Wire and Wireless Communication

7/30/2019 Wire and Wireless Communication

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Wire and Wireless

Communication


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Microwave Communication

Generally described as electromagnetic waves with frequencies that range from 500 MHz to300 GHz or more.

Have relatively short wavelengths due to their inherently high frequencies. At any given radio station, transmitters are normally operating on either the low or the high

band, while receivers are operating on the other hand.

Short Haul category for intrastate or feeder service microwave systems. It is used to carry

information for relatively short distances, such as between cities with the same state.

Long Haul interstate and backbone route application. It is used to carry information for relatively

long distances.

FDM-FM used in early microwave systems

TDM-PCM used to carry information for relatively long distances.

Terminal Stations point at which information terminate or originate.

Microwave radios propagate signals through Earths atmosphere between transmitters and

receivers often located on top of the towers spaced about 15mi to 30 mi apart.

Advantages of Microwave Radio

Radio systems do not require a right-of-way acquisition between stations. Each station requires the purchase or lease of only a small area of land. Because of their high operating frequencies, microwave radio systems can carry large quantity

of information.

High frequencies mean short wavelength, which require relatively small antenna. Radio signals are more easily propagated around physical obstacles such as water and high

mountains.

Fewer repeaters are necessary for amplification. Distances between switching centers are less. Underground facilities are minimized. Minimal crosstalk exists between voice channels. Minimum delay times are introduced. Increased reliability and less maintenance are important factors.

Disadvantages of Microwave Radio

It is more difficult to analyze and design circuits at microwave frequencies. Measuring techniques are more difficult to perfect and implement at microwave frequencies. It is more difficult to implement conventional circuit components at microwave frequencies. Transient is more critical at microwave frequency. Specialized components are necessary. Propagate at straight line, so does limited to LOS.


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FM

Used in microwave radio systems rather than amplitude modulation because AM signals aremore sensitive to amplitude nonlinearities inherent in wideband microwave amplifiers

Less sensitive to random noise and can be propagated with lower transmit powers.Intermodulation Noise

A major factor when designing FM radio systems In an AM, it is caused by repeater amplitude nonlinearity and is a function of signal amplitude. In FM, it is caused primarily by transmission gain and delay distortion and is a function of signal

amplitude and magnitude of the frequency deviation.

FM Microwave radio System

Widely recognized as providing flexible, reliable and economical point-to-point

communications using Earths atmosphere for the transmission medium.

Baseband

The composite signal that modulates the FM carrier and may comprise one or more of the

following:

1. FDM voice-band channels2. TDM voice-band channels3. Broadcast-quality composite video or picture phone4. wideband data

FM Microwave Radio Transmitter

FM Deviator provides the modulation of the IF carrier that eventually becomes the main microwave

carrier.

Pre-emphasis follows the FM deviator and provides an artificial boost in amplitude to the higher

baseband frequencies.

Channel Combining Network

Provides a means of connecting more than one microwave transmitter to a single transmission

line feeding the antenna.

FM Microwave Radio Receiver similar to the conventional FM receiver

FM Microwave Radio Repeaters

Passive Active

Permissible distance between an FM microwave transmitter and its associated microwave

receiver depends on:


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1. Transmitter output power2. Receiver noise threshold (S/N)3. Terrain4. Atmospheric conditions5. System capacity6. Reliability objectives7. Performance expectations.

Microwave Repeater a receiver and a transmitter placed back-to-back or in tandem in the system.

Repeater Stations receives a signal, amplifies and reshapes it, and then retransmits the signal to the

next repeater or terminal stations down line from it.

Types of Microwave Repeaters

1. IF Repeater Here, the received RF carrier is down-converted to an IF frequency, amplified,

reshaped, up-converted to an RF frequency, and then retransmitted.

Signal is never demodulated below IF Baseband intelligence is unmodified by the repeater aka heterodyne repeaters most commonly used

2. Baseband Repeater Here, the received RF carrier is down-converted to an IF frequency, amplified, filtered,

and then further demodulated to baseband.

Baseband frequency is then reconfigured, afterwards it FM modulates an IF carrier,which is up-converted to an RF carrier and then retransmitted.

It is the most complicated system3. RF Repeater

Here, the received microwave signal is not down-converted to IF or baseband. It is simply mixed with a local oscillator frequency in a nonlinear mixer. Signal is neither reconfigured nor reshaped. Signal is just simply converted in frequency and then reamplified and transmitted.

Radio Fades temporary reduction in signal strength due to varying atmospheric condition.

AGC built into radio receivers to compensate the radio signal fades less than 40 dB.

Diversity

Suggests that there is more than on transmission path or method of transmission availablebetween a transmitter and receiver.

Increases the reliability of the system by increasing its availability


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Provides an alternate transmission path for only a single microwave link within the overallcommunications system.

