(f) In the case of radio frequency powered tags designed to operate with a device authorized under this section, the tag may be approved with the device or be considered as a separate device subject to its own authorization. Powered tags approved with a device under a single application shall be labeled with the same identification number as the device.
15.227 Operation within the band 26.96-27.28 MHz.
Previously 26.98-27.28 MHz. - note the overlap with the 26.957-27.283 MHz ISM band and, of course, the 26.995-27.255 MHz RC allocation and the 26.96-27.41 MHz CB band.
13.56 MHz (13560 kHz) x2 = 27.12 MHz (27120 kHz) - and 13.560 MHz x3 = 40.68 MHz
11 meter band 27 MHz operations under Part 15. 26.960 MHz - 27.280 MHz (covers the area between CB channels 1 and 28 - 26.965 MHz to 27.285 MHz)
Part 15 power limit for the 26.96 MHz to 27.28 MHz band is 10,000 microvolts per meter at 3 meters (10,000 microvolts/meter at 3m) or 10mV/m at 3 meters. 10,000 uV/m at 3m may be converted below:
10,000 uV/m at 3 meters = 80 dBuV/m at 3m = 0.03 mW EIRP or -15.23 dBm EIRP = 0.01829 mW ERP or -17.38 dBm ERP
In other words, 0.0182 milliwatts ERP...for a 1/2 wave dipole antenna, roughly. Not a lot of power, but doable. The 22m band offers higher legal Part 15 power levels.
(a) The field strength of any emission within this band shall not exceed 10,000 microvolts/meter at 3 meters. The emission limit in this paragraph is based on measurement instrumentation employing an average detector. The provisions in 15.35 for limiting peak emissions apply.
(b) The field strength of any emissions which appear outside of this band shall not exceed the general radiated emission limits in 15.209.
Many toy R/C transmitters operate under this section of Part 15 and not Part 95. There are others that operate under Part 95. Cheaper equipment operates under the Part 15 limits while other equipment will transmit much higher power (4 watts or 25 watts) if accepted for Part 95 Radio Control Radio Service (RCRS) operation. Toy RC transmitters, some wireless computer mice and other very short range devices generally use 27.145 MHz but may technically use any frequency between 26.980 MHz and 27.280 MHz. This band is shared with the CB radio service (26.960 MHz to 27.410 MHz) and the ISM band (26.957 MHz to 27.283 MHz). While often confused, the ISM band is not the same thing as Part 15. ISM services are covered under Part 18 of the FCC rules (47 C.F.R. 18).
- 26.995 MHz
- 27.045 MHz
- 27.095 MHz
- 27.145 MHz
- 27.195 MHz
- 27.255 MHz
FCC authorization for Part 15 devices does not require that they operate on one of the six channels listed above. Only the higher power RCRS rules require use of the standard channels. FCC ID authorization searches indicate toys, RC cars and other devices approved for frequencies such as 27.05 MHz, 27.052 MHz, 27.0625 MHz, 27.09 MHz, 27.1 MHz, 27.12 MHz, 27.13 MHz 27.14 MHz, 27.1413 MHz, 27.142 MHz, 27.143 MHz, 27.1442 MHz, 27.1448 MHz, 27.146 MHz, 27.14625 MHz 27.147 MHz, 27.148 MHz and 27.15 MHz. The vast majority of 27 MHz Part 15 transmitters operate on 27.145 MHz, however.
Cheaper toy-grade RC devices are often sold in two versions, a 27 MHz version and a 49 MHz version. 27 MHz almost always means 27.145 MHz, and 49 MHz almost always means 49.86 MHz. There are, of course, numerous exceptions to this. Higher grade RC devices operate under the Part 95 provision vs. Part 15.
Older versions of Part 15 allowed for voice and data transmission under 100mw within the 26.99 MHz to 27.26 MHz band (26.990 MHz - 27.260 MHz, previously referred to as the 26.970 MHz - 27.270 MHz band). Prior to updating the rules in the late 1970s, this permitted low-power CB transmissions without a license on the CB frequencies as well as low power 27 MHz walkie talkies (unlicensed) using AM or FM voice on the 27 MHz R/C frequencies. Manufacturing of devices operating under these rules stopped on March 18th, 1977 and marketing of such devices stopped on March 18th, 1978. Use of 100mw devices within the 26.99 MHz - 27.26 MHz band or the 26.97 MHz - 27.27 MHz band became under Part 15 became illegal on March 18th, 1983.
Many older-generation 100mw walkie-talkies operate under this now defunct provision and it is possible to find these devices on the secondhand market, even though they remain technically illegal to use, enforcement is a non-issue.
However, the updated versions continued to allow AM, FM or CW (A3, F3 or A1) emissions on the 26.995 MHz, 27.045 MHz, 27.095 MHz, 27.145 MHz, 27.195 MHz or 27.255 MHz frequencies, with 20 kHz bandwidth limits and the usual 10,000 microvolts/meter at 3 meters requirement.
15.229 Operation within the band 40.66-40.70 MHz.
13.56 MHz (13560 kHz) x3 = 40.68 MHz (40680 kHz)
This band is not used for R/C purposes in the United States. It is used for alarm systems, very short range sensor systems and other purposes. Short-range or toy R/C devices use 26 MHz / 27 MHz, 49 MHz, 72 MHz / 75 MHz or the 2.4 GHz unlicensed band in the United States.
§ 15.229 Operation within the band 40.66-40.70 MHz.
(a) Unless operating pursuant to the provisions in § 15.231, the field strength of any emissions within this band shall not exceed 1,000 microvolts/meter at 3 meters. The 40.660 MHz to 40.700 MHz band is heavily used for short range devices (SRDs) and RC model control in Europe and elsewhere.
(b) As an alternative to the limit in paragraph (a) of this section, perimeter protection systems may demonstrate compliance with the following: the field strength of any emissions within this band shall not exceed 500 microvolts/meter at 3 meters, as determined using measurement instrumentations employing an average detector. The provisions in § 15.35 for limiting peak emissions apply where compliance of these devices is demonstrated under this alternative emission limit.
