rf lecture 1

8/11/2019 RF Lecture 1

1/24

RF Communication Lecture # 1

Dr. Irfan Ahmed

Introduction to RF Propagation

By: Seybold J.S.


2/24

1.1 FREQUENCY DESIGNATIONS

The electromagnetic spectrum is loosely divided into regions as

shown in Table 1.1 [1].


3/24

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).


4/24

ITU-Region MAP


5/24

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


6/24

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


7/24

Example LOS


8/24

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.


9/24

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.


10/24

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


11/24

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.


12/24

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.


13/24

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


14/24

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


15/24

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.



16/24


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.


17/24


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.


18/24


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


19/24

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.


20/24

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.


21/24

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.


22/24


23/24


24/24

rf lecture 1

Documents