TECHNICAL PAPER
Narrow Band Radio
The buzz word these days
when it comes to data
transmission via radio
communications is
definitely spread
spectrum. However, the workhorse remains narrow band
radio communications.
T.L. Kirchner
President
Electronic Systems Technology, Inc.
Transmit Frequency= F1
Cannot Transmit and Receive at the Same Time
Transceiver
Receive Frequency = F1
Transceiver
Simplex System
Transmit Frequency= F1
Cannot Transmit and Receive at the Same Time
Narrow band radio transceivers transmit and receive
digital or analog data over a very narrow bandwidth (a
few kHz) in the licensed private radio frequency (RF)
spectrums. By contrast, with spread spectrum
technology, data is spread across a very wide bandwidth
(several MHz) in the unlicensed 900 MHz radio
spectrum.
This article provides an overview of narrow band radio
communications technology as used in radio telemetry
applications in the United States. It includes a look at
the advantages of narrow band technology, plus some
system design considerations.
Frequencies And The FCC
Examples of common frequencies licensed by the Federal
Communications Commission (FCC) for narrow band
radio telemetry are low band VHF (72 to 76 MHz), midband VHF (150 to 174 MHz), mid-band UHF (421 to 512
MHz), and high band UHF (greater than 800 MHz).
Prior to August 1996, 25 kHz channel spacing was the
norm for the above frequencies. This gave radio
manufacturers 20 kHz of occupied bandwidth to send
data. After August 1996, the FCC required all
manufacturers of new products to design transceivers for
12.5 kHz channel spacing with a usable bandwidth of
11.25 kHz. By the year 2005, the FCC will require 6.25
kHz spacing with a usable bandwidth of 6 kHz. The
main reason for subdividing the frequency spectrum is to
increase the existing number of usable channels. Radios
designed to meet these requirements are given the name
narrow band because the usable bandwidth is very small
compared to the operating frequency.
Bandwidth And Telemetry Applications
Data rate in bits-per-second (bps) is directly proportional
to bandwidth. To go faster with a shrinking bandwidth
requires more complicated modulation schemes from
radio designers. Typical data rates for 25 kHz, 12.5 kHz,
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Transceiver
Receive Frequency = F2
Transceiver
Half Duplex System
Transmit Frequency= F1
Transmit and Receive at the Same Time
Transceiver
Receive Frequency = F2
Transceiver
Full Duplex System
Figure 1: System Configurations
and 6.25 kHz are 9,600, 4,800, and 2,400 bps,
respectively, using existing modulation technology.
Transceivers based on new technology are able to send
19,200 bps for 25 kHz channels, 9,600 bps for 12.5 kHz
channels, and 4,800 bps for 6.25 kHz channels. This
will remain state-of-the-art until even more complex
modulation schemes become practical for consumer
products.
Types Of Narrow Band Radios
There are two major categories of radio transceivers
available to the user: dumb and smart. Dumb radios are
usually fixed frequency radio transceivers that have a
transmit input, receive output, and control lines. This is
the lowest-cost transceiver type, but it requires the
greatest technical know-how from the user when it comes
to interfacing and start-up procedures, and is limited in
system design flexibility.
Smart radios are available with various levels of
intelligence. The low-end category of smart radios will
have frequency agility (i.e., the user's ability to change
operating frequencies in the field) and system diagnostics
(e.g., received signal strength, power output, etc.). Highend radios typically have the above features, plus packet
burst technology with internal protocol drivers for
various manufacturers’ PLCs/RTUs for system design
flexibility and seamless interfacing to the users'
hardware. These features will reduce time and money in
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Narrow Band Radio
the design, installation, and start-up of the RF network
and reduce the technical level required of the integrator.
Repeater
Transceiver
System Configurations
The user has three choices of system configurations for
his/her narrow band telemetry system (Figure 1).
No Line
Of Sight
Transceiver
1.
2.
3.
Simplex-Transmit and receive on the same radio
frequency.
Half-duplex-Transmit on one frequency and receive
on another frequency, but not at the same time.
Full-duplex-Transmit on one frequency and receive
on another at the same time.
The majority of radio telemetry systems use either
simplex or half-duplex radios to support a point-to-point,
point-to-multiple point, or polled-with report-byexception communication scheme (Figure 2).
Point To Point
Transc eiver
1st Poll
Station 1
Transc eiver
3rd Poll
Station 3
Transc eiver
Transc eiver
Station 2
Master
Transc eiver
Polled
Figure 3: Repeater Diagram
Because radio waves won’t penetrate the mountain, you
need a repeater on the top of the mountain to relay the
message.