Provides 100% protection

Most Common Method of Diversity

1. Frequency Diversity Modulating two different RF carrier frequencies with the sane IF intelligence, then

transmitting both RF signals to a given destination.

This arrangement provides complete and simple equipment redundancy and has anadditional advantage of providing two complete transmitter-to-receiver electrical

paths.

2. Space Diversity Uses two or more antennas with the same frequency

3. Polarization Diversity A single RF carrier is propagated with two different electromagnetic polarizations Generally used in conjunction with space diversity

4. Receiver Diversity Uses more than one receiver for a single radio-frequency channel.

5. Quad Diversity Combines frequency, space, polarization, and receiver diversity into one system.

6. Hybrid Diversity Combines the operational advantage of frequency diversity with the improved

protection of space diversity

Protection Switching

Arrangement which provides alternate facilities at avoid a service interruption during periodsof deep fades or equipment failures.

Provides protection for a much larger section of the communications system that generallyincludes several repeaters spanning a distance of 100 miles or more.

Types of Protection Switching Arrangement

1. Diversity A single back-up channel is made available to as much as 11 working channels Offers 100% protection only to the first working channel to fail.

2. Hot Standby Each working radio channel has a dedicated back-up or spare channel.

FM Microwave Radio Stations

Terminal Stations points where baseband signals either originate or terminate. It is composed of

four major stations:


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Wireline Entrance Link serves as the interface between the multiplex terminalequipment and the FM-IF equipment.

Baseband IF section RF section

Transmod (Transmit Modulator) A balanced modulator that, when used in conjunction with a

microwave generator, PA, and BPF, up-converts the IF carrier to an RF carrier and amplifies the RF to

the desired output power.

Microwave Generator provides the RF carrier input to the up-converter

Isolator a unidirectional device often made from a ferrite material. It is used in conjunction with a

channel-coupling network to prevent the output of one transmitter from interfering with the output of

another transmitter.

LNA (Low Noise Amplifier) Commonly a tunnel diode or parametric amplifiers

Radio Wave Propagation

Propagation Methods

Refers to how a radio wave arrives from a radio transmitting antenna into the receivingantenna

1. Ground-Wave Propagation Radio waves follow the curvature of the Earth and can travel at distances beyond the horizon. Primarily used by AM broadcasting Best applicable for VLF, LF and MF or frequencies only up to 2 MHz

2. Sky-Wave (Ionospheric Propagation) Follows the principle of refraction As the radio waves strikes on a certain ionospheric layer, it will be refracted and go back to

Earth at a certain distance for reception.

Used for HF communication systems, including long-distance radio-telephone and soundbroadcasting.

Generally effective above 2 MHz up to 30 MHzLayers of Earths Atmosphere

1. Troposphere Lowest layer from Earths surface up to approximately 10 km above.

2. Stratosphere Second layer in the height that extends from the upper limit of the troposphere at an

approximate elevation of 50 km.

Has constant/non-fluctuating temperature and do not have the capability to refractradio waves.

3. Ionosphere Upper limit of the stratosphere at a distance of approximately 60 to 400 km.


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Most ionized layer.Layers of the Ionosphere

1. D Layer Lowest layer of the ionosphere Average daytime height of 70 km and thickness of 10 km. Can refract VLF and LF waves and thus aids surface wave propagation. In daytime, it absorbs frequencies below 8 or 10 MHz absorbs MF (300 3 MHz)

2. E Layer Exists about 100 km during day and a thickness of about 25 km. Aids MF surface wave propagation Can also refract some HF on daytime up to approximately 20 MHz

3. F1 Layer Exists at a height of 180 km in day Thickness of 20 km at daytime Provides absorption of HF waves.

4. F2 Layer Most important layer and responsible for refracting HF waves At daytime, it has an approximate height of about 250 to 400 km and thickness of

200 km.

Recombines with F!1 at night and having a height of 300 km.Factors Affecting the Ability of Ionosphere to Refract Radio Waves:

1. Ion Density The greater the degree on ionization, the greater the refraction or bending of wave at

any given frequency.

2. Frequency of the Radio Waves The lower the frequency, the more easily the signal is refracted.

3. Angle of Radiation/Angle of Transmission The greater the angle of radiation, the greater the bending of the wave.

Virtual Height Apparent height of the ionized layer, as determined by the time interval between the

transmitted signal and the ionospheric echo at vertical incidence.

Critical FrequencyThe highest frequency that will be returned down to Earth by that layer after

having been beamed vertically straight upward or at normal incidence.

Maximum Usable FrequencyThe highest frequency that will be returned down to Earth over a given

path.

Optimum Working Frequency

Best frequency used to operate a sky-wave link. Frequency that gives the most stable link between transmitters and receivers in sky-

wave propagation.


