We get this question many, many times from our clients. Essentially, they want to make a system that transmits data from point A to point B but don’t know what range they can count on, and how much it will cost to certify, and if they need a license.
Overview of the Rules Governing Intentional Radiators
The FCC has made provisions for unlicensed transmitters throughout the radio spectrum. You can operate just about anywhere, but there are restrictions on the field strength, type of emission, and duty cycle that you need to be aware of before planning a system.
Section 209 (general limits)
Interestingly enough, the FCC allows you to operate any intentional radiator (transmitter) from LF to the GHz region, as long as:
The device’s radiated field strength is less than that called out for unintentional radiators
- The device does not occupy the restricted bands called out in section 205.
- There are a lot of restricted bands, and the fields strength are relatively low. But, this is still a potentially useful mode for short and medium range links. With a good receiver, narrowband width IF, and a short duty cycle, 100-200 ft is easily implemented.
Duty Cycle and Power Averaging
There is an additional factor to be considered, duty cycle. The FCC has stipulated that field strength is measured as an average and peak value, with the maximum peak value being up to 20 dB above the average limit. The rules allow the user to calculate the average field strength by the following formula: FS avg = FS peak-20 log (duty cycle), with the maximum correction limited to 20 dB. So, you can run a lot more power during the burst than the average limit indicates if the duty cycle is less than 10%.
Sections 215-255, Additional Provisions
These sections allow for more power than the 209 limits, but are more restrictive as to the frequencies and types of operation. They can be loosely grouped into four sections:
- Broadcast (AM/FM)
- Data/remote control
- ISM (industrial, scientific, medical)
Broadcast (525-1705 KHz, 88-108 MHz)
Used to transmit voice or data, and are subject to restrictions to power to the PA and/or field strengths. We see these bands used for information services (talking signs) and rebroadcast of audio, and low power radio stations.
Data/Remote Control (70 MHz and up)
This section is intended to address remote control and data transmissions, such as keyless entry and garage door openers. There are a large number of low cost 315 MHz RF ASICs available which makes this band very popular. The restrictions on operation under this section prohibit continuous transmission or even medium duty cycles. There are two subsections:
The transmit on time is limited to a maximum of 5 seconds. Video, voice, and control of toys is prohibited, and the transmitter must be manually triggered. No periodic operation is allowed, except to verify system operation. In this case, the transmit time is limited to 2 seconds per hour (1/1800 duty cycle). It’s a stretch to use this section for anything other than a hand operated remote control or an alarm.
Here, the on time is limited to a maximum of 1 second. The off time shall also be at least 30 times the on time, with no less than 10 seconds between transmissions. The key to using this section is to have the system transmit in short bursts, with a 10 second off time. No restriction on application or modes, but you have to watch the duty cycle. More useful, but the 10 second minimum off time puts a crimp on real time functions.
We see most RC toys running at either 27 or 49 MHz, probably due to the simple circuits at these frequencies and the fairly large field strengths permitted. You can use either band for any mode, as with baby monitors. There used to be a bunch of Motorola receiver ICs (MC3363 et al) that enabled you to make a very cheap simple 27/49 MHz FM system but they are now obsolete. We have located an alternate vendor for a similar part, and still do a few designs for clients who don’t want a complex system.
ISM bands (15.247/15.249)
Section 247 offer the most power and range of any of the unlicensed radiators (up to 4 watts EIRP and miles of range). The biggest drawback is the need to either frequency hop or employ direct sequence spread spectrum, both of which are fairly complex to implement. Section 249 allows unlimited operation in the upper ISM bands, without having to use spread spectrum techniques. But, operation under this mode is determined by the field strength ( essentially low power), limiting the range.
Below is a summary of the 247 rules, these are the hardest to understand and implement thus the need for a “cheat sheet”: FCC Part 15.247 (frequency hopping)
1. Freq range: 902-928 MHz, hopping shall use min of 25 KHz spacing or the 20 dB BW whichever is greater. For less than 250 KHz BW, system shall use at least 50 channels and the avg. time on channel shall be less than 0.4 seconds in a 50 s interval. (section 15.247)
2. Special test requirements None
3. Frequency usage: Should eliminate the end channels to as to not violate the BW rule
4. Frequency tolerance: Not specified, only that the 20 dB BW shall fall inside the 902-928 MHz band. (section 15.215c)
5. Environmental: temp range = Not specified
6. Power output: 30 dBm max spec, with no more than a 6 dBi antenna. Implies Ptransmitter= 30 dBm+cable loss
7. Spurious levels: Emissions outside the band shall be –50 dBc (except for harmonics) or as called out in section 15.209, whichever is the lesser attenuation.
9 KHz-490 KHz: 2400/F(KHz) uV/M, 300 M
490 KHz-1705 KHz: 24000/F(KHz) uV/M, 30 M
1.705-30 MHz: 30 uV/M, 30 M
30-88 MHz: 100 uV/M, 3 M
88-216 MHz: 150 uV/M, 3 M
216-960 MHz: 200 uV/M, 3 M
960 MHz and above: 500 uV/M, 3 M
8. Transmit spectrum. Not specified. See #4.
9. Receive parameters: None, other than the receiver BW shall match the transmitter and shall be synced to the transmit sequence.