What Can Narrow Band Technology
Offer?
•
4th Poll
2nd Poll
Remote
Master
Narrow band radio, because it's licensed, offers the
following advantages over spread spectrum methods
when you need reliable coverage over a distance:
Transc eiver
Transc eiver
Transceiver
•
•
Station 4
Operating range is typically greater than unlicensed,
spread-spectrum radios.
FCC licensing brings order out of chaos.
The spectrum choices available provide range and
reliability.
Operating Range Is Greater
Transc eiver
Transc eiver
Station 1
Transc eiver
Transc eiver
Station 2
Report When Not
Being Polled
Master
Polled With Report By Exception
Station 3
Transc eiver
Station 4
The operating range of a radio system is a function of
five important variables.
1.
Figure 2: Communication Schemes
The cost/performance trade-off of using a full duplex
radio system for telemetry generally makes it impractical.
The older technology telemetry systems will usually be a
half-duplex system if a repeater is needed. The new
state-of-the-art telemetry systems typically are simplex
using narrow packet burst technology for SCADA
systems with or without repeater(s).
What Is A Repeater?
2.
Repeaters are used to overcome line-of-sight blockages in
your radio path, or to extend your radio range. For
example, you might use them to overcome the problem of
there being a mountain between the control room and a
well site you want to control (Figure 3).
Electronic Systems Technology
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RF power-Narrow band systems get a power boost
with respect to spread spectrum radios in that,
typically, the minimum power licensed by the FCC is
in the 2 to 4 W range for almost all of the
frequencies-with the exception of the low band VHF
spectrum, where 1 W is the typical power output. In
addition, in narrow band systems, the RF power is
concentrated in the narrow bandwidth discussed
above. This is the opposite of spread spectrum
technology, in which the RF power is spread across a
much larger bandwidth and limited to 1 W.
Receiver sensitivity-Receiver sensitivity is
fundamental to all radio technologies. The more
sensitive the receiver, the better the range. In theory,
doubling the receiver sensitivity has the same effect
on range as doubling the transmitter power.
However, a word of caution is in order here: what's
important is usable sensitivity; if the environment in
which the radio is being used is very noisy from
natural or man-made noise, the receiver will not be
Narrow Band Radio
3.
4.
5.
able to operate at its most sensitive level. At one
time, the higher frequencies were less noisy than the
lower frequencies-because there were fewer devices
operating in the higher frequencies. But this is not
the case anymore due to the proliferation of emitters
on all frequencies.
Frequency of operation-With narrow band systems,
you have a choice of operating frequencies to better
support your applications. The lower the frequency
of operation, the less will be the absorption of RF
power from environmental factors (e.g., terrain,
weather, foliage, etc.). If all of the items noted here
are equal, a 400 MHz system will have roughly twice
the range as a 900 MHz system.
Antenna gain-The principles of antenna operation
are common to all radio technologies-one of the most
critical being that the higher the antenna gain, the
better the range. Antenna gain will affect the
radiated power output as well as receiver sensitivity.
If you double the antenna gain, you will double your
receiver sensitivity when receiving, and double the
radiated RF power when transmitting. As antenna
gain increases, so does antenna size, but antenna
beamwidth decreases. If the antenna gain is held
constant, the higher the frequency of operation, the
smaller the antenna. Here are some practical
antenna design/specification considerations. Keep
the antenna as small as necessary to reduce cost,
wind loading, etc. Do not specify an antenna on
gain alone. Very high gain antennas have very
narrow vertical and horizontal beams, and may not
be usable in your application. With narrow beams, a
transmitted signal could skip over receivers located
close-in to the transmitter site, and also miss
receivers located outside the horizontal beam
(compass) angle.
System losses-Narrow band operation allows you to
operate at lower frequencies, giving you an
advantage over external losses (feedlines,
connectors, corrosion, etc.). The lower the operating
frequency, the less is the attenuation per foot in the
length of your feedline and connectors, and the fewer
are the effects of corrosion on all components. For
example, typical RG-8 coax (about 1/2-in. dia.) has
5.2 dB loss per 100 ft at 450 MHz and 8.6 dB loss
per 100 ft at 900 MHz. Remember, with every
increased 3 dB of loss, you lose one-half of the power
that with which you started, thus decreasing the
received signal intensity.
Unlicensed operation means the policing of the channel
or band of operation is the responsibility of the user. As
the number of unlicensed frequencies increases, cochannel interference (two or more devices operating on
the same frequency) and the noise floor (residual noise at
the frequency of operation) increases, causing a lower
effective data rate and/or loss of operating range.
