domingo, 28 de noviembre de 2010

A electronic devices and components summary

Power electronic device PowerElectronicDevice Also called the power semiconductor, high-power usually mean the electric current as several dozen to several kiloamperes used for in conversion of electrical energy and electric energy controlling circuit, the voltage is several hundred volts of the above Electronic device. Since latter stage of the eighties, the development of the electric electronic devices and components successively went through rectifier era, inverter era and frequency converter era, promoted the application in a lot of new developing fields of electric electron technology at the same time.

High frequency of collection represented by power MOSFET and IGBT developed in latter stage of the eighties and initial stage of the 1990s, high pressure and heavy current have indicated traditional electric electron technology and already entered modern electric and electronic era in one suit of power semiconductor multiple devices.

Including frequency converter, electric energy quality products and electronic power supply products look like the electricity to the transformation in the inner electric electronic device, machinery, mining and metallurgy, traffic, chemical industry, the light textile is in inner traditional industry and to the spaceflight, the laser, communication, robot are essential in the inner new high-tech industry and efficient utilization energy that the country supports and supports at present.



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electric electronic devices and components develop the tendency

With electric electron being technical and constant to develop at full speed, too towards high-power, apt to drive and the intersection of direction and fast steady development of high frequency modern power electronic device. The technical application of electric electron is expanding constantly too, especially on applications of heavy industry such as the electricity, mine, metallurgy, steel-making,etc., electric electron technology is giving play to its autogenous advantage constantly.

Because the electric electron has very important positions in national economy, for can enable electricity high-efficient while being rational, modern developed country 75% of electric energy use after varying or controlling. It is estimated come will it be the end 21st century, the figure rise to 95% more than. But the electric energy of China changes and still far from reaching and employing electric electron technology the result that could reach. By the end of 2010, the China Power demand will reach 381 million kilowatt-hour, the total capacity of generator installation will reach 852 million kilowatts, therefore we can find out, electric electron technology is in China the potentiality of development is still very huge.

Application of third, electric electron technology in frequency converter and electronic power supply



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Frequency converter with total capacity of market of 200 billion – 300 billion

The frequency converter utilizes on-off action of the electric semiconductor device to transform the power of mains frequency to the electric energy of another frequency and control the device, can realize to exchanging the soft start of the asynchronous machine, speed regulation by frequency variation, improving and operating the precision, changing power factor, excessive load / overvoltage / functions such as overload protection,etc.. Employ the technology of frequency conversion and microelectronic technology, control the electric drive component of the alternating current motor by changing frequency and range of the electrical machinery working power. So, the frequency converter is become " modern industrial vitamin " .

The space of market of the frequency converter is very big, play a very important role in trades such as electricity, textile, printing and dyeing, machinery, petrochemical industry,etc. and project. In China, the market of frequency converter is in a course of high growth. For a long time, the market of Chinese frequency converter has been keeping 12% all the time – -15% growth rate. And the growth rate that will still keep above 10% within the next 5 years, see according to present development speed and market demand, after 10 years at least, the market of frequency converter of China could saturate progressively.

Because of the constant enlargement of the demand of the market of frequency converter, constant growth of development speed, the Chinese manufacturing enterprise increases year by year, foreign famous producers come to China to make the investment and found the factory one after another too. However, the greater disparity that Chinese enterprises and big enterprise of foreign countries also exist technically, as to some upper voltage, high-capacity frequency converter China is still at the stage of developing, key technology under one's control, domestic the intersection of frequency converter and nearly for zero required semi-conductive production of power device, had to depend on import, all these brought challenges at the same time after bringing the opportunity for manufacturer of the domestic electric electronic devices and components.



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Market of electronic power supply with more than 17 billion

The power includes electronic power supply and chemical physical power source, the one correlated to electric electronic technology application is the electronic power supply. The electronic power supply varies and controls the public electric wire netting or a certain electric energy, offer the power supply equipment of the high-quality electric energy to various by electric load.

There is market scale of 5 billion yuan at least every year in the switching power supply. Among them, DC/DC converter module power accounts for 25%, AC/DC rectifier accounts for 75%. Because communication of China undertaking investment centralized, so communication power the proportions accounted for even relatively loud in the total value of the switching power supply, it is estimated, the total value of the national switching power supply reaches more than 10 billion yuan.

And modern UPS has generally adopted modern power electronic devices such as modulation technique and power MOSFET, IGBT of the pulse width,etc., use the modern power electronic device to let the noise of the power be reduced, power and reliability increase. According to statistics, the high-power UPS market annual growth about 5% from 2009 to 2011 years, the market of large middle-power will become main motive force of development of UPS trade.



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Power electronic device of the new materials

1,High-frequency kenotron of high-pressure GaAs

With the constant improvement of the switching frequency of the converter, the requirements for fast recovery diode are thereupon improved too. As everyone knows, the gallium arsenide diode has high-frequency switching characteristic of having an advantage over silicon diode, but because of the reason of respects such as the technology,etc., the withstand voltage of the gallium arsenide diode is lower, real application is limited. In order to meet the application needs of high pressure, high speed, high frequency and low EMI, GaAs high-frequency kenotron has already been developed successfully. Compared with silicon fast recovery diode, such new diode has reverse leakage current that varies with temperature littly, the switching loss is low, such distinctive features as it is good that the opposite direction is for revovering.

2,Power device of the carborundum

SIC carborundum Develop the most mature large energy gap semiconductor material at present, it is very important too in the electric and electronic respect, can make, happen the intersection of characteristic and further more excellent high temperature ' 300 ℃- 500 ℃ , high frequency, heavy-duty, at a high speed, radioprotective device. SIC heavy-duty, high-pressure device is significant to the energy-conservation of apparatuses such as transporting and electric automobile of service message,etc.. Adopt the new device of SIC and appear within the following 5- 10 years, and will produce revolutionary influence on the semiconductor material.

SIC can be used for making radio frequency and microwave power device, high-frequency rectifier, MESFET, MOSFET and JFET,etc.. SIC high-frequency power device been researched and developed successfully in the intersection of Motorola and Company, and apply to microwave and radio frequency and fit; GE is developing SIC power device and high-temperature device; Westinghouse Electric has already produced the Very High Frequency MESFET of making the work in 26GHz frequency; ABB Company is developing the high pressure, high-power SIC rectifier and other SIC low frequency power devices used for industry and electric system.