(c) The field strength of any emissions appearing outside of this band shall not exceed the general radiated emission limits in § 15.209.
(d) The frequency tolerance of the carrier signal shall be maintained within ±0.01% of the operating frequency over a temperature variation of −20 degrees to + 50 degrees C at normal supply voltage, and for a variation in the primary supply voltage from 85% to 115% of the rated supply voltage at a temperature of 20 degrees C. For battery operated equipment, the equipment tests shall be performed using a new battery.
15.233 Operation within the bands 43.71-44.49 MHz, 46.60-46.98 MHz, 48.75-49.51 MHz and 49.66-50.0 MHz.
43.71 MHz to 44.49 MHz (43.72 MHz to 44.48 MHz), 46.60 MHz to 46.98 MHz (46.61 MHz to 46.97 MHz), 48.75 MHz to 49.51 MHz (48.76 MHz to 49.50 MHz and 49.66 MHz to 50.0 MHz (49.67 MHz to 49.9 MHz). Note that the frequencies overlap with the VHF low band business radio band, the military bands, the 49 MHz RC 49 MHz remote control device frequencies and 49 MHz short range devices (walkie-talkies, baby monitors, etc.) The original 10 channel cordless phones used the 46.6 MHz - 47.0 MHz and 49.6 MHz to 50.0 MHz ranges only (46.610 MHz - 46.970 MHz and 46.670 MHz - 49.990 MHz).
The handset/headset transmit frequencies for channels 17, 18, 20, 21 and 22 overlap with the 49.82-49.90 MHz frequencies, including frequencies used by baby monitors. When these cordless phones were popular, issues with interference due to baby monitors was common (and hearing your neighbor's phone call over your baby monitor was also common).
While still legal to own and operate, cordless phones that use this band have all but disappeared.
15.233 Operation within the bands 43.71-44.49 MHz, 46.60-46.98 MHz, 48.75-49.51 MHz and 49.66-50.0 MHz. (a) The provisions shown in this section are restricted to cordless telephones.
(b) An intentional radiator used as part of a cordless telephone system shall operate centered on one or more of the following frequency pairs, subject to the following conditions:
(1) Frequencies shall be paired as shown below, except that channel pairing for channels one through fifteen may be accomplished by pairing any of the fifteen base transmitter frequencies with any of the fifteen handset transmitter frequencies.
(2) Cordless telephones operating on channels one through fifteen must:
(i) Incorporate an automatic channel selection mechanism that will prevent establishment of a link on any occupied frequency; and
(ii) The box or an instruction manual which is included within the box which the individual cordless telephone is to be marketed shall contain information indicating that some cordless telephones operate at frequencies that may cause interference to nearby TVs and VCRs; to minimize or prevent such interference, the base of the cordless telephone should not be placed near or on top of a TV or VCR; and, if interference is experienced, moving the cordless telephone farther away from the TV or VCR will often reduce or eliminate the interference. A statement describing the means and procedures used to achieve automatic channel selection shall be provided in any application for equipment authorization of a cordless telephone operating on channels one through fifteen.
- Base Frequency 43.720 MHz - Handset Frequency 48.760 MHz - Channel 1
- Base Frequency 43.740 MHz - Handset Frequency 48.840 MHz - Channel 2
- Base Frequency 43.820 MHz - Handset Frequency 48.860 MHz - Channel 3
- Base Frequency 43.840 MHz - Handset Frequency 48.920 MHz - Channel 4
- Base Frequency 43.920 MHz - Handset Frequency 49.020 MHz - Channel 5
- Base Frequency 43.960 MHz - Handset Frequency 49.080 MHz - Channel 6
- Base Frequency 44.120 MHz - Handset Frequency 49.100 MHz - Channel 7
- Base Frequency 44.160 MHz - Handset Frequency 49.160 MHz - Channel 8
- Base Frequency 44.180 MHz - Handset Frequency 49.200 MHz - Channel 9
- Base Frequency 44.200 MHz - Handset Frequency 49.240 MHz - Channel 10
- Base Frequency 44.320 MHz - Handset Frequency 49.280 MHz - Channel 11
- Base Frequency 44.460 MHz - Handset Frequency 49.360 MHz - Channel 12
- Base Frequency 44.400 MHz - Handset Frequency 49.400 MHz - Channel 13
- Base Frequency 44.460 MHz - Handset Frequency 49.460 MHz - Channel 14
- Base Frequency 44.480 MHz - Handset Frequency 49.500 MHz - Channel 15
- Base Frequency 46.610 MHz - Handset Frequency 49.670 MHz - Channel 16
- Base Frequency 46.630 MHz - Handset Frequency 49.845 MHz - Channel 17
- Base Frequency 46.670 MHz - Handset Frequency 49.860 MHz - Channel 18
- Base Frequency 46.710 MHz - Handset Frequency 49.770 MHz - Channel 19
- Base Frequency 46.730 MHz - Handset Frequency 49.875 MHz - Channel 20
- Base Frequency 46.770 MHz - Handset Frequency 49.830 MHz - Channel 21
- Base Frequency 46.830 MHz - Handset Frequency 49.890 MHz - Channel 22
- Base Frequency 46.870 MHz - Handset Frequency 49.930 MHz - Channel 23
- Base Frequency 46.930 MHz - Handset Frequency 49.990 MHz - Channel 24
- Base Frequency 46.970 MHz - Handset Frequency 49.970 MHz - Channel 25
15.235 Operation within the band 49.82-49.90 MHz.
49 MHz devices, frequencies used and discussion. This deals exclusively with the 49.820 MHz to 49.900 MHz band covered in 15.235, not the cordless telephones which overlap this band - see 15.233.
RC transmitters generally use 49.830 MHz or 49.860 MHz as 49 MHz "RC channel 1". Children's RC toys are often offered in two versions, a "49 MHz" version and a "27 MHz" version, without any channel numbers indicated. The 27 MHz version generally will operate on 27.145 MHz. These devices are usually operating under Part 15, not Part 95 (with some rare exceptions for the 26 / 27 MHz gear.