Though the procedures and paperwork involved in the
licensing process can be a bit of a nuisance, there are
companies that will do turnkey site licensing. They
charge, on the average, $300 for this service and typically
take four to six weeks to complete the project. There is
licensing still available for more than 90% of the US. In
SCADA systems, you license a site not the individual
radios. On outdoor sites, you should license if possible.
What Is Packet Burst?
All packet systems, whether hardwired or radio, share the
same principle of operation: data is taken from a
standard RS-232C, RS-422, or RS-485 asynchronous port
and transmitted in blocks. Think of this block as an
electronic envelope that we call a packet (Figure 4). The
size of the packet can be defined by the user-usually in
the range of 1 to 2000 bytes of information. Reducing
the size of the packet allows better operation in high EMI
noise environments. Why? Because, by reducing the
packet size, you reduce transmission exposure time to the
radio waves, thereby increasing your probability of a
successful transmission.
Once this packet of data is formed, it is transmitted in a
burst, hence the term packet burst communications. If
more than one packet is required to send the data, a
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Transceiver
FCC Licensing Brings Order To Chaos
Sender
The FCC licensing requirement guarantees you a specific
frequency that you can call your own, and thus, brings
order to the chaos that would reign without it.
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Figure 4: Packet Diagram
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Narrow Band Radio
transceiver will go into full automatic mode and transmit
additional packets. Before a transmitter can transmit its
packet, it first listens to ensure that the air waves are
clear. This listen before transmit scheme is called carrier
sensed multiple access, or CSMA (the same protocol used
in Ethernet). To design a transceiver to communicate
with a network of radios, each radio has to be addressable
(address specific) such that only the transceiver you want
to talk to accepts your information. This CSMA scheme
provides efficient spectrum use by allowing only one unit
in the network to transmit at a given time, allowing many
individual users to share one frequency.
When a packet has been transmitted, every transceiver in
radio range on the same frequency hears it. When
received, the data packet is checked for accuracy. Most
packet burst radios use a 16-bit Cyclic Redundancy
Check (CRC), which is a very sophisticated method of
checking the packet’s data integrity. A 16-bit CRC
insures data integrity greater than one part in one
hundred million.
If the CRC is correct on the received packet, the data is
outputted to the user, and a positive acknowledgment
(ACK) is transmitted back to the sender. If the received
data does not pass the CRC test, the receiving radio will
not transmit an ACK, causing the sending radio to
resend (retry) only the data packet that didn't get
through.
With the CSMA technique, no polling station or token is
required in the radio network; every radio is a master,
meaning that any radio can communicate with any other
radio. The CSMA technique is a very efficient way to
manage a network of radios, and prevent communication
bottlenecks.
noise spectrum, reducing the probability of retries and
keeping the effective data rate high.
System Design Considerations
The major characteristics to review when choosing a
radio transceiver include (Figure 5):
•
•
•
•
•
•
In addition to CSMA, an anti-collision software scheme
is used to recover data if two or more units transmit at
exactly the same time. When this technique is added,
the technical term for this is CSMA-CD (collision
detection). By using the CSMA-CD technique, only one
frequency channel is needed, thereby saving valuable
radio spectrum space.
Because each radio has a specific address, the user is
able to route data through various radios to reach a final
destination. This technique is called digi-repeating.
Digi-repeating allows the user to route data through
existing data nodes to overcome line-of-sight blockages,
without purchasing additional hardware. In addition,
narrow band packet burst radios are very hardened for
operation in high-noise environments, because the
receiver is designed to receive a narrow passband and
reject all other frequencies. Using the listen-beforetransmit feature allows the radio to look for holes in the
Electronic Systems Technology
Effective data rate-Raw data rate is good, but it's the
effective data rate that gets the job done. The
effective data rate is the data rate less allowances for
transmitter turn-on time, data overheads, and how
the radio handles the data format of the PLC or other
controller to which the radio is interfaced.
Transmitter turn-on time-Transmitter turn-on time
will directly affect your polling time. It takes a few
ms to turn on a transmitter. If you are relaying a
signal through several transceivers, turn-on time can
be significant.
Error checking-The type of error checking that you
use will affect the number of retries required. The
number of retries will affect your data throughput.
RF site survey-Site layout and the use of repeaters to
overcome line-of-site blockages.
Lightning protection-Lightning protection can never
be overemphasized. Inadequate protection can mean
more than the financial loss of replacing your
equipment. It can mean the loss of an important
warning transmission when you desperately need it.
Grounding-If you are unfamiliar with proper
grounding procedures, have a professional do it.
