12/20/2013
ASA, CSSA, SSSA International Annual Meeting, Nov 3-6, 2013, Tampa, FL
Symposium: Wireless Technologies and Innovations To Meet Food,
Water, and Energy Challenges: I
Martin A. Hebel
Associate Professor
Electronic Systems Technologies
Southern Illinois University Carbondale
1
12/20/2013










About the Presenter
Wireless Sensor Networks
RF Signal Power & Isotropic Radiators
Free Space Losses
Antenna Types, Design & Gain
Reflections & Multipath Issues
Fresnel Zone & Signal Clearance
Signal Absorption
Wireless Nodes and Mesh Networks
Conclusion
2
12/20/2013



Technologist & Educator at Southern Illinois
University (SIU).
Collaborations using wireless network include:
◦ Agricultural Sciences at SIU, RF signal testing.
◦ University of Florida, wireless citrus vibration monitoring
during harvesting.
◦ USDA, Bushland, Tx, Infrared crop infrared canopy
monitoring for center-pivot irrigation using wireless
networks.
◦ University of Sassari, Italy: Wireless biometric monitoring
using parallel processing.
Primary Author of Getting Started with XBee
Modules on programming for communications
with XBee wireless transceivers.
3
12/20/2013



Wireless Sensor Networks (WSNs) is a collection
of inexpensive, low-power transceivers used to
transmit sensor and control information.
Commonly use Low-Rate, Wireless Personal Area
Network (LR-WPAN) protocol (IEEE-802.15.4),
and ZigBee protocols at higher layers. Optimized
for WSN use.
Other protocols may be used, such as BlueTooth
(IEEE 802.15.1), WiFi (IEEE 802.11), or others.
4
12/20/2013



The LR-WPAN protocols provide point-topoint communications.
The ZigBee protocols (and others) provide
mesh networking and routing of data.
Typically operate in the 2.4GHz ISM band,
though 900Mhz is popular as well.
5
12/20/2013



Signal power is important in gauging the distance
that may covered with the wireless network.
Transmitter power commonly ranges from 1mW
(0 dBm) to 100mW (20 dBm). The allowed
maximum for 2.4Ghz, including antenna gain, is
4W (36dBm).
Common minimum detectable signal power at
the receiver is around 0.1pW (-100 dBm)
6
12/20/2013



dBm is power in decibels (dB) referenced to
1mW, such that 1mW = 0dBm
dBm = 10Log(Power/1mw)
+3dB is a doubling of power
-3dB is a halving of power
A signal strength of -6dBm is -3dB + -3dB
1mW / 2 / 2 = 0.25 mW
+10dB is an increase in power of 10 times
-10dB is a decrease in power of 10 times.
A signal strength of 20dBm is +10dB+ 10dB
1mW x 10 x 10 = 100mW
7
12/20/2013


While a transmitter may transmit 100mW of
power, the power at the receiver is critical in
the communications link.
The isotropic point source is a theoretical
antenna that emits power equally in all
directions (perfectly omni-directional).
8
12/20/2013
◦ The loss of signal in free space is directly
proportional to the square of the distance and
square of the frequency.
◦ Calculating Free Space Loss (FSPL):
◦ d is distance in meters
f is frequency in Hertz
c is speed of light, 3 x 108 m/s
◦ Typically reported in dB: 10Log(FSPL)
9
12/20/2013



For a 2.4GHz signal at 1Km, the loss would
be approximately 100dB.
Based on a transmitter with power of 0dBm to
20 dB and a receiver with a sensitivity of
100dBm, it would appear our signal could be
received at a distance of 1km.
This is based on an isotropic point source
through free space.
10
12/20/2013



Antennas can be omni-directional or directional.
Due to alignment needs of directional antennas,
omni-directional are typically used.
Unlike the theoretical isotropic antenna, real
antennas focus more energy in certain directions.
Antenna power is measured in dBi – power (gain)
referenced to an isotropic source. An antenna of
3dBi would have twice the power in the radiated
directions as compared to the isotropic.
11
12/20/2013

The Dipole antenna is a common
omni-directional antenna.
◦ Typically sized ½ the wavelength of the signal.
The wavelength of a 2.4GHz wave is 12.5cm
( = c / f = speed of light / frequency)
The antenna therefore would be 6.25cm.
Made up from two ¼ wavelength segments.
◦ The radiation pattern is donut shaped.
◦ Typical gain value of 2.15dBi.
(Ref 1)
(Ref 2)
12
12/20/2013




Monopole antennas are essentially ½ a dipole ( /4),
with the ground plane acting as the other pole.
Often called whip or wire antennas (such as
automobile antennas).
Radiation pattern is similar to the dipole, but the
shape is affected by the ground plane size.
Gain is dependent on ground plane size. For small
devices a value of 1.5dBi is typically, but for
broadcast antennas can reach 6dBi.
(Ref 2)
13
12/20/2013