Part 15 power limit for the 49.82-49.9 MHz band is 10,000 microvolts per meter at 3 meters (10,000 microvolts/meter at 3m) or 10mV/m at 3 meters. 10,000 uV/m at 3m may be converted below:
10,000 uV/m at 3 meters = 80 dBuV/m at 3m = 0.03 mW EIRP or -15.23 dBm EIRP = 0.01829 mW ERP or -17.38 dBm ERP
In other words, 0.0182 milliwatts ERP.
49.6 MHz - 50 MHz (49.600 MHz - 49.975 MHz) band (47 CFR 15.235 subband 49.820 MHz - 49.900 MHz) shared with military users and cordless phones (see section above).
The Maxon PC-10 and PC-50 Personal Communicator, Maxon 49-SX, 49-H, 49-FX, 49-B5, 49-HX, 49-EA, Standard Talkman C900, and similar 49MHz radios as well as the RadioShack TRC-503, TRC-506 and TRC-512, Radio Shack (Realistic) Audionic 21-408 VOX radios and similar higher-end radios operates on the same five frequencies listed below. Some radios are single-channel only (generally 49.860 MHz, see RadioShack TRC-509, TRC-502, etc.) while others allow the user to select one of the five channels.
Even current-generation new production baby monitors are available that use the 49 MHz frequencies. Many models offer 2 channels to avoid interference with other nearby monitors. These devices remain in use despite the availability of 902-928 MHz 900 MHz/2.4 GHz, WiFi based, DECT 1.9 GHz or 5.8 MHz band audio monitors or audio/video combination devices.
Analog baby monitors offer low RF radiation exposure due to their use of the VHF 49 MHz frequencies and excellent audio quality from true FM modulation within their service range. 49MHz monitor transmitters generally provide 300-600 feet range (high quality signal) depending on obstructions, antenna location/height and frequency congestion.
The Radio Shack and Maxon 49 MHz walkie talkies generally have a communication range of 600 feet to 900 feet (200-300 meters or 1/8 mile to 1/6 mile) in suburban or open areas. Simple testing with transmitters located indoors at ground level in a heavily built-up urbanized area produced reliable talk range of 300-400 feet [90-120m]. Considerably better radio talk range of and up to 1/4 mile [400m] or even further is possible with outdoor antennas, no obstructions and favorable noise levels.
Experiments with outdoor antennas and upgraded 49 MHz walkie talkie antennas show a range increase of 1.5x to 2x compared to regular rubber duck antennas. Maximum range of consistent coverage increased to approx. 1800-2000 feet from transmitter site. Signal heard (poor copy) past 1/2 mile from transmitter site. Reliable communication range of 3/8 of a mile or so (several city blocks) was achieved. Without the external antenna, range can be expected to be closer to 1/4 of a mile or so handheld radio to handheld radio. This is ideal for the purposes listed below. Militia intra-squad radio (not inter squad radio), communications with somebody 2-3 houses down, etc. when you don't want somebody 1-2 miles away to be able to listen in.
This equipment provides preppers and militia users with a unique short-range low probability of intercept (LPI) radio communications capability that higher-power VHF/UHF services like FRS, GMRS, MURS doesn't provide. The Radio Shack / Realistic TRC-503, TRC-512 and similar 49 MHz walkie talkies specs appear to indicate 16K0F3E emission for voice / 16K0F3D for tone emission. The Maxon PC-50 five channel transceiver apparently uses 4.5 kHz FM deviation, for 18 kHz bandwidth - 18K0F3E emission.
- 49.830 MHz - Channel 1/Channel A
- 49.845 MHz - Channel 2/Channel B
- 49.860 MHz - Channel 3/Channel C
- 49.875 MHz - Channel 4/Channel D
- 49.890 MHz - Channel 5/Channel E
Other Part 15C equipment using 49.82 MHz - 49.9 MHz or a portion of (for example, 49.86 MHz - 49.89 MHz) use different channel plans - 20 kHz channels instead of 15 kHz steps. FM voice room monitors for continuous transmit around 0.0012 mw. Emissions include 20K0F3E regular FM (5 kHz deviation). Other devices use narrower bandwidth, with 3 kHz deviation - 12K0F3E emission, 3.25 kHz deviation - 13K0F3E emission, 3.5 kHz deviation - 14K0F3E emission, 3.75 kHz deviation - 15K0F3E emission, 4 kHz deviation - 16K0F3E emission reported. Older baby monitors and other devices report wider FM - up to 30 kHz bandwidth 30K0F3E for some applications. At least one baby monitor on the market uses AM mode with 15 kHz bandwidth on two non-standard frequencies - see below.
