What is MURS? Multi-Use Radio System is a
communications method that is an unlicensed two-way
radio service that operates under rules similar to those of
today’s CB radio. As long as you don’t use a continuous
carrier, you can talk and send data on it. MURS was
established in 2000 and operates under Part 95 with a
power limit of two watts.
MURS is open to the public and can be used for
personal and business communications tasks within the
US. All commonwealth areas and islands under the control
of the US are also legal. You can use MURS on a boat in
international and domestic waters as long as the captain
approves. Don’t do MURS on an aircraft, and you can’t
take advantage of MURS on the public telephone
network. You also can’t set up a MURS repeater.
If you are a representative of a foreign government
and have ideas about using MURS as a “spy network,”
don’t get caught because that’s not allowed. Building a
“Hogan’s Heroes” jail house MURS radio set is not
permitted either. Only sanctioned and licensed MURS
equipment can operate on the MURS frequencies. Don’t
feed a store and forward computing system with it either.
MURS is spread over five channels. The first three are
narrow band FM channels with 2. 5 kHz deviation.
Channels 4 and 5 are wide band FM channels with 5 kHz
deviation. The radios we will use in this project are “color
dot” devices. That means their
frequencies are designated by a
channel name which is described as
the color of a dot. Our radios fall into
the Blue Dot frequency of 154.570
MHz (channel 4).
Our MURS system consists of a
transmitter module and a receiver
module mounted on a set of common
carrier boards. The complete MURS
receiver module is shown in Photo 1.
The communications protocol is
implemented as half-duplex. There is
no automatic collision detection. So,
that’s something we will have to
handle either manually or via firmware
in a host microcontroller.
NBEK Carrier Board
NBEK is short for Narrow Band
Evaluation Kit. As you can see in
Schematic 1, the NBEK carrier board is
under the spell of a Microchip
PIC16F628. The PIC is officially called
the NBEK IC. The NBEK IC is
THE DESIGN CYCLE
Make Mine MURS
■ BY FRED EADY
60 November 2015
The IoT (Internet of Things) is hogging the spotlight here lately. That’s a good thing, as I am a
believer in using electronic devices for the betterment of mankind. I’m also for it because I
love to design and build all things electronic. When you get right down to it, those Io T
devices can’t do much without their radio equipment. The “things” working and
communicating out there in the IoT world are really cool. However, sometimes a “thing” may
not be the thing you need to get it done. In some cases, all you really need is a good old
fashioned radio to do the job.
■ Photo 1. Under the supervision of an eight-bit PIC, the MURS NBEK carrier
board provides all of the power and data interface circuitry for the radio board.