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Lowest Usable Frequency The lower limit of the range on frequencies that provide useful

communications between the two given points by ionospheric refraction.

Gyrofrequency The frequency where the periodic time of the wave is equal to the time required for

one complete revolution about the magnetic axis, the path of the electrons becomes a very wide single

loop.

Critical Angle Highest angle of refraction that will return the wave to the Earth.

Skip Distance The minimum distance over which communication at a given frequency (usually MUF)

can be established by means of the sky-wave.

Skip Zone The area that lies between the outer limit of the ground wave range and the inner edge of

energy return from the ionosphere.

Hop Refers to a single reflection of a radio wave from the ionosphere back to the Earth.

Multihop Multiple reflection and refractions thus increasing the coverage along Earths ground in

sky-wave propagation.

Factors Affecting Optimum Operating Frequency

1. Location and Geography2. Seasonal Variations

Brought about by the revolution of the Earth around the sun.3. Diurnal Variations

Brought about by the rotation of the Earth in its own axis.4. Cyclical Variations

Ionospheric Irregularities

1. Sudden Ionospheric Disturbances (SID) Caused by solar flares which are gigantic emissionof hydrogen from the sun.

2. Travelling Ionospheric Disturbances (TID) Seriously affect the accuracy of high-frequencydirection finders due to irregularities of electron densities in the ionosphere.

3. Ionospheric Storms Caused by particle emissions from the sun.4. Fading The fluctuation of signal strength at the receiver.

Types of Fading

a. Interference Fadingb. Polarization Fadingc. Focusing and Defocusingd. Absorption Fadinge. Selective Fading

Forms of Selective Fading


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a. Rayleigh Fading Occurs when the signal received by an antenna is the result of reflectedsignals from a number of nearby objects and there is no signal path between transmitter and

receiver.

b. Rician Fading Similar to Rayleigh fading, but now there is also a direct radio path betweenthe transmitter and receiver.

c. Multipath Fading Occured when the time delay of the reflected signals is long compared tothe modulation periodic time.

3. Space-Wave (Tropospheric) Propagation Line-of-sight propagation Compulsory when frequency generally exceeds 30 MHz and beyond up to 300 GHz. Used for TV broadcasting and for mobile systems operating in the VHF, UHF and SHF bands.

Irregularities of Space Wave Propagation

1. Superrefraction or Ducting Occurs when the refractive index of the air decreases with height b=much more rapidly

than normal.

Duct a region in which superrefraction occurs.

Formed in the troposphere when a layer of cool air becomes trapped underneath a layerof warmer air of when a layer of cooled air becomes sandwich between two layers of

warmer air.

2. Subrefraction Reduces signal strength by bending the ray away from the receiving point.

Scatter Propagation Modes

1. Tropospheric Scatter Wave (troposcatter) Propagation Least method of propagation and used only when the other methods are not available. Operates on UHF band (between 350 MHz to 10 GHz) with common frequencies of 0.9

GHz, 2 GHz and 5 GHz.

2. Ionospheric Scatter Propagation (Ionoscatter) Works much similar to troposcatter, except that it uses E-Layer as scattering medium with

some assistance of D and F layers.

3. Backscatter Form of ionospheric propagation via the E and F layers. Characterized by rapid fluttering and fading.

4. Sidescatter Propagation Similar to backscatter except that the ground scatter zone is merely somewhat off the

direct line between the participants.

Observed frequently on the 14-MHz band.5. Trans-equatorial (TE) scatter Propagation

Usually about 4000 km (2500 miles) either side of the geomagnetic equator. Due to equators more exposure to sun.


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Minor Propagation Modes

1. Auroral Propagation Happens during the existence of Aurora Borealis Phenomenon., fluorescence at E-layer

height, that is a curtain of ions capable of reflecting radio wave in the frequency range of

about 20 MHz.

2. Sporadic-E (ES) Propagation Ionization at E Layer that affects mainly lower amateur frequencies. At its season this extends the single-hop maximum range to about 1400 miles and

double-hop maximum range to about 1400 to 2500 miles.

3. Gray Line Propagation Propagation of radio waves to the what is called gray line.Gray Line a band around the earth between the sunlit portion and darkness. It also called as

terminator or twilight zone.

4. Meteor-Burst Propagation A type of propagation af VHF and UHF waves that utilizes the phenomenon of scattering of

a radio signal from the ionization trails caused by meteors entering the Earths

atmosphere.

Waiting time time between useful trails.

Suns Role on Radio Wave Propagation

1. Sunspots The tendency of the sun to have grayish-black blemishes, seemingly at randomplaces, on its fiery surface

2. Solar Flux A measure of the energy received per unit time, per unit area per unit frequencyinterval.

3. Solar Flare A sudden eruption on the sun that causes high-speed atomic particles to beejected far into space from the surface of the sun.