Theoretical anylysis indicates, SIC power devices very close to the ideal power device. We can predict, the research and development of different SIC devices will become one of the main trends of the power device research field. But we want sober seeing, SIC material and mechanism, theory and manufacture process of the power device have a large number of problems that need solving, it should really bring the new revolution to electric and electronic technical field, estimate that also needs wait of time.


The application of the electric electronic devices and components has already got deeply to all respects of industrial production and social life, actual to need to promote the constant innovation of the device greatly. The power electronic device is entering the power electronic device era of new generation taking new device as body, it will basically replace the traditional device. " a generation of electric electron technology that a generation of devices determine, " As the governing factor of the electric and electronic technical development, research and development and key technological break-through of the power electronic device, will inevitablely promote the technical rapid development of electric electron, thus has promoted the rapid development of traditional industry and new high-tech industry based on electric electron technology.



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Microwave Superconductivity

Superconductivity is a phenomenon occurring in certain materials generally at very low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field (the Meissner effect). It was discovered by Heike Kamerlingh Onnes in 1911. Applying the principle of Superconductivity in microwave and millimeter-wave (mm-wave) regions, components with superior performance can be fabricated. Major problem during the earlier days was the that the cryogenic burden has been perceived as too great compared to the performance advantage that could be realized. There were very specialized applications, such as low-noise microwave and mm-wave mixers and detectors, for the highly demanding radio astronomy applications where the performance gained was worth the effort and complexity. With the discovery of high temperature superconductors like copper oxide, rapid progress was made in the field of microwave superconductivity.

This topic describes the properties of superconductivity that can be exploited in microwave and mm-wave technologies to yield components with appreciable performance enhancement over conventional systems. Superconducting signal transmission lines can yield low attenuation, zero-dispersion signal transmission behavior for signals with frequency components less than about one tenth the superconducting energy gap. No other known microwave device technology can provide a similar behavior. Superconductors have also been used to make high speed digital circuits, josephsons junction and RF and microwave filters for mobile phone base stations.



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Microwave Devices Help Broadcast HD Content

The way the world delivers digital information is converging. Electronic News Gathering and Broadcasting applications as well as entertainment and live events broadcasting can capture digital content with more flexibility and a higher degree of freedom with HD Roaming.

BMS Europe GmbH is at the cutting-edge of new technology. They provide technology for wireless HD transmission for fixed or mobile operation.

BMS is debuting its new DR6000 MK2 receiver. a 6-way diversity receiver for reception of COFDM RF video signals in a 360-degree view and in a challenging OB environment.

In conjunction with the new NANO-Transmitter Series, BMS provides broadcast wireless solutions extendable for citywide coverage.

The new DR6000 MK2 features also include 2- / 6-way high performance FFT-MRC Diversity, antenna 2 / 6 Input, low delay 40ms end-to-end delay, MPEG4 H.264 ready, MPEG2 HD MPEG-2 422P@HL decoding, repeater Integrated up-converter with auto re-modulation setup, COM Server TCP/IP access for remote control, and GPS data output for the tracking antenna.

The internal decoder provides professional features such as ASI, SDI, HD-SDI, component or composite and analog/embedded audio outputs.

Optionally, the DR6000 MK2 can also be operated as single repeater within an auto-modus for automatically configuring all COFDM settings and presetting. With the in-build 6-way spectrum-analyzer the DR6000 MK2 becomes an essential tool in the broadcast world.

These products integrate digital microwave technology with functionality, ultra small form factor, low power consumption, light weight and future H.264 capabilities to operate in all major bands for Europe or in other desired frequency ranges.

Free HD roaming, high flexibility, without compromise in reliability, enables customers to provide their digital content easily. With these products professionals in multiple markets can now present technologically savvy broadcasts to audiences with an ever-demanding desire for HD content.

BMS Inc designs, develops, manufactures, and distributes microwave transmission systems. BMS offers a broad range of microwave communication products and systems developed for electronic news gathering and entertainment, law enforcement, unmanned aerial vehicles, and military surveillance applications.



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Nanotubes transistor developments

Carbon nanotubes hold great promise due to their extraordinary electrical, mechanical, optical, thermal, and chemical properties. Their current applications range from improving consumer electronics, to medicine delivery to cells, to strengthening airplane components. Carbon nanotubes come in many different forms and purities. They range from flexible, thin, few-walled or single-walled nanotubes (SWNTs) to rigid, long, thick, multi-walled nanotubes (MWNTs), with a spectrum of characteristics.

Researchers at Stanford University, Cornell University and Purdue University have jointly produced a carbon nanotube transistor that they claim has better properties than silicon transistors of an equivalent size. The device uses zirconium oxide rather than silicon dioxide, which has a lower dielectric constant, as the gate insulator. Highest performance carbon nanotube field-effect transistors were made to date by integrating zirconia gate insulators. They obtained 70 mV/decade sub threshold swings approaching the theoretical limit for transistors. The scientists used semi conducting single-walled nanotubes (SWNTs) to make p-type field-effect-transistors (FETs). They formed the zirconia gate insulators by atomic layer deposition, creating zirconia films of about 8 nm thick without significantly degrading key transistor performance parameters of the nanotubes, such as mobility. The team converted p-type ZrO2/SWNT-FETs to n-type transistors by heating them in molecular hydrogen at 400°C for one hour. The properties of the n-type transistors, although good, were not as ideal as the p-type FETs. The researchers also made a NOT logic gate, i.e. an inverter, by connecting a p- and n-type ZrO2/SWNT-FET. The device had a high voltage gain.

Zurich researchers have built a transistor whose crucial element is a carbon nanotube, suspended between two contacts, with outstanding electronic properties. A novel fabrication approach allowed the scientists to construct a transistor with no gate hysteresis. This opens up new ways to manufacture nano-sensors and components that consume particularly little energy.
Researchers of University of California at Irvine developed a device which consists of a single-walled carbon nanotube sandwiched between two gold electrodes to operate at extremely fast microwave frequencies. This has resulted is an important effort to develop nano electronic components that could be used to replace silicon in a range of electronic applications.

Researchers from the University of California, San Diego and Clemson University synthesized Y-shaped carbon nanotubes to make transistors. The nanotube transistors were initially grown as straight nanotube elements. Titanium-modified iron catalyst particles added to the synthesis mixture were then attached to the straight nanotubes, nucleating additional growth, which continued in a fashion similar to branches growing from a tree trunk. The nascent nanotubes assumed a Y-shape with the catalyst particle gradually becoming absorbed at the junction of the stem and two branches. When electrical contacts are attached to the nanotube structures, electrons travel into one arm of the Y, hop onto the catalyst particle, and then hop to the other arm and flow outward. The movement of electrons through the Y-junction can be finely controlled, or gated, by applying a voltage to the stem, a replication of the function of existing transistors.

