Communiqué
MURS
The Radiometrix MURS radios. The NiM1B transceiver modules are on the left and the QPX1-154-5 transceiver is on the right. These and others are distributed by Lemos International.

MURS: Another Potential IoT Wireless Option

The best alternative for long-range applications may be hiding in plain sight.

Most Internet of Things (IoT) projects are adopting the well-known wireless technologies like Wi-Fi, Bluetooth, Z-Wave, ZigBee, and other IEEE 802.15.4-based standards. If longer range is needed, designers are choosing Semtech’s LoRa, as well as other technologies like Sigfox, 802.11f or Weightless (white spaces), and even LTE-M or NB-IoT cellular.

It seems as though almost everyone has simply overlooked what may be a better option for long-range applications: MURS. It’s probably because few know about this underreported, underused, and enigmatic option. MURS is an excellent choice for telemetry and remote control operations, and could be an option for the IoT.

MURS stands for Multi-Use Radio Service. It is one of several communications alternatives under the Federal Communications Commission (FCC) Personal Radio Services category. Get a copy of the Code of Federal Regulations (CFR) 47, Part 95 online from the FCC for details. Listed there are several personal radio services you can use as an individual or as a business.

You have probably heard of some like Citizens Band (CB) Radio or the Family Radio Service (FRS). But are you familiar with the General Mobile Radio Service (GMRS) or MURS? I doubt it. Anyway, if you want to consider MURS, you should have a copy of this regulation just to familiarize yourself with the guidelines. See Part 95 Subpart J, 95.1301 through 95.1317 and 95.632 through 95.639. The FCC recently revised Part 95 and the new guidelines for MURS are covered in 95.2700 through 95.2799. Some sections of Part 90 are also applicable.

Unlike the other IoT standards that operate in the already crowded 902 to 928 MHz or 2.4 to 2.4835 GHz unlicensed bands, MURS operates on five frequencies in the VHF band.

Channel

Frequency (MHz)

Max. Bandwidth (kHz)

1

151.82

11.25

2

151.88

11,25

3

151.94

11.25

4

154.57

20

5

154.60

20

 

These are shared frequencies, as are most other license-free spectrum. The required procedure is to listen before you transmit. Most commercial MURS radios have this feature.

Given the principles of physics, lower frequencies travel farther than the higher frequency waves for a given power level and other factors. They also have non-line of sight potential. That means MURS is an ideal choice for long range applications. And supporting that is a maximum power output of 2 watts and the ability to use external gain antennas, gives MURS a significant advantage over other so-called long-range technologies.

Modulation is restricted to AM or NBFM for voice and ASK or FSK for data. Yes, voice communications is an option which does not exist with the other IoT standards, although it is certainly not needed in an IoT application. One downside of MURS is that the data rate is restricted by the bandwidth available in each channel. A maximum rate is probably about 5 kb/s, but that is more than sufficient for many telemetry, remote control, or IoT uses.

Two key restrictions are that the MURS radios cannot be connected to the switched telephone network. In addition, no repeaters are signal boosters are allowed. Internet connectivity can no doubt be accommodated by an interim gateway that may or not be needed depending upon the application. And, of course, MURS is not an international standard. For U.S.-only applications it is worth considering.

Another possible disadvantage is the need for larger antennas. A standard quarter wave vertical should be 18-19 in., but this can be shortened by using a loading coil or a helix design. You can use a remote outdoor antenna as long as it is less than 60 ft. off the ground or less than 20 ft. from the top of any structure it is attached to. The good news is that you may use a gain antenna of any type to boost effective radiated power (ERP). Yagi and collinear antennas can greatly extend the range.

For new designs you will typically use existing modules. One example is the Radiometrix NiM1B transceiver module. (see photo). This module comes in versions for 154.57 MHz or 154.60 MHz with narrowband frequency modulation (NBFM) and a data rate up to 5 kb/s. The transmit power is 13 dBm or 20 mW. The receiver sensitivity is −120 dBm. This provides a great link budget and should be good for a range of at least several kilometers—and up to 10 miles or so, depending upon antenna height, type, and terrain.

The Radiometrix modules are made in the UK and distributed by Lemos International in the U.S. Lemos also carries the QPX1-154-5 MURS transceiver module that uses the top two MURS channels with 2 watts of power. (Again, see the photo). Its internal PLL synthesizer also allows operation anywhere in the 135-175 MHz range making it useful for other applications. Receive sensitivity is −118 dBm. A RSSI output is available. Data rate maximum is 5 kb/s. Estimated range is 10 km or more.

For more details, go to the Lemos site and take a look at the extensive line of modules available. If you want to get an IoT or other wireless project up and running fast, the module route is the way to go. And keep in mind, that MURS may now be one of your wireless options.


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