4. Maunder Minimum Long period with a lack of any solar activity.Microwave System Parameters

Free Space Path Loss Loss incurred by an electromagnetic wave as it propagates in a straight line

through a vacuum with no absorption or reflection of energy from nearby objects.

= 4 2

where:

LP= free-space path loss (unitless)

D = distance (km)


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= wavelength (m)

Also,

= 92.4 + 20 log + 20 log = 32.4 + 20 log + 20 log = 96.6 + 20 log 20 log

Path Clearance and Antenna Heights

Fresnel Zone a region near an object in which diffraction effects are significant

First Fresnel Zone all points from which a wave could be reflected with an additional path length of

one-half wavelength from an ellipse.

Beyond this First Fresnel Zone region interference will be alternatively destructive and constructive.

Fresnel showed that the destructive contribution of some of these zones beyond the First Fresnel Zone

will be offset by the constructive contribution of other zones and thus the reaction of the reflector

responsible for a reflection will be only that of the First Fresnel Zone.

= 121 + 2where:

Fn= nth Fresnel Zone radius (m)

d1= distance of P from one end (m)

d2= distance of P from the other end (m)

= wavelength of the transmitted signal m


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Fade Margin

Aka link margin Essentially a fudge factor included in system gain equation that considers the non-ideal and

less predictive characteristics of radio wave propagation, such as multipath loss and terrain

sensitivity.

= 30 log + log1 70where:

FM = fade margin (dB)

D = distance (km)

f = frequency (GHz)

R = reliability in decimal

1-R = reliability objective for one-way 40km route

A = roughness factor

= 4 over water or very smooth terrain

= 1 over an average terrain

= 0.25 over a very rough, mountainous terrain

B = factor to convert a worst-month probability

= 1 to convert an annual availability to a worst-month basis

= 0.5 for hot humid areas

= 0.25 for average inland areas

= 0.125 for very dry or mountainous areas


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System Gain

= = + + +

where: FM = fade margin for a given reliability objective (dB)

LP = free-space path loss incurred as a signal propagates from the transmitter to the receiver

antennas through Earths atmosphere dB

Lf= transmission line loss (dB)

Lb= total coupling or branching loss (dB)

At= transmit antenna gain relative to isotropic radiator (dB)

Ar= receive antenna gain relative to isotropic radiator (dB)

Pt= transmitter output power (dBm/dBW)

F

(GHz)

Feeder Loss (Lf) Branching Loss (Lb) Antenna Gain (Ar or At)

TypeLoss

(dB/100m)

Frequency

Diversity

Space

Diversity

Diameter

(meter)Gain (dB)

1.8Air-filled

Coax5.4 4 2

1.2

2.4

3.0

2.7

4.8

25.2

31.2

33.2

34.7

37.2

7.4EWP 64elliptical

waveguide

4.7 3 2

1.2

1.52.4

3.0

3.7

37.1

38.843.1

44.8

46.5

8.0

EWP 69

elliptical

waveguide

6.5 3 2

1.2

2.4

3.0

3.7

4.8

37.8

43.8

45.6

47.3

49.8

WaveGuides

A hollow metal tube designed to carry microwave energy from one place to anotherAdvantages of a Waveguides

1. Less copper loss2. No Skin Effect3. Less dielectric loss4. Large power handling capability


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Disadvantages

1. Physical size is the primary lower-frequency limitation of waveguides.2. Waveguides are difficult to install because of their rigid, hollow-pipe shape.3. Increase the costs and decrease the practicality of waveguide systems at any other than

microwave frequencies.

Waveguide Cut-Off Frequency

The lowest frequency of operation.EM Wave propagation on Waveguide

Mode the way in which the electromagnetic field propagate along the waveguide.

Classification of Mode

1. Transverse Electric (TE) The E field exists across the guide and no E lines extent lengthwise along the guide.

2. Transverse Magnetic (TM) H lines loops in plane perpendicular to the walls of the guide, and no part of an H line is

lengthwise along the guide.

Mode Numbering Systems for Rectangular Waveguides: TEm, n or TMm, n

m number of half-wave patterns in the a dimension

n number of half-wave patterns in the b dimension

Dominant Mode mode which gives the lowest cut-off frequency

For the cut-off wavelength:

= 2 2 + 2

= 2


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Mode Numbering Systems for Circular Waveguides:

TEm, n or TMm, n

m number of full-wave patterns around the circumference

n number of half-wave patterns across the diameter

= 2 = 1.7where:

r = internal radius of the waveguide

d= diameter of waveguide

kr = solution of a Bessel Function equation (1.84 commonly)

Waveguide Input/Output Methods

1. Probea /4 vertical antenna at the signal frequency which is inserted in the waveguide one-quarter wavelength from the end which is closed.
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2. Loops also used to couple the microwave signal into the waveguide.