Printable transistors

The semi conducting properties of carbon nanotubes can be exploited to create printable transistors with extremely high performance. Specifically, researchers have shown CNT-based transistors employing a sparse nanotube network to achieve mobility of 1 cm2/V-s, while those using an aligned array of single-walled nanotubes can reach as high as 480 cm2/V-s. Nanotubes also prove to be useful additives to polymer-based TFTs and help to overcome some of the shortcomings of those devices. Beyond their performance, such devices are compatible with solution-based printing techniques, which enable dramatic cost savings in such devices as LCDs and OLED-based displays



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Microwave Synthesis And Ramen Noodles

Microwaves are a low frequency light, at least compared to visible light, say, or ionizing radiation like gamma rays. Thus, microwaves are quite harmless. A microwave oven baths the food in an oscillating electro-magnetic field. Molecules with permanent electrical dipole moments wiggle in the field and thus heat up the food.

Ok, lets google "microwave danger". Wow, a barrage of pseudo-science about how microwaves slowly kill you and your family. Microwaves supposedly distort the molecules and natural energy in the food and of course, they were invented by the Nazis during the third Reich – who else would come up with such evil? Anyways, they are not natural like the sun, because "microwaves from the sun are based on principles of pulsed direct current (DC) that don't create frictional heat" ???

A few diamonds in the rough: Serious studies quantifying carcinogens in microwave food. Of course, these you obtain much more when frying or eating from the BBQ grill. Large organic molecules in baby milk may isomerize in the microwave oven. What is the conclusion? No big problem, we are in the know, physics has immunized me against rhetoric about supposedly "natural" stuff and why natural should be good by default. I am not expert in all fields, but there is the scientific method, peer review, and so on. Surely, if there was a problem, we would know by now. Or would we?

In our lab, we synthesize nanometer sized particles. Certain particles, like the one pictured below, we cannot make without microwaves. High power? Nope - usual kitchen appliance on the low or warm setting, a few minutes max. Do we know why the microwaves have this effect? I have not the slightest idea! My paper needs references, so I waffle on about the "microwave effect": is it thermal or non-thermal?

Nano-porous micro-carbon spheres with only about several hundred nanometers diameter having metal nanoparticles in its interior, highly catalytic (read: bio active). We make these with help of a conventional microwave oven, 1014 at a time, easily scalable. The metals involved here are from ionic solutions with low concentrations (like in foods) and not much reducer is used either. Vitamin C is such a reducer and at high concentrations in many foods. We do not know why microwaves are necessary or what such compounds would do if digested, nor are there test for unknown nano-compounds in microwaved food.

Our novel results should "inspire much future research" – so I write, and it is accepted by peer review. What is it: Do we know microwaves sufficiently or do we need much future research? Would a chemical test for carcinogens or isomers find that the molecules are actually present in form of nanoparticles? No! Do we know what novel nanoparticles do inside of us, given that biology is basically naturally evolved nanotech? Not a clue! Our papers claim potentially "strong catalytic activity due to non-trivial morphologies, crystallographic structures, and size effects".

Do we know what kind of nanoparticles or refolded proteins (involved in prion diseases and apparently also Alzheimer's) develop inside a burrito or pizza that was poorly defrosted and cooked on the high power setting in a microwave oven? I do not. I probably ate more than two thousand of those as a busy graduate student.

What do I know? On one hand, my Pleistocene mind and my kind of body evolved by natural selection and cooking over fire has been around for a relatively long while, but microwaves not. I also know that I too often think I know although I do not. I could write a series of posts listing all the facts I deemed obvious in the light of basic scientific understanding and that I had to personally give up because some stupid, ugly tidbit of data hit me over the head. On the other hand, I do not want to present a toehold to esoteric pseudoscience. I do not fear "big pharma" conspiring with some evil microwave oven military complex conglomerates trying to shut me up, but I do fear being quoted in support of the "teach the controversy" strategy of creationists, global warming denialists, and suchlike.

Yesterday, my dearest visits me in my office. She puts dry Ramen noodles into a bowl, adds cold water, and off it goes into the microwave oven. She does neither consider that the water has had no time to enter the hard noodles, nor that the microwave frequency is not actually tuned to only excite the water. She is a lay person, she respects science, and scientists say that microwave cooking is fine. Piping hot noodles in front of the computer screen on my desk, the steam swirls the transmission electron microscope images of our lab's newest samples. Bon Appetit!



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Does gravity travel at the speed of light?

To begin with, the speed of gravity has not been measured directly in

the laboratory---the gravitational interaction is too weak, and such

an experiment is beyond present technological capabilities. The

"speed of gravity" must therefore be deduced from astronomical

observations, and the answer depends on what model of gravity one uses

to describe those observations.

In the simple Newtonian model, gravity propagates instantaneously: the
force exerted by a massive object points directly toward that object's
present position. For example, even though the Sun is 500 light
seconds from the Earth, Newtonian gravity describes a force on Earth
directed towards the Sun's position "now," not its position 500
seconds ago. Putting a "light travel delay" (technically called
"retardation") into Newtonian gravity would make orbits unstable,
leading to predictions that clearly contradict Solar System

In general relativity, on the other hand, gravity propagates at the
speed of light; that is, the motion of a massive object creates a
distortion in the curvature of spacetime that moves outward at light
speed. This might seem to contradict the Solar System observations
described above, but remember that general relativity is conceptually
very different from Newtonian gravity, so a direct comparison is not
so simple. Strictly speaking, gravity is not a "force" in general
relativity, and a description in terms of speed and direction can be
tricky. For weak fields, though, one can describe the theory in a
sort of Newtonian language. In that case, one finds that the "force"
in GR is not quite central---it does not point directly towards the
source of the gravitational field---and that it depends on velocity as
well as position. The net result is that the effect of propagation
delay is almost exactly cancelled, and general relativity very nearly
reproduces the Newtonian result.

