rf lecture 1
TRANSCRIPT
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RF Communication Lecture # 1
Dr. Irfan Ahmed
Introduction to RF Propagation
By: Seybold J.S.
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1.1 FREQUENCY DESIGNATIONS
The electromagnetic spectrum is loosely divided into regions as
shown in Table 1.1 [1].
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During World War II, letters were used to designate various frequency bands, particularly
those used for radar. These designations were classified at the time, but have found their
way into mainstream use. The band identifiers may be used to refer to a nominal frequency
range or specific frequency ranges. Table 1.2 shows the nominal band designations and
the official radar band designations in Region 2 as determined by international
agreement through the International Telecommunications Union (ITU).
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ITU-Region MAP
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1.2 MODES OF PROPAGATION
For most RF propagation modeling, it is sufficient to visualize the electromagnetic wave
by a ray (the Poynting vector) in the direction of propagation.
In free space, electromagnetic waves are modeled as propagating outward from the
source in all directions, resulting in a spherical wave front. Such a source is called an
isotropic radiator and in the strictest sense, does not exist.
As the distance from the source increases, the spherical wave (or phase) front converges
to a planar wave front over any finite area of interest, which is how the propagation is
modeled.
The power density on the surface of an imaginary sphere surrounding the
RF source can be expressed as
where d is the diameter of the imaginary sphere, P is the total power at the source, and S is
the power density on the surface of the sphere in watts/m2.
P = EH
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1.2.1 Line-of-Sight Propagation and the Radio Horizon
When considering line-of-sight (LOS)
propagation, it may be necessary toconsider the curvature of the earth
(Figure 1.1). The curvature of the
earth is a fundamental geometric
limit on LOS propagation. In
particular, if the distance between the
transmitter and receiver is largecompared to the height of the
antennas, then an LOS may not exist.
The simplest model is to treat the
earth as a sphere with a radius
equivalent to the equatorial radius of
the earth.
where d is the distance to the radio horizon
in miles and h is in feet
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Example LOS
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1.2.2 Non-LOS Propagation
The mechanisms of non-LOS propagation vary considerably, based on
the operating frequency. At VHF and UHF frequencies, indirect
propagation is often used. Examples of indirect propagation are cell phones, pagers, and some
military communications. An LOS may or may not exist for these
systems.
In the absence of an LOS path, diffraction, refraction, and/or multipath
reflections are the dominant propagation modes Diffractionis the phenomenon of electromagnetic waves bending at
the edge of a blockage, resulting in the shadow of the blockage being
partially filled-in.
Refractionis the bending of electromagnetic waves due to in-
homogeniety in the medium.
Multipathis the effect of reflections from multiple objects in the field
of view, which can result in many different copies of the wave arriving
at the receiver.
The over-the-horizon propagation effects are loosely categorized as
sky waves, tropospheric waves, and ground waves.
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Sky waves are based on ionospheric reflection/refraction
and are discussed presently. Tropospheric waves are those electromagnetic waves that
propagate through and remain in the lower atmosphere.
Ground waves include
Surface waves, which follow the earths contour
Space waves, which include direct, LOS propagation as
well as ground-bounce propagation.
1.2.2 Non-LOS Propagation contd.
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1.2.2.1 Ind irect o r Obstructed Propagation
Indirect propagation aptly describes terrestrial propagation
where the LOS is obstructed
In such cases, reflection from and diffraction around buildingsand foliage may provide enough signal strength
HFfrequencies can penetrate buildings and heavy foliage quite
easily
Above UHF, indirect propagation becomes very inefficient and isseldom used
When the features of the obstruction are large compared to the
wavelength, the obstruction will tend to reflect or diffractthe wave
rather than scatterit
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1.2.2.2 Tropospheric Propagation
The troposphere is the first (lowest) 10 km of the atmosphere,
where weather effects exist.
Tropospheric propagation consists of reflection (refraction) of RF
from temperature and moisture layers in the atmosphere.
Tropospheric propagation is less reliable than ionospheric
This effect is sometimes called ducting, although technically
ducting consists of an elevated channel or duct in the atmosphere.
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1.2.2.3 Ionospheric Propagation
The ionosphereis an ionized plasma around the earth that is essential to
sky-wave propagation and provides the basis for nearly all HF
communications beyond the horizon. It is also important in the study of satellite communications at higher
frequencies since the signals must transverse the ionosphere, resulting in
refraction, attenuation, depolarization, and dispersion due to frequency
dependent group delay and scattering.
HFcommunication relying on ionospheric propagation was once thebackbone of all long-distance communication.
Over the last few decades, ionospheric propagation has become primarily
the domain of shortwavebroadcasters and radio amateurs.
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1.2.3 Propagation Effects as a Function of Frequency
Very low frequency (VLF) band covers 330kHz
The VLF band only permits narrow bandwidths to be used (theentire band is only 27kHz wide)
The primarily mode of propagation in the VLF range is ground-
wavepropagation
VLF has been successfully used with underground antennas for
submarinecommunication
The low frequency dictates that large antennas are required to
achieve a reasonable efficiency
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1.2.3 Propagation Effects as a Function of Frequency contd
The low-(LF) and medium-frequency (MF) bands, cover the range
from 30kHz to 3MHz.
Both bands use ground-wavepropagation and some sky wave. While the wavelengths are smaller than the VLF band, these bands
still require very large antennas.
These frequencies permit slightly greater bandwidth than
the VLFband.
Uses include broadcast AM radio and theWWVB time referencesignal that is broadcast at 60 kHz for automatic (atomic) clocks.