3. Slot or apertures are sometimes used when very loose (inefficient) coupling is desired.In this method energy enters through a small slot in the waveguide and the E field expands

into the waveguide. The E lines expand first across the slot and then across the interior of the

waveguide.

Minimum reflections occur when energy is injected or removed if the size of the slot is

properly proportioned to the frequency of the energy.

Waveguide Terminations

Electromagnetic energy is often passed through a waveguide to transfer the energy from a source into

space. As previously mentioned, the impedance of a waveguide does not match the impedance of

space, and without proper impedance matching, standing waves cause a large decrease in the

efficiency of the waveguide.
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Any abrupt change in impedance causes standing waves, but when the change in impedance at the

end of a waveguide is gradual, almost no standing waves are formed. Gradual changes in impedance

can be obtained by terminating the waveguide with a funnel-shaped HORN, such as the three types

illustrated in figures. The type of horn used depends upon the frequency and the desired radiation

pattern.

A waveguide may also be terminated in a resistive load that is matched to the characteristic

impedance of the waveguide. The resistive load is most often called a DUMMY LOAD, because its only

purpose is to absorb all the energy in a waveguide without causing standing waves.
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WaveGuide Propagation Modes

Group Velocity the actual speed at which a signal travels down the guide.

= 1 2

2

where:

vg= group velocity

= free-space wavelength

a= larger dimension of the interior cross section

But since

= 2

where:

fc= cut-off frequency

f = operating frequency


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Phase Velocity the rate at which the wave appears to move along the wall of the guide, based on the

way the phase angle varies along the walls.

= 1 22

= 2

Waveguide Plumbing

Since waveguides are really only hollow metal pipes, the installation and the physical handling

of waveguides have many similarities to ordinary plumbing. In light of this fact, the bending, twisting,

joining, and installation of waveguides is commonly called waveguide plumbing. Naturally, waveguides

are different in design from pipes that are designed to carry liquids or other substances. The design of

a waveguide is determined by the frequency and power level of the electromagnetic energy it will

carry. The following paragraphs explain the physical factors involved in the design of waveguides.

WAVEGUIDE BENDS

The size, shape, and dielectric material of a waveguide must be constant throughout its length for

energy to move from one end to the other without reflections. Any abrupt change in its size or shape

can cause reflections and a loss in overall efficiency. When such a change is necessary, the bends,

twists, and joints of the waveguides must meet certain conditions to prevent reflections.

Waveguides may be bent in several ways that do not cause reflections. One way is the gradual bend

shown in figure 1-46. This gradual bend is known as an E bend because it distorts the E fields. The E

bend must have a radius greater than two wavelengths to prevent reflections.

Another common bend is the gradual H bend (figure 1-47). It is called an H bend because the H fields

are distorted when a waveguide is bent in this manner. Again, the radius of the bend must be greater

than two wavelengths to prevent reflections. Neither the E bend in the "a" dimension nor the H bend

in the "b" dimension changes the normal mode of operation.


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A sharp bend in either dimension may be used if it meets certain requirements. Notice the two 45-

degree bends in figure 1-48; the bends are 1/4 apart. The reflections that occur at the 45-degree

bends cancel each other, leaving the fields as though no reflections have occurred.

Sometimes the electromagnetic fields must be rotated so that they are in the proper phase to match

the phase of the load. This may be accomplished by twisting the waveguide as shown in figure 1-49.

The twist must be gradual and greater than 2.

The flexible waveguide (figure 1-50) allows special bends which some equipment applications might

require. It consists of a specially wound ribbon of conductive material, most commonly brass, with the

inner surface plated with chromium. Power losses are greater in the flexible waveguide because the

inner surfaces are not perfectly smooth. Therefore, it is only used in short sections where no other

reasonable solution is available.


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WAVEGUIDE JOINTS

Since an entire waveguide system cannot possibly be molded into one piece, the waveguide must be

constructed in sections and the sections connected with joints. The three basic types of waveguide

joints are the PERMANENT, the SEMIPERMANENT, and the ROTATING JOINTS. Since the permanent

joint is a factory-welded joint that requires no maintenance, only the semi-permanent and rotating

joints will be discussed.

Sections of waveguide must be taken apart for maintenance and repair. A semi-permanent joint, called

a CHOKE JOINT, is most commonly used for this purpose. The choke joint provides good

electromagnetic continuity between sections of waveguide with very little power loss.