This cancellation may seem less strange if one notes that a similar
effect occurs in electromagnetism. If a charged particle is moving at
a constant velocity, it exerts a force that points toward its present
position, not its retarded position, even though electromagnetic
interactions certainly move at the speed of light. Here, as in
general relativity, subtleties in the nature of the interaction
"conspire" to disguise the effect of propagation delay. It should be
emphasized that in both electromagnetism and general relativity, this
effect is not put in _ad hoc_ but comes out of the equations. Also,
the cancellation is nearly exact only for *constant* velocities. If a
charged particle or a gravitating mass suddenly accelerates, the
*change* in the electric or gravitational field propagates outward at
the speed of light.

Since this point can be confusing, it's worth exploring a little
further, in a slightly more technical manner. Consider two
bodies---call them A and B---held in orbit by either electrical or
gravitational attraction. As long as the force on A points directly
towards B and vice versa, a stable orbit is possible. If the force on
A points instead towards the retarded (propagation-time-delayed)
position of B, on the other hand, the effect is to add a new component
of force in the direction of A's motion, causing instability of the
orbit. This instability, in turn, leads to a change in the mechanical
angular momentum of the A-B system. But *total* angular momentum is
conserved, so this change can only occur if some of the angular
momentum of the A-B system is carried away by electromagnetic or
gravitational radiation.

Now, in electrodynamics, a charge moving at a constant velocity does
not radiate. (Technically, the lowest order radiation is dipole
radiation, which depends on the acceleration.) So to the extent that
that A's motion can be approximated as motion at a constant velocity,
A cannot lose angular momentum. For the theory to be consistent,
there *must* therefore be compensating terms that partially cancel the
instability of the orbit caused by retardation. This is exactly what
happens; a calculation shows that the force on A points not towards
B's retarded position, but towards B's "linearly extrapolated"
retarded position. Similarly, in general relativity, a mass moving at
a constant acceleration does not radiate (the lowest order radiation
is quadrupole), so for consistency, an even more complete cancellation
of the effect of retardation must occur. This is exactly what one
finds when one solves the equations of motion in general relativity.

While current observations do not yet provide a direct
model-independent measurement of the speed of gravity, a test within
the framework of general relativity can be made by observing the
binary pulsar PSR 1913+16. The orbit of this binary system is
gradually decaying, and this behavior is attributed to the loss of
energy due to escaping gravitational radiation. But in any field
theory, radiation is intimately related to the finite velocity of
field propagation, and the orbital changes due to gravitational
radiation can equivalently be viewed as damping caused by the finite
propagation speed. (In the discussion above, this damping represents
a failure of the "retardation" and "non-central, velocity-dependent"
effects to completely cancel.)

The rate of this damping can be computed, and one finds that it
depends sensitively on the speed of gravity. The fact that
gravitational damping is measured at all is a strong indication that
the propagation speed of gravity is not infinite. If the
calculational framework of general relativity is accepted, the damping
can be used to calculate the speed, and the actual measurement
confirms that the speed of gravity is equal to the speed of light to
within 1%. (Measurements of at least one other binary pulsar system,
PSR B1534+12, confirm this result, although so far with less

Are there future prospects for a direct measurement of the speed of
gravity? One possibility would involve detection of gravitational
waves from a supernova. The detection of gravitational radiation in
the same time frame as a neutrino burst, followed by a later visual
identification of a supernova, would be considered strong experimental
evidence for the speed of gravity being equal to the speed of light.
However, unless a very nearby supernova occurs soon, it will be some
time before gravitational wave detectors are expected to be sensitive
enough to perform such a test.



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Coaxial Cable Phase Matching

This application note is intended as a general guideline in the selection and specification of coaxial cables for applications requiring phase matching and or phase tracking versus temperature. The match can be specified in electrical degrees as in the suggested best match curves of Figure 1 or in nanoseconds of time delay.

M/A-COM Phase Matching
Figure 1. Suggested Phase match limits

M/A-COM Antennas, Cables and Waveguides

The M/A-COM Antenna, Cable and Waveguide (ACW) product line has developed hundreds of various cable sizes/constructions each optimized for different applications. Most utilize solid or air-spaced PTFE dielectrics and fall into four general families that can be characterized by their Velocity of Propagation (Vp):

69% Velocity of Propagation
76% Velocity of Propagation
80% Velocity of Propagation
82% Velocity of Propagation

Cables with a solid extruded PTFE dielectric core have a nominal Vp of 69% and are generally the strongest mechanically but worst for insertion loss and phase changes as a function of temperature. Cables with a high air content and a nominal Vp of 82% are generally weaker mechanically but the best electrically with very low insertion loss and excellent phase versus temperature characteristics.

Cables with intermediate values of Vp, Vp of 76% for example, are offered as a balance between mechanical and electrical parameters and have proven to be very reliable in over 40 years of service in demanding military programs. Typical phase-temperature characteristics for these families are illustrated in Figure 2.

M/A-COM Phase Versus Temperature Data
Figure 2. Phase versus temperature characteristics for common cable families

Although based on M/A-COM cables, these curves are also representative of cables from all cable manufacturers. The nominal Vp for any particular cable type is given in its data sheet.

Coaxial Cable Phase Matching

The closeness of the match in phase matched cable sets is dependent upon several parameters. These include:

1. Highest Frequency of Operation

In general, the higher the operating frequency, the more difficult it is to achieve a close match. The match limits of Figure 1 are generally offered with minimal extra cost for the additional fabrication and testing required. More stringent matches may require even greater effort and cost, or the use of phase adjustable connectors.

2. Length of Cable Assembly

The longer the cable assembly, the more difficult is the matching task. Thus, longer assemblies require wider phase match windows. Conversely, short assemblies can often be provided with tighter phase match windows. The suggested "Best Match" limits can readily be achieved for assemblies of the specified lengths, and sometimes longer. With increased

manufacturing effort, and corresponding increase in price, tighter limits can be achieved. It is often necessary to balance system requirements and financial restrictions to arrive at the best solution.

3. Variation of Velocity of Propagation

Cables which have identical physical lengths but different Vp's will have different electrical lengths. Tight control of Vp eases the matching process and results in assemblies with more similar physical lengths. For high Vp flexible coaxial cable assemblies the Vp can usually be held to the nominal value within ±1% about half of the specified ±2% range. For long assemblies, adjustment of the physical length to achieve match can result in a variation of several inches. Thus, it is best to specify the electrical match and minimum mechanical length with length being the variable to achieve the desired phase match. Specifying both a tight phase match tolerance, and a tight length tolerance, decreases cable yield and increases cost.