Improper grounding can help lightning damage your
equipment, and can be responsible for injecting
unwanted noise into your system.
ANTENNA
ANTENNA FEEDLINE
RECOMMENDATIONS
1. Up to 50 ft. use RG-8
Coax.
2. Over 50 ft. use 1/2”
heliax.
RS-232C,422, or 485
INTERFACE CABLE
RG-8 Coax
Transceiver
User’s
Device
Earth
Ground
LIGHTNING ARRESTOR
12 VDC POWER SUPPLY
Figure 5: Typical Site Diagram
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Narrow Band Radio
•
•
•
•
Weatherproofing coax connections-If you've never
weatherproofed a coax connection, once again, have
a professional do it. When water or corrosive gases
get into coax connections, it's like putting a dead
short circuit on your signal.
Antenna types-When you have a site survey done, the
field engineer will probably specify antennas. As we
said earlier, high gain antennas are not always an
asset. Yagi antennas provide high gain in a narrow
horizontal beam width, say 45°. If your intended
receiver doesn't move around, the Yagi may be a
good choice. Vertical antennas emit signals equally
in every compass direction. However, high gain
verticals (and high gain Yagis) can reach out farther,
but have patterns that will produce comparatively
weaker received signals at nearby receivers. This
effect is known as the cone of silence.
Antenna height-Higher antenna placement can mean
better coverage over obstacles in the path of the
antenna-such as buildings, mountains, and trees.
The FCC may impose restrictions on antenna heights
in certain locales. Check your license.
Feedline length-Higher antennas can mean
improved signals, but the gain in signal from a
higher antenna could easily be offset by the increased
loss in feedline. If the improved signal level is
crucial, use low-loss feedline, such as Andrew
Corp.'s Heliax(r). Heliax will be several times more
expensive than the RG-8 coax cable you get at Radio
Shack(r), but in addition to having much lower loss
at high frequencies (500 MHz and above), it should
have a longer life expectancy.
I didn't mention modulation type because all modern data
radios use frequency modulation (FM) for greater
immunity to natural/man-made noise.
What is a RF Site Survey?
A radio site is an on-site analysis of the customer's
existing frequency spectrum and topography to determine
how the radio system will perform in a given
geographical area. Numerous factors can make the
difference between a reliable and marginal radio system,
including environmental conditions, antenna placement,
and blockages to the line of site between antennas. Just
remember: you must go beyond a computer model and
actually gather data from the site. Subtle factors such as
buildings and environmental changes are not analyzed by
computer models, and thus, are often ignored. These
items can only be found by a hands-on site analysis.
The first step in a successful site survey is a review of
terrain maps of the site. These maps will give the field
engineer an idea of possible repeater locations, and of
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equipment that will be required for the field testing.
Preparation can be as valuable as analysis of the data, if
applied correctly.
When the field engineer arrives on the site, he or she will
perform an analysis of the available frequency spectra.
All emitters will be logged by frequency and amplitude to
determine your best frequency of operation. When the
test frequency is determined, the engineer will proceed to
all remote locations to create a working model of the
radio system. During this testing, the engineer will
gather data on: received signal strength; antenna type,
height, and placement; and reliability of data transfer
between all stations. This modeling of a working system
will give realistic data on site performance, in addition to
field strength data.
When the site survey is completed, the engineer will
compile all data into a site survey report. This report will
contain all information gathered from the site survey, and
can be used as a how to manual for the installation of the
radio system.
A site survey is suggested for all applications of a radio
system, but is highly recommended for sites with
multiple stations spread over a large area. An on-site
analysis can answer the questions that arise during the
design and installation of a radio network, prior to them
becoming a problem.
Get It Right The First Time
In conclusion, if you're planning a licensed radio system,
you need to have radio engineering professionals look at
your plans and your site. You may have more than one
vendor in mind. Work up a request for proposal and/or
bid spec. Carefully look over the numbers you get in
reply. Use common sense. Remember, it's better to
specify a little more than what you really need than to go
back and do the job all over.
This document is copyrighted by Electronic Systems Technology
(EST) with all rights reserved. Under the copyright laws, this
document may not be copied, in whole or part, without the written
consent of EST. Under the law, copying includes translating into
another language. EST, EST logo, and ESTeem are registered
trademarks of Electronic Systems Technology, Inc. Simultaneously
published in the United States and Canada. All rights reserved. For
more information contact: Electronic Systems Technology, Inc., 415
North Quay Street, Kennewick, WA 99336
Ph: (509) 735-9092 Fax: (509) 783-5475
www.esteem.com
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