A cross-sectional view of a choke joint is shown in figures 1-51A and 1-51B. The pressure gasket shown

between the two metal surfaces forms an airtight seal. Notice in figure 1-51B that the slot is exactly

1/4 from the "a" wall of the waveguide. The slot is also 1/4 deep, as shown in figure 1-51A, and

because it is shorted at point (1), a high impedance results at point (2). Point (3) is 1/4 from point (2).

The high impedance at point (2) results in a low impedance, or short, at point (3). This effect creates a

good electrical connection between the two sections that permits energy to pass with very little

reflection or loss.

Whenever a stationary rectangular waveguide is to be connected to a rotating antenna, a rotating joint

must be used. A circular waveguide is normally used in a rotating joint. Rotating a rectangular

waveguide would cause field pattern distortion. The rotating section of the joint, illustrated in figure 1-

52, uses a choke joint to complete the electrical connection with the stationary section. The circular

waveguide is designed so that it will operate in the TM0,1 mode. The rectangular sections are attached

as shown in the illustration to prevent the circular waveguide from operating in the wrong mode.


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Distance "O" is 3/4 so that a high impedance will be presented to any unwanted modes. This is the

most common design used for rotating joints, but other types may be used in specific applications.

WAVEGUIDE DEVICES

The discussion of waveguides, up to this point, has been concerned only with the transfer of energy

from one point to another. Many waveguide devices have been developed, however, that modify the

energy in some fashion during transit. Some devices do nothing more than change the direction of the

energy. Others have been designed to change the basic characteristics or power level of the

electromagnetic energy.

This section will explain the basic operating principles of some of the more common waveguide

devices, such as DIRECTIONAL COUPLERS, CAVITY RESONATORS, and HYBRID JUNCTIONS.

Directional Couplers

The directional coupler is a device that provides a method of sampling energy from within a waveguide

for measurement or use in another circuit. Most couplers sample energy travelling in one direction

only. However, directional couplers can be constructed that sample energy in both directions.

These are called BIDIRECTIONAL couplers and are widely used in radar and communications systems.

Directional couplers may be constructed in many ways. The coupler illustrated in figure 1-53 is

constructed from an enclosed waveguide section of the same dimensions as the waveguide in which

the energy is to be sampled. The "b" wall of this enclosed section is mounted to the "b" wall of thewaveguide from which the sample will be taken. There are two holes in the "b" wall between the

sections of the coupler. These two holes are 1/4 apart. The upper section of the directional coupler

has a wedge of energy-absorbing material at one end and a pickup probe connected to an output jack

at the other end. The absorbent material absorbs the energy not directed at the probe and a portion

of the overall energy that enters the section.

Figure 1-54 illustrates two portions of the incident wavefront in a waveguide. The waves travel down

the waveguide in the direction indicated and enter the coupler section through both holes. Since both


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portions of the wave travel the same distance, they are in phase when they arrive at the pickup probe.

Because the waves are in phase, they add together and provide a sample of the energy traveling down

the waveguide. The sample taken is only a small portion of the energy that is traveling down the

waveguide. The magnitude of the sample, however, is proportional to the magnitude of the energy in

the waveguide. The absorbent material is designed to ensure that the ratio between the sample

energy and the energy in the waveguide is constant. Otherwise the sample would contain no useful

information.

Cavity Resonators

In ordinary electronic equipment a resonant circuit consists of a coil and a capacitor that are

connected either in series or in parallel. The resonant frequency of the circuit is increased by reducing

the capacitance, the inductance, or both. A point is eventually reached here the inductance and the

capacitance can be reduced no further. This is the highest frequency at which a conventional circuit

can oscillate.

The upper limit for a conventional resonant circuit is between 2000 and 3000 megahertz. At these

frequencies, the inductance may consist of a coil of one-half turn, and the capacitance may simply bethe stray capacitance of the coil. Tuning a one-half turn coil is very difficult and tuning stray

capacitance is even more difficult. In addition, such a circuit will handle only very small amounts of

current.

By definition, a resonant cavity is any space completely enclosed by conducting walls that can contain

oscillating electromagnetic fields and possess resonant properties. The cavity has many advantages

and uses at microwave frequencies. Resonant cavities have a very high Q and can be built to handle

relatively large amounts of power. Cavities with a Q value in excess of 30,000 are not uncommon. The

high Q gives these devices a narrow bandpass and allows very accurate tuning. Simple, rugged

construction is an additional advantage.

Waveguide Junctions

You may have assumed that when energy traveling down a waveguide reaches a junction, it simply

divides and follows the junction. This is not strictly true. Different types of junctions affect the energy

in different ways. Since waveguide junctions are used extensively in most systems, you need to

understand the basic operating principles of those most commonly used.

The T JUNCTION is the most simple of the commonly used waveguide junctions. T junctions are divided

into two basic types, the E-TYPE and the H-TYPE. HYBRID JUNCTIONS are more complicated

developments of the basic T junctions. The MAGIC-T and the HYBRID RING are the two most commonlyused hybrid junctions.