Consider as an example a ten foot assembly for use up to 18 GHz. Further, assume that it has a nominal Vp of 82% but due to manufacturing or material variations, the Vp can range from 81.0% to 83.0%. Recall that in free space

C=f * λ

Where C is the speed of light approximately 3 * 108 meters/second, f is the frequency in Hertz, and λ is the free space wavelength in meters. At 18 GHz, the wavelength is 0.0167 meters. Within the coaxial cable, the effective wavelength is Vp * λ or 0.0137 meters. Our hypothetical ten foot cable with Vp of 82% is 223.1 wavelengths long. Each wavelength is a 360 degree phase shift so the electrical length is around 80,316 degrees.

If we repeat the calculation with Vp reduced to its 81% limit, the effective wavelength is 0.0135 meters. The same ten foot length is now 225.8 wavelengths long with a corresponding phase shift of around 81,288 degrees.

The M/A-COM Cable Catalog program includes a calculator that illustrates this phenomenon by calculating the phase difference between two identical length cables having different Vps. The numbers are a bit different from the sample calculation because more decimal places are used in the program. Note the dramatic effect a small change in Vp has on the electrical length. It also calculates the length change required to phase match the cable pair. The required length adjustments are shown in Figure 3 below.

Phase Variation Comparison Phase Variation Comparison
Figure 3a. Comparison of cables with Vp = 82.0% and 81.0%Figure 3b. Comparison of cables with Vp = 82.0% and 83.0%

Figure 3. Length adjustment to match 10' cables with Vp ranging from 81.0% to 83.0%.

Note that phase matching under these conditions requires a length tolerance of ±1.5 inches, which is not obvious. Our intuition might say use ±0.12 inches to achieve a good match. But a tight length restriction only limits the amount of cable that can be used to fabricate the matched cables and doesn't assure a match. In fact, as shown in this example, cables with precisely the same length have a phase variation due to different Vps of approximately 980°.

4. Temperature

The electrical length of a Teflon dielectric coaxial cable assembly changes as a complex function of temperature as shown by the phase-temp curves of Figure 2. Note that over most temperature ranges the higher Vp cables exhibit smaller phase changes than the lower Vp cables. This is also important in Phase Tracking, which is discussed in the next section.

Suppose two ten foot assemblies are perfectly matched to each other at room temperature. Now suppose one cable of the pair is used in a temperature controlled area while the second cable is used in an area where the temperature varies from -55°C to +125°C. Using the formulas given in Section 3 above, combined with the phase-temperature changes given in Figure 2, we can determine the electrical length at any temperature.

Consider a solid Teflon dielectric with Vp of 69%. At room temperature the phase shift is 95,482°. At -55°C the length increases by 294°; at +125°C it decreases by 162°.

If the dielectric were air-spaced Teflon with Vp of 82%, the numbers are quite different. The room temperature phase shift is 80,345°. At -55°C the length increases by 26°; at +125°C it increases by 57°; and at 30°C the length decreases by 1°.

Again, this calculation has been automated with a calculator, which is illustrated in Figure 4.

Phase Variation
Figure 4. Phase shift change over temperature range Clearly, special precautions must be taken to maintain the match when the cables of a matched set are exposed to different temperatures.

5. Connectors

It is much easier to phase match cable assemblies with the same connectors on both ends than assemblies with different connectors. That doesn't mean that an assembly with straight connectors can't be matched to one with

angled connectors; or one with TNC connectors matched to one with SMA connectors. It just adds to the difficulty and uncertainty of the match.

In some applications it is necessary to account for the phase changes which occur during installation.

Often the system software does this. It can also be accomplished through the use of phase adjustable connectors attached directly to the cable assembly.

6. Test Equipment Accuracy

It is highly recommended that a Vector Automatic Network Analyzer or PNA (Agilent 8510 for example) be used for the measurement of electrical length. To achieve a high degree of accuracy, the test equipment and cable assemblies must be stabilized in a temperature-controlled room. At M/A-COM we do the final phase trimming and ATP testing in the same temperature controlled room.

Cable Matched Sets

When the cables of a matched set are bent into different shapes in their installed condition, test fixtures simulating the installation bends should be used during the matching process.

Matching in Sets

There are two ways of phase matching sets of cables:

Matched to a Standard
Matched to other cables in the set

Matching to a Standard

The phase standard could consist of either a "Gold" hardware standard or an unchanging software standard; i.e. a known electrical length in degrees at a specific frequency. Cable assemblies that are phase matched to a gold standard are completely interchangeable. Similarly, cable assemblies that are phase matched to a software standard (known electrical length) are also completely interchangeable. In addition the use of software standards is more cost effective since they don't require extra material to produce physical standards. With this approach any cable of a set can be replaced without replacing all cables of the set.

Matching as a Set

Cable assemblies matched as a set are only guaranteed to be matched to other cables in the same set. There is no guarantee that the cables in any one set will match those of another set, especially if they are manufactured at different times. This approach results in the lowest cost because cable yields are highest. The drawback is that should any one cable of a set have to be replaced, the entire set may be replaced.

Specifying Phase Matched Sets

To produce phase matched sets the manufacturer needs as much of above information as you can provide. At a minimum we need to know which cables make up the set, the highest frequency of operation and the desired match. We also need to know if phase standards are required. For critical applications we need to know the bends of the installed configuration so the matching is achieved simulating the installed configuration. This is especially true of long cables where one or more cables might be coiled while others are relatively straight.

Phase Tracking

Phase tracking is primarily influenced by three parameters:


Temperature changes

The overall phase tracking due to temperature changes is dependent upon whether all assemblies in the set are exposed to the same thermal environment. The absolute phase change is dependent primarily upon the velocity of propagation. In general, the less the absolute phase changes, the better the phase tracking over temperature. Thus, higher Vp cables are less sensitive to phase temperature changes and track better. This was shown in the examples above.


The overall phase tracking due to bends is extremely difficult to predict. For static installations, it depends upon the number of bends, the angular arc they encompass and the proximity to other bends. For dynamic installations, it depends upon the similarity of the flexure cycle each cable experiences.


Prior to matching the cables of a phase-matched set it is necessary to thermally stress relieve them to assure good tracking. For example, assume that the first time a cable assembly is exposed to 125°C its phase shift changes by 10 degrees. The second time this might be reduced to 8 degrees; the third time, 7.5 degrees; the fourth time, 7.2 degrees; etc. Thus, thermal cycling artificially ages or stabilizes the assembly. All M/A-COM phase matched cable assemblies are preconditioned prior to final matching.