- 49.830 MHz - Channel 1
- 49.850 MHz - Channel 2
- 49.870 MHz - Channel 3
- 49.890 MHz - Channel 4
FCC certification records for room monitors (continuous transmit) sold in 2019 indicate use of FM voice mode for modulation and “CW” transmit with four user-selectible channels:
- 49.830 MHz - Channel 1
- 49.845 MHz - Channel 2
- 49.860 MHz - Channel 3
- 49.875 MHZ - Channel 4
Other 2/4 channel baby monitors sold use an alphanumeric channel numbering scheme:
- 49.850 MHz - Channel 1A
- 49.870 MHz - Channel 2A
- 49.830 MHz - Channel 1B
- 49.890 MHz - Channel 2B
Modern equipment generally uses 2 switchable channels. Some of these transmitters use a simple 16.6xxx MHz crystal with a frequency tripler circuit (16.610 MHz x3 = 49.830 MHz - 16.61415 MHz x3 = 49.8425 MHz, 16.615 MHz x3 = 49.845 MHz, 16.61575 MHz x3 = 49.84725 MHz, 16.616 MHz x3 = 49.848 MHz, 16.1616 MHz x3 = 49.8484 MHz, 16.61667 MHz x3 = 49.850 MHz, 16.620 MHz x3 = 49.860 MHz etc.) and simple FM modulation. Frequency steps, channel plans and channel spacing is random and channel labeling or naming is arbitrary and not consistent from manufacturer to manufacturer or even model number to model number - including:
- 49.845 MHz - Channel X
- 49.875 MHz - Channel Y
- 49.830 MHz - Channel A
- 49.860 MHz - Channel B
- 49.845 MHz - Channel A
- 49.890 MHz - Channel B
- 49.840 MHz - Channel A
- 49.870 MHz - Channel B
- 49.850 MHz - Channel I
- 49.870 MHz - Channel II
- 49.825 MHz - Channel 1
- 49.845 MHz - Channel 2
- 49.870 MHz - Channel 1
- 49.890 MHz - Channel 2
- 49.840 MHz - Channel 1
- 49.860 MHz - Channel 2
- 49.8425 MHz - Channel 1 (49.842 MHz or 49.843 MHz offset)
- 49.8625 MHz - Channel 2 (49.862 MHz or 49.863 MHz offset)
- 49.840 MHz - Channel A
- 49.885 MHz - Channel B
- 49.830 MHz - Channel A
- 49.875 MHz - Channel B
- 49.840 MHz - Channel A
- 49.876 MHz - Channel B (specifically designed to operate off-frequency from 49.875 MHz - 49.876 MHz per FCC ID documentation)
- 49.840 MHz - Channel A
- 49.890 MHz - Channel B
Or, for devices with two channel sets (A/B channels or X/Y channels) with 30 kHz channel spacing:
- 49.830 MHz - Channel A
- 49.860 MHz - Channel B
- 49.850 MHz - Channel X
- 49.880 MHz - Channel Y
FCC authorization for equipment using AM mode instead of FM has been found for baby monitor use. The channels / frequencies listed in the authorization are:
- 49.870 MHz - Channel A (actually closer to 49.885 MHz, 49.8875 MHz, 49.888 MHz or 49.890 MHz)
- 49.810 MHz - Channel B - actual center frequency 49.8225 MHz (49.820 MHz or 49.825 MHz on 5 kHz step receivers)
It is unknown how this equipment (FCC ID L2555005) was approved for use under Part 15. The authorization specifically mentions use of amplitude modulation (AM) for voice transmission on the nominal frequencies of 49.81 MHz and 49.87 MHz. Test reports show the transmitter actually transmits closer to 49.825 MHz for Channel B and 49.880 MHz for Channel A. The Part 15 FCC approval test report indicates the center frequencies were actually 49.8225 MHz or 49.825 MHz and 49.880 MHz or 49.8825 MHz. Testing documentation shows a 16.8 kHz bandwidth for Channel A [center frequency 49.888 MHz) and a 15.6 kHz bandwidth for Channel B [center frequency 49.828 MHz].
15 kHz wide band AM modulation appears to be used. 15K0A3E emission... According to the schematic provided with the authorization paperwork, the device in question uses two crystals for frequency generation - 16.610 MHz x3 (for 49.830 MHz) for Channel B and 16.630 MHz x3 (for 49.890 MHz) for Channel A. Testing indicates rather loose frequency tolerance (49.828 MHz instead of 49.830 MHz and 49.888 MHz instead of 49.890 MHz).
Other equipment use random 10 kHz / 20 kHz channel spacing, with offsets from the "standard" 15 kHz channel steps.
- 49.830 MHz - Channel 1
- 49.850 MHz - Channel 2
- 49.860 MHz - Channel 3
- 49.870 MHz - Channel 4
- 49.880 MHz - Channel 5
- 49.890 MHz - Channel 6
Other room monitor equipment with the following available channels is/was on the market:
- 49.835 MHz - Channel A
- 49.850 MHz - Channel B
- 49.865 MHz - Channel C
- 49.830 MHz - Channel A
- 49.860 MHz - Channel B
- 49.890 MHz - Channel C
In addition to monitors, baby monitors, wireless microphones and similar devices, there were/are wireless doorbells and intercom systems sold that use the 49 MHz Part 15 band. One such device uses the following frequencies:
- 49.835 MHz - Channel 1/A
- 49.860 MHz - Channel 2/B
- 49.885 MHz - Channel 3/C
Even with the 900 MHz, 2.4 GHz, 5.8 GHz and 1.9 GHz DECT bands available, the 49 MHz band is still heavily used by room monitors, wireless headphones, wireless intercom/doorbell systems, etc.
Covered under FCC Part 15 47 CFR 15.235 Operation within the band 49.82-49.90 MHz.
Sometimes frequencies may be off by several kHz. For example, 49.845 MHz may be closer to 49.840 MHz, 49.847 MHz or even 49.850 MHz, 49.860 MHz may be closer to 49.855 MHz, 49.8625 MHz, 49.865 MHz, etc. Apparently some manufacturers did/do this intentionally, likely to reduce interference issues. Certain name-brand baby monitors, for example, purposely operate off-frequency. For example, FCC authorization documents indicate a baby monitor system designed to operate on 49.862 MHz (49.8625 MHz). Another baby monitor features two channels, 49.840 MHz and 49.876 MHz. Third party testing and circuit designs specify 49.876 MHz instead of 49.875 MHz or 49.880 MHz. 49.8875 MHz instead of 49.885 MHz or 49.890 MHz. The specifications also call for a 25 kHz wide channel (20K0F3E FM voice). Another single channel design uses 49.87125 MHz (appears to be closer to 49.872 MHz than 49.871 MHz) instead of 49.870 MHz or 49.875 MHz. These variations appear to be deliberately designed into the product to reduce interference from other 49 MHz devices. Since a specific transmitter is designed to be paired to a specific receiver only, it is advantageous to use a randomly offset frequency (the FM capture effect also helps a lot).
49 MHz R/C transmitters usually use 49.830 MHz, 49.860 MHz or 49.890 MHz. As noted earlier, frequencies may vary +/- several kHz. Baby monitors are usually found on 49.83 MHz, 49.86 MHz or 49.89 MHz. It's common for a set of R/C toys to be sold with one toy using a 49 MHz RC frequency and the other using a 26 MHz or 27 MHz RC frequency.