E-TYPE T JUNCTION It is called an E-type T junction because the junction arm extends from the main

waveguide in the same direction as the E field in the waveguide.


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H-TYPE T JUNCTION. is called an H-type T junction because the long axis of the "b" arm is parallel to

the plane of the magnetic lines of force in the waveguide. Again, for simplicity, only the E lines are

shown in this figure. Each X indicates an E line moving away from the observer. Each dot indicates an E

line is moving toward the observer.

MAGIC-T HYBRID JUNCTION. The magic-T is a combination of the H-type and E-type T junctions. The

most common application of this type of junction is as the mixer section for microwave radar

receivers.

In summary, when an input is applied to arm b of the magic-T hybrid junction, the output signals from

arms a and c are 180 degrees out of phase with each other, and no output occurs at the d arm.

Unfortunately, when a signal is applied to any arm of a magic-T, the flow of energy in the output arms

is affected by reflections. Reflections are caused by impedance mismatching at the junctions. These

reflections are the cause of the two major disadvantages of the magic-T. First, the reflections

represent a power loss since all the energy fed into the junction does not reach the load which the

arms feed. Second, the reflections produce standing waves that can result in internal arching. Thus the

maximum power a magic-T can handle is greatly reduced.


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HYBRID RING.A type of hybrid junction that overcomes the power limitation of the magic-T is the

hybrid ring, also called a RAT RACE. The hybrid ring, illustrated in figure 1-71A, is actually a

modification of the magic-T. It is constructed of rectangular waveguides molded into a circular pattern.

The arms are joined to the circular waveguide to form E-type T junctions. Figure 1-71B shows, in

wavelengths, the dimensions required for a hybrid ring to operate properly.

The hybrid ring is used primarily in high-powered radar and communications systems to perform two

functions. During the transmit period, the hybrid ring couples microwave energy from the transmitter

to the antenna and allows no energy to reach the receiver. During the receive cycle, the hybrid ring

couples energy from the antenna to the receiver and allows no energy to reach the transmitter. Any

device that performs both of these functions is called a DUPLEXER. A duplexer permits a system to use

the same antenna for both transmitting and receiving.

Ferrite Devices

A FERRITE is a device that is composed of material that causes it to have useful magnetic properties

and, at the same time, high resistance to current flow. The primary material used in the construction

of ferrites is normally a compound of iron oxide with impurities of other oxides added. The compound

of iron oxide retains the properties of the ferromagnetic atoms, and the impurities of the other oxides

increase the resistance to current flow. This combination of properties is not found in conventional

magnetic materials. Iron, for example, has good magnetic properties but a relatively low resistance to


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current flow. The low resistance causes eddy currents and significant power losses at high frequencies.

Ferrites, on the other hand, have sufficient resistance to be classified as semiconductors.

FERRITE ISOLATORS.An isolator is a ferrite device that can be constructed so that it allows

microwave energy to pass in one direction but blocks energy in the other direction in a waveguide.

This isolator is constructed by placing a piece of ferrite off-center in a waveguide, as shown in figure 1-

75. A magnetic field is applied by the magnet and adjusted to make the electron wobble frequency of

the ferrite equal to the frequency of the energy traveling down the waveguide. Energy traveling down

the waveguide from left to right will set up a rotating magnetic field that rotates through the ferrite

material in the same direction as the natural wobble of the electrons. The aiding magnetic field

increases the wobble of the ferrite electrons so much that almost all of the energy in the waveguide is

absorbed and dissipated as heat. The magnetic fields caused by energy traveling from right to left

rotate in the opposite direction through the ferrite and have very little effect on the amount of

electron wobble. In this case the fields attempt to push the electrons in the direction opposite the

natural wobble and no large movements occur. Since no overall energy exchange takes place, energy

traveling from right to left is affected very little.

Microwave Antennas

1. Horn Antenna Not practical at low frequencies because of size Can be E-plane, H-plane, pyramidal or conical Moderate gain, about 20 dBi Common as feed antennas for dishes

Types of Horn Antenna

a. Sectoral Horn flaring the waveguide in only one direction.


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b. Pyramidal Horn Flared in both dimensions.

c. Conical Horn Most appropriate with circular waveguide.

Gain

= 7.52

=

4

2

where:

dE= E-plane

dH= H-plane

k = constant derived from how uniformly the phase and amplitude of the electromagnetic fields

are distributed across the aperture

A = aperture of horn

Beamwidth

= 80 = 70


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= 56 where:

B = beamwidth

W = width of horn (m)

H= H-plane beamwidth

E= E-plane beamwidth

dH= H-plane aperture

dE= E-plane aperture

Aperture Area the area of the rectangle formed by opening of the horn

Flare Angle typically about 20 to 60

Beamwidth the angle formed by extending lines from the center of the antenna response curve to

the 3-dB-down points.