The tracking deviation is dependent primarily on the similarity of the installation for each cable in the set. To achieve the best phase tracking it is necessary that all cables be installed in a similar manner, be exposed to the same thermal environment and/or be flexed together.

Critical Applications

For critical applications where the ultimate tracking is required, the cables of the phase-matched set should be stranded into a bundle and enclosed within a protective sheath. If possible, the sheath should be a thermal blanket that maintains the temperature near 30°C where temperature sensitivity is minimal.



C.I: 17.557.095



The propagation constant of an electromagnetic wave is a measure of the change undergone by the amplitude of the wave as it propagates in a given direction. The quantity being measured can be the voltage or current in a circuit or a field vector such as electric field strength or flux density. The propagation constant itself measures change per metre but is otherwise dimensionless.

The propagation constant is expressed logarithmically, almost universally to the base e, rather than the more usual base 10 used in telecommunications in other situations. The quantity measured, such as voltage, is expressed as a sinusiodal phasor. The phase of the sinusoid varies with distance which results in the propagation constant being a complex number, the imaginary part being caused by the phase change.



C.I: 17.557.095


domingo, 25 de julio de 2010

Reflection of Waves from Boundaries

Reflection of Waves from Boundaries

These animations were inspired in part by the figures in chapter 6 of Introduction to Wave Phenomena by A. Hirose and K. Lonngren, (J. Wiley & Sons, 1985, reprinted by Kreiger Publishing Co., 1991)

When an object, like a ball, is thrown against a rigid wall it bounces back. This "reflection" of the object can be analyzed in terms of momentum and energy conservation. If the collision between ball and wall is perfectly elastic, then all the incident energy and momentum is reflected, and the ball bounces back with the same speed. If the collision is inelastic, then the wall (or ball) absorbs some of the incident energy and momentum and the ball does not bounce back with the same speed.

Waves also carry energy and momentum, and whenever a wave encounters an obstacle, they are reflected by the obstacle. This reflection of waves is responsible for echoes, radar detectors, and for allowing standing waves which are so important to sound production in musical instruments.

Wave pulse traveling on a string:

The animation
shows a wave pulse travelling on a string. The speed, c, with which the wave pulse travels along the string depends on the elastic restoring force (tension, T) and inertia (mass per unit length, ) according to


Reflection from a HARD boundary:

The animation shows a wave pulse on a string moving from left to right towards the end which is rigidly clamped. As the wave pulse approaches the fixed end, the internal restoring forces which allow the wave to propagate exert an upward force on the end of the string. But, since the end is clamped, it cannot move. According to Newton's third law, the wall must be exerting an equal downward force on the end of the string. This new force creates a wave pulse that propagates from right to left, with the same speed and amplitude as the incident wave, but with opposite polarity (upside down).
=> at a fixed (hard) boundary, the displacement remains zero and the reflected wave changes its polarity (undergoes a 180o phase change) 


Reflection from a SOFT boundary:

The animation shows a wave pulse on a string moving from left to right towards the end which is free to move vertically (imagine the string tied to a massless ring which slides frictionlessly up and down a vertical pole). The net vertical force at the free end must be zero. This boundary condition is mathematically equivalent to requiring that the slope of the string displacement be zero at the free end (look closely at the movie to verify that this is true). The reflected wave pulse propagates from right to left, with the same speed and amplitude as the incident wave, and with the same polarity (right-side up).
=> at a free (soft) boundary, the restoring force is zero and the reflected wave has the same polarity (no phase change) as the incident wave 

Reflection from an impedance discontinuity:

When a wave encounters a boundary which is neither rigid (hard) nor free (soft) but instead somewhere in between, part of the wave is reflected from the boundary and part of the wave is transmitted across the boundary. The exact behavior of reflection and transmission depends on the material properties on both sides of the boundary. One important property is the characteristic impedance of the material. The characteristic impedance of a material is the product of mass density and wave speed, . If a wave with amplitude  in medium 1 encounters a boundary with medium 2, the amplitudes of the reflected and transmitted waves are determined by:

In the animations below, two strings of different densities are connected so that they have the same tension. The density of the thick string is 4 times that of the thin string. If the speed of waves on a string is related to density and tension by:

how do the wave speeds compare for the two strings?

From high speed to low speed (low density to high density)
 In this animation the incident wave is travelling from a low density (high wave speed) region towards a high density (low wave speed) region.

=> How do the amplitudes of the reflected and transmitted waves compare to the amplitude of the incident wave?
=> How do the polarities of the reflected and transmitted waves compare to the polarity of the incident wave?
=> How do the widths of the reflected and transmitted waves compare to the width of the incident wave? 


From low speed to high speed (high density to low density)
In this animation the incident wave is travelling from a high density (low wave speed) region towards a low density (high wave speed) region.

=> How do the amplitudes of the reflected and transmitted waves compare to the amplitude of the incident wave?
=> How do the polarities of the reflected and transmitted waves compare to the polarity of the incident wave?
=> How do the widths of the reflected and transmitted waves compare to the width of the incident wave? 


Romero Mora Loren C.I:18762881
Tomado de:

Propagation, attenuation and phase constants

Propagation, attenuation and phase constants

The propagation constant is separated into two components that have very different effects on signals:


The real part of the propagation constant is the attenuation constant and is denoted by Greek lowercase letter  (alpha). It causes a signal amplitude to decrease along a transmission line. The natural units of the attenuation constant are Nepers/meter, but we often convert to dB/meter in microwave engineering. To get loss in dB/length, multiply Nepers/length by 8.686. Note that attenuation constant is always a positive number, if it was negative you'd violate the First Law of Thermodymamics (you never get something for nothing!)

The phase constant is denoted by Greek lowercase letter  (beta) adds the imaginary component to the propagation constant. It determines the sinusoidal amplitude/phase of the signal along a transmission line, at a constant time. The phase constant's "natural" units are radians/meter, but we often convert to degrees/meter. A transmission line of length "l" will have an electrical phase of l, in radians or degrees. To convert from radians to degrees, multiply by 180/.
The two parts of the propagation constant have radically different effects on a wave. The amplitude of a wave (frozen in time) goes as cosine(l). In a lossless transmission line, the wave would propagate as a perfect sine wave. In real life there is some loss to the transmission line, and that is where the attenuation constant comes in. The amplitude of the signal decays as Exp(-l). The composite behavior of the propagation constant is observed when you multiply the effects of  and .
The graph below represents wave propagation in a fairly lossy line, we made it lossy so you could observe the familiar exponential curve of amplitude decay. In this graph, =1 and =0.05. In practice we usually want to minimize loss, but this example is a very lossy line!