The 49.82 MHz to 49.90 MHz band allows for personal hobby transmitters up to 100mw output with any modulation as long as the modulation stays within the 49.82-49.9 MHz band. This provision within Part 15 opens up opportunities for 49 MHz beacons, provided an effective antenna is used.
§ 15.235 Operation within the band 49.82–49.90 MHz. (a) The field strength of any emission within this band shall not exceed 10,000 microvolts/meter at 3 meters. The emission limit in this paragraph is based on measurement instrumentation employing an average detector. The provisions in §15.35 for limiting peak emissions apply. (b) The field strength of any emissions appearing between the band edges and up to 10 kHz above and below the band edges shall be attenuated at least 26 dB below the level of the unmodulated carrier or to the general limits in §15.209, whichever permits the higher emission levels. The field strength of any emissions removed by more than 10 kHz from the band edges shall not exceed the general radiated emission limits in §15.209. All signals exceeding 20 microvolts/meter at 3 meters shall be reported in the application for certification. (c) For a home-built intentional radiator, as defined in §15.23(a), operating within the band 49.82–49.90 MHz, the following standards may be employed: (1) The RF carrier and modulation products shall be maintained within the band 49.82–49.90 MHz. (2) The total input power to the device measured at the battery or the power line terminals shall not exceed 100 milliwatts under any condition of modulation. (3) The antenna shall be a single element, one meter or less in length, permanently mounted on the enclosure containing the device. (4) Emissions outside of this band shall be attenuated at least 20 dB below the level of the unmodulated carrier. (5) The regulations contained in §15.23 of this part apply to intentional radiators constructed under the provisions of this paragraph. (d) Cordless telephones are not permitted to operate under the provisions of this section.
15.236 - Operation of wireless microphones in the bands 54-72 MHz, 76-88 MHz, 174-216 MHz, 470-608 MHz and 614-698 MHz.
Covers the requirements for wireless microphones operating within the VHF and UHF TV bands over the air TV bands. TVBDs TV Band Devices, including the 600 MHz band re-allocation and TV band repacking (reallocation).
See also: 169-173 MHz VHF wireless microphones (under Part 90) as well as wireless microphones operating in the 49 MHz band, the 72-76 MHz band (auditory listening devices), the 88-108 MHz FM band, the 902-928 MHz band, the 2.4 GHz band and the higher bands.
15.237 - Operation in the bands 72.0-73.0 MHz, 74.6-74.8 MHz and 75.2-76.0 MHz.
a) The intentional radiator shall be restricted to use as an auditory assistance device.
(b) 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 above specified frequency ranges.
(c) The field strength within the permitted 200 kHz band shall not exceed 80 millivolts/meter at 3 meters. The field strength of any emissions radiated on any frequency outside of the specified 200 kHz band shall not exceed the general radiated emissions limits specified in § 15.209. The emission limits in this paragraph are based on measurement instrumentation employing an average detector. The provisions in § 15.35 for limiting peak emissions apply.
The portions of the 72.0 MHz to 73.0 MHz band, the 74.6 MHz to 74.8 MHz band and 75.2 MHz to 76.0 MHz band are shared with Remote Control R/C RC devices 72 MHz RC and 75 MHz RC as well as licensed systems including wireless clock systems, fixed data links, industrial remote control systems, audio backhaul links, and numerous other systems.
15.239 - Operation in the band 88-108 MHz.
Low power operations within the FM broadcast band. This includes in-car Bluetooth devices and similar FM transmitters.
(a) 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.
(b) The field strength of any emissions within the permitted 200 kHz band shall not exceed 250 microvolts/meter at 3 meters. The emission limit in this paragraph is based on measurement instrumentation employing an average detector. The provisions in § 15.35 for limiting peak emissions apply.
(c) The field strength of any emissions radiated on any frequency outside of the specified 200 kHz band shall not exceed the general radiated emission limits in § 15.209.
15.240 - Operation in the band 433.5-434.5 MHz.
Voice transmissions prohibited for Part 15 devices. In some countries, this overlaps with the 433 MHz LPD433 service. In the USA (and other countries), it may also overlap with the 70cm amateur radio band.
Operation under the provisions of this section is restricted to devices that use radio frequency energy to identify the contents of commercial shipping containers. Operations must be limited to commercial and industrial areas such as ports, rail terminals and warehouses. Two-way operation is permitted to interrogate and to load data into devices. Devices operated pursuant to the provisions of this section shall not be used for voice communications.
The peak level of any emissions within the specified frequency band shall not exceed 55,000 microvolts per meter measured at a distance of 3 meters.
The field strength of any emissions radiated within the specified frequency band shall not exceed 11,000 microvolts per meter measured at a distance of 3 meters. The emission limit in this paragraph is based on measurement instrumentation employing an average detector.
15.241 - Operation in the band 174-216 MHz.
This section regulated biomedical telemetry devices operating in the 174 MHz to 216 MHz band.
(a) Operation under the provisions of this section is restricted to biomedical telemetry devices.
(b) Emissions from the device shall be confined within a 200 kHz band which shall lie wholly within the frequency range of 174-216 MHz.
(c) The field strength of any emissions radiated within the specified 200 kHz band shall not exceed 1500 microvolts/meter at 3 meters. The field strength of emissions radiated on any frequency outside of the specified 200 kHz band shall not exceed 150 microvolts/meter at 3 meters
15.245 - Operation within the bands 902-928 MHz, 2435-2465 MHz, 5785-5815 MHz, 10500-10550 MHz, and 24075-24175 MHz.
Subsection is for remote sensing, alarms, etc.
(a) Operation under the provisions of this section is limited to intentional radiators used as field disturbance sensors, excluding perimeter protection systems.
(b) The field strength of emissions from intentional radiators operated within these frequency bands shall comply with the following:
- 902 MHz - 928 MHz - 500 millivolts/meter at 3 meters
- 2435 MHz - 2465 MHz - 500 millivolts/meter at 3 meters - 2.435 GHz - 2.465 GHz
- 5785 MHz - 5815 MHz - 500 millivolts/meter at 3 meters - 5.785 GHz - 5.815 GHz
- 10500 MHz - 10550 MHz - 2500 millivolts/meter at 3 meters - 10.5 GHz - 10.55 GHz
- 24075 MHz - 24175 MHz - 2500 millivolts/meter at 3 meters - 24.075 GHz - 24.175 GHz
15.247 - Operation within the bands 902-928 MHz, 2400-2483.5 MHz, and 5725-5850 MHz.