Horn Antennas are usually employed in conjunction with parabolic reflectors.

Parabolic Reflectors

A large dish-shaped structure made of metal and screen mesh. Collimate EM waves into a narrow beam of energy.

Gain of Parabolic Reflector

= 6 2

Beamwidth of Parabolic Reflector

= 80


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Parabolic Reflector Feed Arrangements

a. Horn Feed

b. Cassegrain Feeds

2. Helical AntennaMade up of six to eight turns of heavy wire or tubing to form a coil or helix.


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= 1523

where:

G = gain (in ratio)

N = number of turns in the helix

S = turn spacing in meters

D = diameter of the helix in meters

= wavelength in meters

Beamwidth

= 52 3. Patch Antenna Consists of a thin metallic patch placed a small fraction of a wavelength above a

conducting ground plane separated by a dielectric.

4. Slot Antenna

Microwave Devices

Transmission lines for microwave signals that are constructed on the PCB.


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1. Microstrip a flat conductor separated from a large conducting ground plane by an insulatingdielectric.

2. Stripline a flat conductor sandwiched between two ground planes.

Circulators and Isolators Uses ferrites in their operation

Isolator a device that allows a signal to pass in only one direction.

Circulator a very useful device that allows the separation of signals.

Microwave Solid State Devices

Problems on Conventional Solid State Devices on Microwave

1. Exhibits stray inductance and capacitance2. Transit time

1. Point-Contact Diode Oldest microwave semiconductor A piece of semiconductor device and a fine wire (made of tungsten) which makes contact with

semiconductor material.

Has an extremely low capacitance Ideal for low-signal applications and are widely used in microwave mixers and detectors Extremely delicate and cannot withstand high power


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2. Schottky or hot-carrier diodes Extremely small and has a tiny junction capacitance Has low bias threshold voltage

3. Varactor Diode Familiar from its lower frequency use as a means of providing a capacitance that can be

changed by varying the voltage that reverse biases the diode.

4. Step-Recovery or Snap-Off Varactor At forward bias, it conducts as any diode, but a charge is stored in the depletion region. At reverse bias, charge keeps the diode on momentarily. Then suddenly turns off abruptly. Produces an extremely high intensity reverse current pulse.

5. Gunn Diode Also known as Transferred-Electron Device (TED) One of the simpler transit-time devices. Exhibits a negative resistance characteristics


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6. Tunnel Diode Produces a narrow range of negative resistance when forward-biased Used to produce low-power microwave oscillation

7.

IMPATT Diode Impact Avalanche and Transit Time Four-layer PN junction device Operates in reverse-breakdown region

8. TRAPATT Diode Trapped Plasma avalanche triggered transit Time Operate on higher power rating

9. Yttrium Iron Garnet Device A type of ferrite YIG sphere can be used in place of a resonant cavity as a microwave resonant circuit.

Microwave Tubes

1. Magnetron The oldest microwave tube design


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High power, fixed-frequency oscillations, but noted for stability or ease of modulation butsimple, rugged, and relatively efficient.

Commonly used in radar transmitters, where they can generate peak power levels in themegawatt range.

Consists of a circular anode into which has been machined an even number of resonantcavities

In the center of the anode, called the interaction chamber, is a circular cathode that emitselectrons when heated.

Cyclotron Frequency The rate at which electrons move around the cathode.

Slow-Wave Structure The circular arrangement of resonant cavities that require for the movement

of the wave around the tube at a rate much slower than the speed of light.

Pulsed Magnetron used in radar system

CW Magnetron used in heating purposes in microwave ovens.

2.

Klystron A microwave tube using cavity resonators to provide velocity modulation of the electron beam

and produce amplification

Preferred tube for high-power, high-stability amplification of signals at frequencies from UHFto about 30GHz.

Commonly found on UHF TV transmittersVelocity Modulation the speeding up and slowing down of the electron beam.

Buncher Cavity the input cavity that produces bunches of electrons.

Catcher Cavity cavity closer to the collector on which the output signal is taken.

Types of Klystron

1. Reflex Klystron2. Multicavity Klystron


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3. Travelling-Wave Tube One of the most versatile microwave RF power amplifiers Has an extremely wide bandwidth of operation Has the ability to generate hundreds and even thousands of watts of microwave power. Not resonant at a single frequency Consists of a cathode, an anode, a filament heater, collector plate and helix Also produces velocity modulation which produce density modulation Used as power amplifiers in satellite transponder.

Helix coil which provides a path for the RF signal that will slow down its propagation

Wire and Wireless Communication

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