Phase constant versus wavenumber

We have a separate page on wavenumber. Phase constant and wavenumber are often treated as the same thing. Indeed, for TEM transmission lines (coax and stripline), the phase constant and wavenumber are equal. Wavequide is one case where you need to understand the difference between the two.

Wavenumber is denoted by lower case "k", and is a measure of how many cycles a wave has in a given length, for a traveling wave that is frozen in time.

Phase constant, phase velocity, frequency and wavelength

Let's examine the relationships between phase constant, frequency, phase velocity and wavelength, Recall that there are 2 radians in a wavelength, therefore the relationship between phase constant and wavelength is simply:


Romero Mora Loren C.I:18762881
Tomado De:

Reflection of Sound

Reflection of Sound:

The reflection of sound follows the law "angle of incidence equals angle of reflection", sometimes called the law of reflection. The same behavior is observed with light and other waves, and by the bounce of a billiard ball off the bank of a table. The reflected waves can interfere with incident waves, producing patterns of constructive and destructive interference. This can lead to resonances called standing waves in rooms. It also means that the sound intensity near a hard surface is enhanced because the reflected wave adds to the incident wave, giving a pressure amplitude that is twice as great in a thin "pressure zone" near the surface. This is used in pressure zone microphones to increase sensitivity. The doubling of pressure gives a 6 decibel increase in the signal picked up by the microphone. Reflection of waves in strings and air columns are essential to the production of resonant standing waves in those systems.



Phase Change Upon Reflection:
The phase of the reflected sound waves from hard surfaces and the reflection of string waves from their ends determines whether the interference of the reflected and incident waves will be constructive or destructive. For string waves at the ends of strings there is a reversal of phase and it plays an important role in producing resonance in strings. Since the reflected wave and the incident wave add to each other while moving in opposite directions, the appearance of propagation is lost and the resulting vibration is called a standing wave.
When sound waves in air (pressure waves) encounter a hard surface, there is no phase change upon reflection. That is, when the high pressure part of a sound wave hits the wall, it will be reflected as a high pressure, not a reversed phase which would be a low pressure. Keep in mind that when we talk about the pressure associated with a sound wave, a positive or "high" pressure is one that is above the ambient atmospheric pressure and a negative or "low" pressure is just one that is below atmospheric pressure. A wall is described as having a higher "acoustic impedance" than the air, and when a wave encounters a medium of higher acoustic impedance there is no phase change upon reflection.
On the other hand, if a sound wave in a solid strikes an air boundary, the pressure wave which reflects back into the solid from the air boundary will experience a phase reversal - a high-pressure part reflecting as a low-pressure region. That is, reflections off a lower impedance medium will be reversed in phase.
Besides manifesting itself in the "pressure zone" in air near a hard surface, the nature of the reflections contribute to standing waves in rooms and in the air columns which make up musical instruments.
The conditions which lead to a phase change on one end but not the other can also be envisioned with a string if one presumes that the loose end of a string is constrained to move only transverse to the string. The loose end would represent an interface with a smaller effective impedance and would produce no phase change for the transverse wave. In many ways, the string and the air column are just the inverse of each other. 


Romero Mora Loren C.I:18762881

Standing wave ratio

Standing wave ratio

In telecommunications, standing wave ratio (SWR) is the ratio of the amplitude of a partial standing wave at an antinode (maximum) to the amplitude at an adjacent node (minimum), in an electrical transmission line.

The SWR is usually defined as a voltage ratio called the VSWR, for voltage standing wave ratio. For example, the VSWR value 1.2:1 denotes a maximum standing wave amplitude that is 1.2 times greater than the minimum standing wave value. It is also possible to define the SWR in terms of current, resulting in the ISWR, which has the same numerical value. The power standing wave ratio (PSWR) is defined as the square of the VSWR.

Relationship to the reflection coefficient
The voltage component of a standing wave in a uniform transmission line consists of the forward wave (with amplitude Vf) superimposed on the reflected wave (with amplitude Vr).

Reflections occur as a result of discontinuities, such as an imperfection in an otherwise uniform transmission line, or when a transmission line is terminated with other than its characteristic impedance. The reflection coefficient Γ is defined thus:

  \Gamma = {V_r \over V_f}.

Γ is a complex number that describes both the magnitude and the phase shift of the reflection. The simplest cases, when the imaginary part of Γ is zero, are:
  • Γ = − 1: maximum negative reflection, when the line is short-circuited,
  • Γ = 0: no reflection, when the line is perfectly matched,
  • Γ = + 1: maximum positive reflection, when the line is open-circuited.
For the calculation of VSWR, only the magnitude of Γ, denoted by ρ, is of interest. Therefore, we define:

ρ = | Γ |

At some points along the line the two waves interfere constructively, and the resulting amplitude Vmax is the sum of their amplitudes:

V_\max = V_f + V_r = V_f + \rho V_f = V_f (1 + \rho).\,

At other points, the waves interfere destructively, and the resulting amplitude Vmin is the difference between their amplitudes:

V_\min = V_f - V_r = V_f - \rho V_f = V_f ( 1 - \rho).\,

The voltage standing wave ratio is then equal to:

VSWR = {V_\max \over V_\min} = {{1 + \rho} \over {1 - \rho}}.

As ρ, the magnitude of Γ, always falls in the range [0,1], the VSWR is always ≥ +1.
The SWR can also be defined as the ratio of the maximum amplitude of the electric field strength to its minimum amplitude, i.e. Emax / Emin.

Further analysis

To understand the standing wave ratio in detail, we need to calculate the voltage (or, equivalently, the electrical field strength) at any point along the transmission line at any moment in time. We can begin with the forward wave, whose voltage as a function of time t and of distance x along the transmission line is:

V_f(x,t) = A \sin (\omega t - kx),\,

where A is the amplitude of the forward wave, ω is its angular frequency and k is the wave number (equal to ω divided by the speed of the wave). The voltage of the reflected wave is a similar function, but spatially reversed (the sign of x is inverted) and attenuated by the reflection coefficient ρ:

V_r(x,t) = \rho A \sin (\omega t + kx).\,

The total voltage Vt on the transmission line is given by the superposition principle, which is just a matter of adding the two waves:

V_t(x,t) = A \sin (\omega t - kx) + \rho A \sin (\omega t + kx).\,

Using standard trigonometric identities, this equation can be converted to the following form:

V_t(x,t) = A \sqrt {4\rho\cos^2 kx+(1-\rho)^2} \cos(\omega t + \phi),\,


 {\tan \phi}={{(1+\rho)}\over{(1-\rho)}}\cot(kx).

