Are License-Free Radio Frequency Bands Really Free?

by Glenn Roenigk, Senior System Design Engineer

The world is exploding with wireless control and communication applications. One of the reasons is the availability for RF spectrum to operate license-free. One of the license-free bands that is generally indicated is the industrial, scientific and medical (ISM) radio band.

This band was originally reserved internationally for non-commercial use of RF electromagnetic fields for industrial, scientific and medical purposes. Because of the demand by consumers to have remote operation and data communication without the necessity of making physical connections, parts of these bands have become very commercialized.

The bands have become viable for many applications. These applications include, but are not limited to, garage door openers, remote car starters, building environmental controls such as heating, ventilation and air conditioning (HVAC) to such interesting applications as asset location or even “people finders.”

So are these bands really available license-free? I guess the operative word here is free. The answer is yes and no. The yes part of the answer is that these bands can be used as long as nobody complains. In other words, no license is required to operate a device using these frequencies. Once a complaint is issued, that’s where the no part of the answer comes in. To avoid the liability of a complaint that the governing agencies must follow up on, most suppliers of these products go through a rigorous testing program, usually with a lab that has experience with the many regulations that encumber the certification process. These labs manage the process from Test Plan development to Radio Configuration for Testing through the Administrative Filing process. This process is obviously not free. So to be safe for your product to operate without complaint, engage an experienced lab which the Future Electronics technical staff can help you with, or follow the guidelines established by the governing agencies.

In the United States, your radio must be certified with the FCC with the issuance of a Grant of Authorization by the FCC. In Canada, Industry Canada must issue a Certification Certificate. In Europe, Self-Certification by the manufacturer is based on the utilization of Harmonized Frequencies. In Australia, Self-Certification is managed by an ACMA Auditing process. There are additional agencies around the world as well.

 

Common Materials Can Affect RF Propagation in These License-Free Bands

Common materials used in buildings and the natural environment treat RF electromagnetic radiation with differing attenuation properties. The National Institute for Science & Technology (NIST) in the US performed tests to determine some of these properties. This is a quick summary of some of their findings.

The test set-up used for these tests is indicated above and some of these test results are indicated in the table below

 

 

These are just a few of the materials examined in the report. The way to use this data is to calculate the free space attenuation [dB = 20 * LOG ((4 * Pi * Range)/Wavelength)] at the frequency of interest [Wavelength = (speed of light)/(frequency of interest)] and removing the loss indicated in the table above. There is wide variation in the loss numbers because of the many factors that come into play such as water content, material density, spacing or thickness of components within the material, etc. Click here for a full copy of the NIST report.

 

An Overview of the FCC Rules for ISM Band Wireless Equipment In the USA, management of the electro-magnetic spectrum is the responsibility of the Federal Communications Commission (FCC). The FCC regulates not only the “intentional radiation” of radio transmitters, but also the “unintentional radiation” of noise from all sorts of electrical equipment. The FCC regulations appear in title 47 of the United States Code of Federal Regulations (47CFR), and radio spectrum issues are the subject of part 15 of the FCC rules, or 47CFR15 in brief.   FCC Rules Applicable to ISM-SS Radios: FCC 15.107, Conducted limits FCC 15.109, Radiated emission limits FCC 15.205, Restricted bands of operation FCC 15.207, Conducted limits FCC 15.209, Radiated emission limits; general requirements FCC 15.245, 15.247, 15.249, Operation within the bands 902-928MHz, 2400-2483.5MHz, 5725-5875MHz, and 24.0-24.25GHz   Summary of Power Output Rules for ISM bands: Maximum transmitter output is 1W (30 dBm) (47CFR15.247.b.1) Maximum EIRP is 4W (36dBm) (47CFR15.247.b.3) i.e. for every dB of antenna gain above 6dBi, transmitter output must be reduced by 1dBm; per this rule, a 24dBi antenna limits the output power to 12dBm which is 16mW For fixed point to point operation in ISM2.4, peak output need only be reduced by 1dBm for every 3dBi of antenna gain above 6 (47CFR15.247.b.3.i) i.e. per this rule, a 24dBi antenna may be fed by 24dBm or 256mW In ISM5.8, you can apply all the antenna gain you want (47CFR15.247.b.3.ii) with no reduction in output power.   The responsibility for staying within these power limits falls on the operator or the installer.   The Federal Communication Commission (FCC) governs the rules for frequency spectrum use within the US. The following list contains a few links to agencies in other parts of the world: Canada: http://spectrum.ic.gc.ca/tafl/tafindxe.html - Canada and US rules are very similar. Europe : http://www.etsi.org/home_a.htm Japan : http://www.tele.soumu.go.jp/e/index.htm   In the USA, management of the electromagnetic spectrum is the responsibility of the Federal Communications Commission (FCC), http://www.fcc.gov. The FCC regulates not only the "intentional radiation" of radio transmitters, but also the "unintentional radiation" of noise from all sorts of electrical equipment. The FCC regulations appear in title 47 of the United States Code of Federal Regulations (47CFR), and radio spectrum issues are the subject of part 15 of the FCC rules, or 47CFR15 in brief. The FCC website has a lot of material online, including FCC rules (47CFR), http://www.fcc.gov/oet/info/rules/ FCC Rules Applicable to ISM-SS Radios, use this table of contents: http://www.access.gpo.gov/nara/cfr/waisidx_01/47cfr15_01.html To download a full copy of the NIST report, please click here

 

Glenn Roenigk is a graduate of Penn State University in Electrical Engineering, with 36 years of research and development experience, seven of which have been with Future Electronics in the Technology Centers (first the Engineering Resource Center, then with the System Design Center).