See also: ISM, cordless phone systems (900 MHz band, 2.4 GHz band and 5.8 GHz band digital cordless phones may operate under 15.247. Wireless intercoms, full duplex headset comms systems, long-range telemetry, non Part 95 remote control, datalink, video backhaul systems for UAV systems, drones, etc.). Line of sight (air to ground) radio links on 900 MHz are capable of very long range, often 40 km or more.
(a) Operation under the provisions of this Section is limited to frequency hopping (FH) and digitally modulated intentional radiators that comply with the following provisions:
(1) Frequency hopping systems shall have hopping channel carrier frequencies separated by a minimum of 25 kHz or the 20 dB bandwidth of the hopping channel, whichever is greater. Alternatively, frequency hopping systems operating in the 2400-2483.5 MHz band may have hopping channel carrier frequencies that are separated by 25 kHz or two-thirds of the 20 dB bandwidth of the hopping channel, whichever is greater, provided the systems operate with an output power no greater than 125 mW. The system shall hop to channel frequencies that are selected at the system hopping rate from a pseudo randomly ordered list of hopping frequencies. Each frequency must be used equally on the average by each transmitter. The system receivers shall have input bandwidths that match the hopping channel bandwidths of their corresponding transmitters and shall shift frequencies in synchronization with the transmitted signals.
(i) For frequency hopping systems operating in the 902-928 MHz band: if the 20 dB bandwidth of the hopping channel is less than 250 kHz, the system shall use at least 50 hopping frequencies and the average time of occupancy on any frequency shall not be greater than 0.4 seconds within a 20 second period; if the 20 dB bandwidth of the hopping channel is 250 kHz or greater, the system shall use at least 25 hopping frequencies and the average time of occupancy on any frequency shall not be greater than 0.4 seconds within a 10 second period. The maximum allowed 20 dB bandwidth of the hopping channel is 500 kHz.
(ii) Frequency hopping systems operating in the 5725-5850 MHz band shall use at least 75 hopping frequencies. The maximum 20 dB bandwidth of the hopping channel is 1 MHz. The average time of occupancy on any frequency shall not be greater than 0.4 seconds within a 30 second period.
(iii) Frequency hopping systems in the 2400-2483.5 MHz band shall use at least 15 channels. The average time of occupancy on any channel shall not be greater than 0.4 seconds within a period of 0.4 seconds multiplied by the number of hopping channels employed. Frequency hopping systems may avoid or suppress transmissions on a particular hopping frequency provided that a minimum of 15 channels are used.
2) Systems using digital modulation techniques may operate in the 902-928 MHz, 2400-2483.5 MHz, and 5725-5850 MHz bands. The minimum 6 dB bandwidth shall be at least 500 kHz.
(b) The maximum peak conducted output power of the intentional radiator shall not exceed the following:
(1) For frequency hopping systems operating in the 2400-2483.5 MHz band employing at least 75 non-overlapping hopping channels, and all frequency hopping systems in the 5725-5850 MHz band: 1 watt. For all other frequency hopping systems in the 2400-2483.5 MHz band: 0.125 watts.
(2) For frequency hopping systems operating in the 902-928 MHz band: 1 watt for systems employing at least 50 hopping channels; and, 0.25 watts for systems employing less than 50 hopping channels, but at least 25 hopping channels, as permitted under paragraph (a)(1)(i) of this section.
(3) For systems using digital modulation in the 902-928 MHz, 2400-2483.5 MHz, and 5725-5850 MHz bands: 1 Watt. As an alternative to a peak power measurement, compliance with the one Watt limit can be based on a measurement of the maximum conducted output power. Maximum Conducted Output Power is defined as the total transmit power delivered to all antennas and antenna elements averaged across all symbols in the signaling alphabet when the transmitter is operating at its maximum power control level. Power must be summed across all antennas and antenna elements. The average must not include any time intervals during which the transmitter is off or is transmitting at a reduced power level. If multiple modes of operation are possible (e.g., alternative modulation methods), the maximum conducted output power is the highest total transmit power occurring in any mode. See Motorola DTR series 900 MHz frequency hopping hopsets.
(4) The conducted output power limit specified in paragraph (b) of this section is based on the use of antennas with directional gains that do not exceed 6 dBi. Except as shown in paragraph (c) of this section, if transmitting antennas of directional gain greater than 6 dBi are used, the conducted output power from the intentional radiator shall be reduced below the stated values in paragraphs (b)(1), (b)(2), and (b)(3) of this section, as appropriate, by the amount in dB that the directional gain of the antenna exceeds 6 dBi.
(c) Operation with directional antenna gains greater than 6 dBi.
(1) Fixed point-to-point operation:
(i) Systems operating in the 2400-2483.5 MHz band that are used exclusively for fixed, point-to-point operations may employ transmitting antennas with directional gain greater than 6 dBi provided the maximum conducted output power of the intentional radiator is reduced by 1 dB for every 3 dB that the directional gain of the antenna exceeds 6 dBi.
(ii) Systems operating in the 5725-5850 MHz band that are used exclusively for fixed, point-to-point operations may employ transmitting antennas with directional gain greater than 6 dBi without any corresponding reduction in transmitter conducted output power.
Numerous systems operate under the provisions of Part 15.247, especially in the 900 MHz band and the 2.4 GHz band. Examples include video senders, baby monitors, cordless phones (see also: DECT) and others.