This form of the equation shows, if we ignore some of the details, that the maximum voltage over time Vmot at a distance x from the transmitter is the periodic function

This varies with x from a minimum of A(1 − ρ) to a maximum of A(1 + ρ), as we saw in the earlier, simplified discussion. A graph of Vmot against x, in the case when ρ = 0.5, is shown below. The maximum and minimum Vmot in a periods are Vmin and Vmax and are the values used to calculate the SWR.

Standing wave ratio for a range of ρ. In this graph, A and k are set to unity.

It is important to note that this graph does not show the instantaneous voltage profile along the transmission line. It only shows the maximum amplitude of the oscillation at each point. The instantaneous voltage is a function of both time and distance, so could only be shown fully by a three-dimensional or animated graph.

Practical implications of SWR

The most common case for measuring and examining SWR is when installing and tuning transmitting antennas. When a transmitter is connected to an antenna by a feed line, the impedance of the antenna and feed line must match exactly for maximum energy transfer from the feed line to the antenna to be possible. The impedance of the antenna varies based on many factors including: the antenna's natural resonance at the frequency being transmitted, the antenna's height above the ground, and the size of the conductors used to construct the antenna.

When an antenna and feedline do not have matching impedances, some of the electrical energy cannot be transferred from the feedline to the antenna. Energy not transferred to the antenna is reflected back towards the transmitter. It is the interaction of these reflected waves with forward waves which causes standing wave patterns. Reflected power has three main implications in radio transmitters: Radio Frequency (RF) energy losses increase, distortion on transmitter due to reflected power from load and damage to the transmitter can occur.

Matching the impedance of the antenna to the impedance of the feed line is typically done using an antenna tuner. The tuner can be installed between the transmitter and the feed line, or between the feed line and the antenna. Both installation methods will allow the transmitter to operate at a low SWR, however if the tuner is installed at the transmitter, the feed line between the tuner and the antenna will still operate with a high SWR, causing additional RF energy to be lost through the feedline.

Many amateur radio operators consider any impedance mismatch a serious matter.However, this is not the case. Assuming the mismatch is within the operating limits of the transmitter, the radio operator needs only be concerned with the power loss in the transmission line. Power loss will increase as the SWR increases, however the increases are often less than many radio amateurs might assume. For example, a dipole antenna tuned to operate at 3.75MHz—the center of the 80 meter amateur radio band—will exhibit an SWR of about 6:1 at the edges of the band. However, if the antenna is fed with 250 feet of RG-8A coax, the loss due to standing waves is only 2.2dB. Feed line loss typically increases with frequency, so VHF and above antennas must be matched closely to the feedline. The same 6:1 mismatch to 250 feet of RG-8A coax would incur 10.8dB of loss at 146MHz.

Romero Mora Loren C.I:18762881
Tomado de:

standing-wave ratio

standing-wave ratio

Standing-wave ratio (SWR) is a mathematical expression of the non-uniformity of an electromagnetic field (EM field) on a transmission line such as coaxial cable. Usually, SWR is defined as the ratio of the maximum radio-frequency (RF) voltage to the minimum RF voltage along the line. This is also known as the voltage standing-wave ratio (VSWR). The SWR can also be defined as the ratio of the maximum RF current to the minimum RF current on the line (current standing-wave ratio or ISWR). For most practical purposes, ISWR is the same as VSWR.

Under ideal conditions, the RF voltage on a signal transmission line is the same at all points on the line, neglecting power losses caused by electrical resistance in the line wires and imperfections in the dielectric material separating the line conductors. The ideal VSWR is therefore 1:1. (Often the SWR value is written simply in terms of the first number, or numerator, of the ratio because the second number, or denominator, is always 1.) When the VSWR is 1, the ISWR is also 1. This optimum condition can exist only when the load (such as an antenna or a wireless receiver), into which RF power is delivered, has an impedance identical to the impedance of the transmission line. This means that the load resistance must be the same as the characteristic impedance of the transmission line, and the load must contain no reactance (that is, the load must be free of inductance or capacitance). In any other situation, the voltage and current fluctuate at various points along the line, and the SWR is not 1.

When the line and load impedances are identical and the SWR is 1, all of the RF power that reaches a load from a transmission line is utilized by that load. When the load is an antenna, the utilization takes the form of EM-field radiation. If the load is a communications receiver or terminal, the signal power is converted into some other form, such as an audio-visual display. If the impedance of the load is not identical to the impedance of the transmission line, the load does not absorb all the RF power (called forward power) that reaches it. Instead, some of the RF power is sent back toward the signal source when the signal reaches the point where the line is connected to the load. This is known as reflected power or reverse power.

The presence of reflected power, along with the forward power, sets up a pattern of voltage maxima (loops) and minima (nodes) on the transmission line. The same thing happens with the distribution of current. The SWR is the ratio of the RF voltage at a loop to the RF voltage at a node, or the ratio of the RF current at a loop to the RF current at a node. In theory, there is no limit to how high this ratio can get. The worst cases (highest SWR values) occur when there is no load connected to the end of the line. This condition, known as an unterminated transmission line, is manifested when the end of the line is either short-circuited or left open. In theory, the SWR is infinite in either of these cases; in practice, it is limited by line losses, but can exceed 100. This can give rise to extreme voltages and currents at certain points on the line.

The SWR on a transmission line is mathematically related to (but not the same as) the ratio of reflected power to forward power. In general, the higher the ratio of reflected power to forward power, the greater is the SWR. The converse is also true. When the SWR on a transmission line is high, the power loss in the line is greater than the loss that occurs when the SWR is 1. This exaggerated loss, known as SWR loss, can be significant, especially when the SWR exceeds 2 and the transmission line has significant loss to begin with. For this reason, RF engineers strive to minimize the SWR on communications transmission lines. A high SWR can have other undesirable effects, too, such as transmission-line overheating or breakdown of the dielectric material separating the line conductors.

In some situations, such as those encountered at relatively low RF frequencies, low RF power levels, and short lengths of low-loss transmission line, a moderately high SWR does not produce significant SWR loss or line overheading, and can therefore be tolerated.

Romero Mora Loren A. C.I: 18762881
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