WWVBis a NISTtime signalradio station near Fort Collins,
Colorado.
http://en.wikipedia.org/wiki/NISThttp://en.wikipedia.org/wiki/Time_signalhttp://en.wikipedia.org/wiki/Fort_Collins,_Coloradohttp://en.wikipedia.org/wiki/Fort_Collins,_Coloradohttp://en.wikipedia.org/wiki/Fort_Collins,_Coloradohttp://en.wikipedia.org/wiki/Fort_Collins,_Coloradohttp://en.wikipedia.org/wiki/Time_signalhttp://en.wikipedia.org/wiki/NIST -
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The high-frequency (HF), band covers 330MHz.
Most HF communication is via sky wave. There are few remaining commercial
uses due to unreliability, but HF sky waves were once the primary means of long-
distance communication.
One exception is international AM shortwave broadcasts, which still rely on
ionospheric propagation to reach most of their listeners.
The HF band includes citizens band (CB) radio at 27MHz.CB radio is an example
of poor frequency reuse planning. While intended for short-range communication,
CB signals are readily propagated via sky wave and can often be heard hundreds
of miles away.
The advantages of the HF band include inexpensiveand widely available
equipment and reasonably sized antennas, which was likely the original
reason for the CB frequency selection
Several segments of the HF band are still used for amateur radio and for militaryground and over-the-horizon communication.
1.2.3 Propagation Effects as a Function of Frequency contd
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1.2.3 Propagation Effects as a Function of Frequency contd
The very high frequency (VHF) and ultra-high frequency (UHF)cover frequencies
from 30MHz to 3GHz .
In these ranges, there is very little ionospheric propagation, which makes them
ideal for frequency reuse.
For the most part, VHF and UHF waves travel by LOS and ground-bounce
propagation.
VHF and UHF systems can employ moderately sized antennas, making these
frequencies a good choice for mobile communications.
Applications of these frequencies include broadcast FM radio, aircraft radio,cellular/PCS telephones, pagers, public service radio such as police and fire
departments, and the Global Positioning System (GPS).
These bands are the region where satellite communication begins since the signals
can penetrate the ionosphere with minimal loss.
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1.2.3 Propagation Effects as a Function of Frequency contd
The super-high-frequency (SHF) frequencies include 330GHz
and use strictly LOS propagation.
In this band, very small antennas can be employed, or, moretypically,moderately sized directional antennas with high gain
Applications of the SHF band include satellite communications,
direct broadcast satellite television, and point-to-point links.
Precipitation and gaseous absorption can be an issue in these
frequency ranges, particularly near the higher end of the range
and at longer distances.
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1.2.3 Propagation Effects as a Function of Frequency contd
The extra-high-frequency (EHF) band covers 30300GHz and
is often called millimeter wave.
In this region, much greater bandwidths are available. Propagation is strictly LOS, and precipitation and gaseous
absorption are a significant issue.
The Atacama Large Millimeter/sub-millimeter Array(ALMA) is
an astronomical interferometerof radio telescopesin the Atacama
desertof northern Chile
http://en.wikipedia.org/wiki/Astronomical_interferometerhttp://en.wikipedia.org/wiki/Radio_telescopehttp://en.wikipedia.org/wiki/Atacama_deserthttp://en.wikipedia.org/wiki/Atacama_deserthttp://en.wikipedia.org/wiki/Chilehttp://en.wikipedia.org/wiki/Chilehttp://en.wikipedia.org/wiki/Atacama_deserthttp://en.wikipedia.org/wiki/Atacama_deserthttp://en.wikipedia.org/wiki/Radio_telescopehttp://en.wikipedia.org/wiki/Astronomical_interferometer -
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1.3 WHY MODEL PROPAGATION?
The goal of propagation modeling is often to determine the probability of
satisfactory performance of a communication system or other system that is
dependent upon EM wave propagation.
If the modeling is too conservative, excessive costs may be incurred, whereas too
liberalof modeling can result in unsatisfactory performance. Thus the fidelity of
the modeling must fit the intended application.
For communication planning, the modeling of the propagation channel is for the
purpose of predicting the received signal strength at the end of the link.
In addition to signal strength, there are other channel impairments that candegrade link performance. These impairments include
delay spread
(smearing in time) due to multipath
rapid signal fading within a symbol (distortion of the signal spectrum).
These effects must be considered by the equipment designer, but are notgenerally considered as part of communication link planning. Instead, it is
assumed that the hardware has been adequately designed for the channel.
In some cases this may not hold true and a communication link with sufficient
receive signal strength may not perform well.
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1.4 MODEL SELECTION AND APPLICATION
The selection of the model to be used for a particular application often
turns out to be as much art as it is science
Corporate culture may dictate which models will be used for a given
application Generally, it is a good idea to employ two or more independent models if
they are available and use the results as bounds on the expected
performance.
The process of propagation modeling is necessarily a statistical one, and
the results of a propagation analysis should be used accordingly
There may be a temptation to shop different models until one is found
that provides the desired answer. Needless to say, this can lead to
disappointment
Even so, it may be valuable for certain circumstances such as highlycompetitive marketing or proposal development
It is important that the designer not be lulled into placing too much
confidence in the results of a single model.
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1.4.1 Model Sources
Many situations of interest have relatively mature models based upon large
amounts of empirical data collected specifically for the purpose of
characterizing propagation for that application
There are also a variety of proprietary models based on data collected for
very specific applications
For more widely accepted models, organizations like the International
Telecommunications Union (ITU) provide recommendations for modeling
various types of propagation impairments While these models may not always be the best suited for a particular
application, their wide acceptance makes them valuable as a benchmark.
There exist a number of commercially available propagation modeling
software packages. Most of these packages employ standard modeling
techniques.
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