For example, a video/audio baby monitor transmitter operating in the 2403.5 MHz - 2475.5 MHz band or the 2.4035 GHz - 2.4755 GHz band using frequency hopping 49 hopping frequencies available. Transmitter output is 55.2 mW. Dwell time on each channel is 3.9 milliseconds. 49 individual channels and GFSK modulation - 2.08 MHz wide signals or 2M08G2D modulation. Standardized frequency hopping channel spacing of 1.5 MHz (channel 1 is 2403.5 MHz or 2.4035 GHz, channel 2 is 2405 MHz or 2.405 GHz, channel 3 is 2406.5 MHz or 2.4065 GHz, up to channel 48 is 2474 MHz or 2.474 MHz and channel 49 is 2475.5 MHz or 2.4755 GHz). Under this example, channel 32 is the center frequency 2450 MHz 2.45 GHz. FCC ID is 2AF2R-HB32TX.
15.249 - Operation within the bands 902-928 MHz, 2400-2483.5 MHz, 5725-5875 MHz, and 24.0-24.25 GHz.
(b) Fixed, point-to-point operation as referred to in this paragraph shall be limited to systems employing a fixed transmitter transmitting to a fixed remote location. Point-to-multipoint systems, omnidirectional applications, and multiple co-located intentional radiators transmitting the same information are not allowed. Fixed, point-to-point operation is permitted in the 24.05-24.25 GHz band subject to the following conditions:
(1) The field strength of emissions in this band shall not exceed 2500 millivolts/meter.
(2) The frequency tolerance of the carrier signal shall be maintained within ±0.001% of the operating frequency over a temperature variation of −20 degrees to +50 degrees C at normal supply voltage, and for a variation in the primary supply voltage from 85% to 115% of the rated supply voltage at a temperature of 20 degrees C. For battery operated equipment, the equipment tests shall be performed using a new battery.
(3) Antenna gain must be at least 33 dBi. Alternatively, the main lobe beamwidth must not exceed 3.5 degrees. The beamwidth limit shall apply to both the azimuth and elevation planes. At antenna gains over 33 dBi or beamwidths narrower than 3.5 degrees, power must be reduced to ensure that the field strength does not exceed 2500 millivolts/meter.
Non-fixed point-to-point systems are limited to the following field strengths:
- 902 MHz - 928 MHz - 50 millivolts/meter at 3 meters or 50mV/m at 3 meters (50,000 microvolts/meter at 3 meters)
- 2400 MHz - 2483.5 MHz - 50 millivolts/meter at 3 meters or 50mV/m at 3 meters (50,000 microvolts/meter at 3 meters)
- 5725 MHz - 5875 MHz - 50 millivolts/meter at 3 meters or 50mV/m at 3 meters (50,000 microvolts/meter at 3 meters)
- 24.0 GHz - 24.25 GHz - 250 millivolts/meter at 3 meters or 250mV/m at 3 meters (250,000 microvolts/meter at 3 meters)
15.258 - Operation in the bands 116-123 GHz, 174.8-182 GHz, 185-190 GHz and 244-246 GHz.
(a) Operation on board an aircraft or a satellite is prohibited.
(b) Emission levels within the 116-123 GHz, 174.8-182 GHz, 185-190 GHz and 244-246 GHz bands shall not exceed the following equivalent isotropically radiated power (EIRP) limits as measured during the transmit interval:
(1) The average power of any emission shall not exceed 40 dBm and the peak power of any emission shall not exceed 43 dBm; or
(2) For fixed point-to-point transmitters located outdoors, the average power of any emission shall not exceed 82 dBm and shall be reduced by 2 dB for every dB that the antenna gain is less than 51 dBi. The peak power of any emission shall not exceed 85 dBm and shall be reduced by 2 dB for every dB that the antenna gain is less than 51 dBi. The provisions in this paragraph (b)(2) for reducing transmit power based on antenna gain shall not require that the power levels be reduced below the limits specified in paragraph (b)(1) of this section.
(3) The peak power shall be measured with a detection bandwidth that encompasses the entire occupied bandwidth within the intended band of operation, e.g., 116-123 GHz, 174.8-182 GHz, 185-190 GHz or 244-246 GHz. The average emission levels shall be measured over the actual time period during which transmission occurs.
(4) Transmitters with an emission bandwidth of less than 100 MHz must limit their peak radiated power to the product of the maximum permissible radiated power (in milliwatts) times their emission bandwidth divided by 100 MHz. For the purposes of this paragraph (b)(4), emission bandwidth is defined as the instantaneous frequency range occupied by a steady state radiated signal with modulation, outside which the radiated power spectral density never exceeds 6 dB below the maximum radiated power spectral density in the band, as measured with a 100 kHz resolution bandwidth spectrum analyzer. The center frequency must be stationary during the measurement interval, even if not stationary during normal operation (e.g., for frequency hopping devices).
Part 15 Subpart D - Unlicensed Personal Communications Services UPCS DECT 1920-1930 MHz Band
Sections 15.301 - 15.323 1.9 GHz band DECT 6.0 specification. See the page for DECT for more information.
15.323 Specific requirements for devices operating in the 1920-1930 MHz band. (a) Operation shall be contained within the 1920-1930 MHz band. The emission bandwidth shall be less than 2.5 MHz. The power level shall be as specified in § 15.319(c), but in no event shall the emission bandwidth be less than 50 kHz.
(c) Devices must incorporate a mechanism for monitoring the time and spectrum windows that its transmission is intended to occupy. The following criteria must be met:
(1) Immediately prior to initiating transmission, devices must monitor the combined time and spectrum windows in which they intend to transmit for a period of at least 10 milliseconds for systems designed to use a 10 milliseconds or shorter frame period or at least 20 milliseconds for systems designed to use a 20 milliseconds frame period.
(2) The monitoring threshold must not be more than 30 dB above the thermal noise power for a bandwidth equivalent to the emission bandwidth used by the device.
(3) If no signal above the threshold level is detected, transmission may commence and continue with the same emission bandwidth in the monitored time and spectrum windows without further monitoring. However, occupation of the same combined time and spectrum windows by a device or group of cooperating devices continuously over a period of time longer than 8 hours is not permitted without repeating the access criteria.
(4) Once access to specific combined time and spectrum windows is obtained an acknowledgment from a system participant must be received by the initiating transmitter within one second or transmission must cease. Periodic acknowledgments must be received at least every 30 seconds or transmission must cease. Channels used exclusively for control and signaling information may transmit continuously for 30 seconds without receiving an acknowledgment, at which time the access criteria must be repeated.