Chips Antenna are very low profile, but have
uneven radiation patterns and gain values of
-1.5dBi are common.
14
12/20/2013


Directional antenna focus energy in certain
directions.
Yagi-Uda antenna.
◦ Gains of 15dBi or more can be achieved.
(Ref 2)
◦ Parabolic dish antennas can achieve gains of 60dBi.
15
12/20/2013

With antenna gain, the power budget includes
the gains at the transmitter and receiver.
PR = PT + GT – FSPL + GR

For 2.4GHz at 1km, with a 100mW
transmitter and through free space, using
dipole antennas on each end:
PR = 20dBm – 100dB + 2.15dB + 2.15dB
= -75.7dBm
16
12/20/2013



The longer the wavelength (lower the
frequency), the further it goes.
The longer the wavelength (lower the
frequency), the better it travels through and
around objects.
The shorter the wavelength (higher the
frequency), the more data it can transport.
* Adapted from Ref 3.
17
12/20/2013


Based on only free-space loss, it would
appear that 2 antennas, 1km apart, with lineof-sight (LOS), would only have a loss of
around 100dB.
Other factors such as reflections (even with
directional antennas), will affect the power
budget.
18
12/20/2013



Reflections reaching the receivers can
dramatically affect the received signal due to
signals arriving on different paths causing
multipath issues.
Waves from the source arriving in different paths
can cause constructive interference (waves aid
each other), and destructive interference (wave
cancel each other).
Time delays based on distances travelled on
different paths can also lead to data issues.
19
12/20/2013

Major sources of reflections are metals and
water (moisture in materials or covering
materials, such as dew). These can be the
most difficult to plan for.
(Ref. 4)
20
12/20/2013


Fresnel Zones define the areas of highest
reflections. While different zones exist,
typically the 1st zone is of most concern.
If 60% of the radius (r) of the zone is kept
clear, it will be a good approximation of free
space.
(Ref. 4)
21
12/20/2013


The radius for 2.4GHz can be found with the
formula:
At a distance of 1km, rm calculates to be
5.6 meters. Keeping at least 60% clear, would
require a clearance height of at least
3.35 meters.
22
12/20/2013


The signal strength
at receivers for
2.4GHz devices
were tested for
distances up to 200
meters with varying
antenna heights
from 0.25m to
2.0m (Ref 5).
Tested with a 1mW
transmitter (0dBm)
over a mowed,
grass plain.
(Ref. 5)
23
12/20/2013



The permittivity, or dielectric constant, of a
material defines how well it pass RF signals.
At 2.4GHz, water is a major absorber of the
energy. Moisture in materials can dramatically
affect signal strength.
To test non-metallic materials to find out if they
absorb 2.4GHz energy, try to heat the material in
a microwave oven – which operates at 2.4GHz.
24
12/20/2013


Researchers at the citrus research center at
the University of Florida noted lower receiver
power levels in the early morning hours as
compared to later in the day.
This was attributed to dew in the area which
burnt off as the morning progressed.
25
12/20/2013


Nodes on the WSN can use
inexpensive RF transceiver
(such as the XBee)
containing the protocol
stack.
For the device application, it
is coupled with a
microcontroller
programmed to interact
with the real-world devices
and communicate to the
transceiver.
(Ref. 6)
26
12/20/2013

Wireless Sensor
Networks may use
a mesh network
architecture, such
as ZigBee, to span
distances. Router
Nodes may be used
to move data
between end
nodes.
(Ref. 5)
27
12/20/2013



When implementing a wireless system, some
losses due to distance and gains based on
antennas used can be estimated and factored
into the planning.
Issues of reflections and multipath signals, can
be reduced by understanding Fresnel zones.
Moisture, being a major absorber and reflector of
RF energy, is of major concern in agricultural
environments.
28
12/20/2013






1: http://en.wikipedia.org/wiki/Dipole_antenna
2: http://www.antenna-theory.com/antennas/dipole.php
3: (2013) Wireless Networking in the Developing World, 3rd Ed.
http://wndw.net
4: Hebel, M. A., Bricker, G. (2010). Getting started with Xbee RF
Modules. Parallax, Inc., Rocklin, CA.
5: Hebel, M. A., Tate, R. F, & Watson, D. G. (2007). Results of
wireless sensor network transceiver testing for agricultural
applications. ASABE International Meeting, Minneapolis, Mn.
6: Hebel, M. A. (2006). Meeting Wide-Area Agricultural Data
Acquisition And Control Challenges Through Zigbee Wireless
Network Technology. 4th Annual WCCA Conference, Orlando, FL.
29