(5) If access to spectrum is not available as determined by the above, and a minimum of 20 duplex system access channels are defined for the system, the time and spectrum windows with the lowest power level may be accessed. A device utilizing the provisions of this paragraph must have monitored all access channels defined for its system within the last 10 seconds and must verify, within the 20 milliseconds (40 milliseconds for devices designed to use a 20 milliseconds frame period) immediately preceding actual channel access that the detected power of the selected time and spectrum windows is no higher than the previously detected value. The power measurement resolution for this comparison must be accurate to within 6 dB. No device or group of co-operating devices located within 1 meter of each other shall during any frame period occupy more than 6 MHz of aggregate bandwidth, or alternatively, more than one third of the time and spectrum windows defined by the system.
(6) If the selected combined time and spectrum windows are unavailable, the device may either monitor and select different windows or seek to use the same windows after waiting an amount of time, randomly chosen from a uniform random distribution between 10 and 150 milliseconds, commencing when the channel becomes available.
(7) The monitoring system bandwidth must be equal to or greater than the emission bandwidth of the intended transmission and have a maximum reaction time less than 50xSQRT (1.25/emission bandwidth in MHz) microseconds for signals at the applicable threshold level but shall not be required to be less than 50 microseconds. If a signal is detected that is 6 dB or more above the applicable threshold level, the maximum reaction time shall be 35xSQRT (1.25/emission bandwidth in MHz) microseconds but shall not be required to be less than 35 microseconds.
(8) The monitoring system shall use the same antenna used for transmission, or an antenna that yields equivalent reception at that location.
(9) Devices that have a power output lower than the maximum permitted under this subpart may increase their monitoring detection threshold by one decibel for each one decibel that the transmitter power is below the maximum permitted.
(10) An initiating device may attempt to establish a duplex connection by monitoring both its intended transmit and receive time and spectrum windows. If both the intended transmit and receive time and spectrum windows meet the access criteria, then the initiating device can initiate a transmission in the intended transmit time and spectrum window. If the power detected by the responding device can be decoded as a duplex connection signal from the initiating device, then the responding device may immediately begin transmitting on the receive time and spectrum window monitored by the initiating device.
(11) An initiating device that is prevented from monitoring during its intended transmit window due to monitoring system blocking from the transmissions of a co-located (within one meter) transmitter of the same system, may monitor the portions of the time and spectrum windows in which they intend to receive over a period of at least 10 milliseconds. The monitored time and spectrum window must total at least 50 percent of the 10 millisecond frame interval and the monitored spectrum must be within 1.25 MHz of the center frequency of channel(s) already occupied by that device or co-located co-operating devices. If the access criteria is met for the intended receive time and spectrum window under the above conditions, then transmission in the intended transmit window by the initiating device may commence.
(12) The provisions of (c)(10) or (c)(11) of this section shall not be used to extend the range of spectrum occupied over space or time for the purpose of denying fair access to spectrum to other devices.
(d) Emissions outside the band shall be attenuated below a reference power of 112 milliwatts as follows: 30 dB between the band and 1.25 MHz above or below the band; 50 dB between 1.25 and 2.5 MHz above or below the band; and 60 dB at 2.5 MHz or greater above or below the band. Emissions inside the band must comply with the following emission mask: In the bands between 1B and 2B measured from the center of the emission bandwidth the total power emitted by the device shall be at least 30 dB below the transmit power permitted for that device; in the bands between 2B and 3B measured from the center of the emission bandwidth the total power emitted by an intentional radiator shall be at least 50 dB below the transmit power permitted for that radiator; in the bands between 3B and the band edge the total power emitted by an intentional radiator in the measurement bandwidth shall be at least 60 dB below the transmit power permitted for that radiator. B” is defined as the emission bandwidth of the device in hertz. Compliance with the emission limits is based on the use of measurement instrumentation employing peak detector function with an instrument resolution bandwidth approximately equal to 1.0 percent of the emission bandwidth of the device under measurement.
(e) The frame period (a set of consecutive time slots in which the position of each time slot can be identified by reference to a synchronizing source) of an intentional radiator operating in this band shall be 20 milliseconds or 10 milliseconds/X where X is a positive whole number. Each device that implements time division for the purposes of maintaining a duplex connection on a given frequency carrier shall maintain a frame repetition rate with a frequency stability of at least 50 parts per million (ppm). Each device which further divides access in time in order to support multiple communication links on a given frequency carrier shall maintain a frame repetition rate with a frequency stability of at least 10 ppm. The jitter (time-related, abrupt, spurious variations in the duration of the frame interval) introduced at the two ends of such a communication link shall not exceed 25 microseconds for any two consecutive transmissions. Transmissions shall be continuous in every time and spectrum window during the frame period defined for the device.
(f) The frequency stability of the carrier frequency of the intentional radiator shall be maintained within ±10 ppm over 1 hour or the interval between channel access monitoring, whichever is shorter. The frequency stability shall be maintained over a temperature variation of −20° to + 50 °C at normal supply voltage, and over a variation in the primary supply voltage of 85 percent to 115 percent of the rated supply voltage at a temperature of 20 °C. For equipment that is capable only of operating from a battery, the frequency stability tests shall be performed using a new battery without any further requirement to vary supply voltage.
Historic CFR 47 Part 15 FCC Regulations
Part 15 grew from a single page in 1949 to two pages in 1953, then barely 4 pages in 1963, to 11 pages in 1964, and remained that long through 1969.
Historic - Cordless telephones
Early cordless telephones used these frequencies for the base units: 1.665, 1.690, 1.695, 1.710, 1.725, 1.730, 1.750, 1.755, 1.770 MHz.
And for the handsets: 46.610, 46.630, 46.670, 46.710, 46.730, 46.770, 46.830, 46.870, 46.930, 46.970, 49.670, 49.770, 49.830, 49.845, 49.860, 49.875, 49.890, 49.930, 49.970, 49.990 MHz
Both used frequency modulation.
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