line parameter for group2

8/7/2019 Line Parameter for Group2

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Presentation by: GROUP 2Members are:

OLAOGUN, ADENIYI O. RH20100592LATEEF, AMINAT A. RH20100587OLARINOYE DAVID O. RH20100431ADEYEMI, ADEBAYO L. RH20100750


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CLASSIFICATION OF TRANSMISSION LINES A transmission line is a material medium or structure that forms a path fordirecting the transmission of energy from one place to another, such aselectromagnetic waves or acoustic waves, as well as electric powertransmission.

Transmission lines are classified as follows:Short:-When the length of the line is less than about 80Km, the effect of shuntcapacitance and conductance is neglected and the line is designated as a shorttransmission line. For these lines the operating voltage is less than 20KV.

Medium:- The length of the line is in between 80km - 240km and the operating linevoltage will be in between 21KV-100KV.In this case the shunt capacitance can beassumed to be lumped at the middle of the line or half of the shunt capacitance may be considered to be lumped each end of the line. The two representations of mediumlength lines are termed as nominal-T and nominal- respectively.


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Classification of Transmission Lines (Contd)

L ong :-L ines more than 240Km long and line voltage above 100KV requirecalculations in terms of distributed parameters. Such lines are known as longtransmission lines. This classification on the basis of length is more or lessarbitrary and the real criterion is the degree of accuracy required.


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The Classification of Transmission according toOperating Frequencies

The elegance of the transmission line is that in the ideal

world its behaviour is independent of frequency. Designers use transmission lines to deliverhigh speed signals with minimum degradation. This isthe ideal condition; however, in the real world there aresome minor differences as the operating frequency changes.


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The Classification of Transmission according toOperating Frequencies

y The primary physical characteristic that causes

transmission lines to deviate from the ideal is thedielectric constant(Er), of the base material. However, with the following graph shown below, by modelling atypical transmission line on FR4 from 1 to 5GHz theeffect of dielectric constant change is minimal inimpedance terms.


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The graph below shows typical variations in the Er of FR4 with frequency. Er varies from about 4.15 down to around 4 at 5Ghz and remains essentially flatabove 5Ghz (actual figures depend on glass-resin ratios)

The Classification of Transmission according toOperating Frequencies (Contd)


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The Classification of Transmission according toOperating Frequencies (Contd)

y The measurement of Zo can prove difficult, even with a Vector Network Analyser (VNA) . For example, the

connectors at both ends of the line must becharacterised for each trace configuration. The VNA requires considerable expertise if misleading readingsare to be avoided.

y

A micro-strip circuit uses a thin flat conductor which isparallel to a ground plane.y A strip-line circuit uses a flat strip of metal which is

sandwiched between two parallel ground planes.


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y From the graph of a Zo vs Er for a surface microstripstructure below you can see the modelled change inimpedance is very small (approximately 1 Ohm for anominal 50 Ohm Zo) over the range of Er from 4 to 4.2.


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Th e effects are similar for a symmetrical stripline structure (below).


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y The p rimary line constants are parameters thatdescribe the characteristics of copper (or otherconductive material) transmission lines in terms of the

physical electrical properties of the line. The primary line constants are only relevant to copper lines and are tobe contrasted with the secondary line constants, whichcan be derived from them, and are more generally applicable.


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y The secondary line constants can be used, for instance,to compare the characteristics of a waveguide to a copperline, whereas the primary constants have no meaning fora waveguide.


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EXPLANATION ON LINE PARAMETERS WHICH ARE:

ATTENUATION COEFFICIENTPHASE CONSTANT

PROPAGATION CONSTANT


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ATTEN UATION COEFFI CIENTAttenuation constant

y In telecommunications, the term attenuation constant , also called attenuation parameter or coefficient , is the attenuation of an electromagnetic wave propagating through a medium per unitdistance from the source. It is the real part of the propagation constant and is measured in nepers per metre. A neper is approximately 8.7dB. Attenuation constant can be defined by the amplitude ratio;

y The attenuation constant for copper (or any other conductor) lines can be calculated from the primaryline coefficients as shown above. For a line meeting the distortionless condition, the attenuationconstant is given by;

y L osses in the dielectric depend on the loss tangent ( tan ) of the material, which depends inversely onthe wavelength of the signal and is directly proportional to the frequency.

y The propagation constant per unit length is defined as the natural logarithmic of ratio of the sending endcurrent or voltage to the receiving end current or voltage.


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PHAS E

CON S TA NT

y In electromagnetic theory, the ph ase constant , also called ph ase ch ange constant , parameter or coefficientis the imaginary component of the propagation constant for a plane wave. It represents the change in phase permeter along the path travelled by the wave at any instant and is equal to the angular wave number of the wave. Itis represented by the symbol and is measured in units of radians per meter.From the definition of angular wave number;

y This quantity is often (strictly speaking incorrectly) abbreviated to wave number. Properly, wave number is givenby,

which differs from angular wave number only by a constant multiple of 2 , in the same way that angularfrequency differs from frequency.

y For a transmission line, the Heaviside condition of the telegrapher's equation tells us that the wave number mustbe proportional to frequency for the transmission of the wave to be undistorted in the time domain. Thisincludes, but is not limited to, the ideal case of a lossless line. The reason for this condition can be seen by considering that a useful signal is composed of many different wavelengths in the frequency domain. For there tobe no distortion of the waveform, all these waves must travel at the same velocity so that they arrive at the far endof the line at the same time as a group. Since wave phase velocity is given by;

it is proved that is required to be proportional to . In terms of primary coefficients of the line, this yields fromthe telegrapher's equation for a distortionless line the condition;

However, practical lines can only be expected to approximately meet this condition over a limited frequency band.


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PROPA GATION CON S TA NTD efinitiony The propagation constant, symbol , for a given system is defined by the ratio of the amplitude at the

source of the wave to the amplitude at some distance x , such that,

y Since the propagation constant is a complex quantity we can write;

Where:y , the real part, is called the attenuation constanty , the imaginary part, is called the phase constant

which is a sinusoid that varies in phase as varies but does not vary in amplitude because it requiresbase e, so the attenuation is likewise in base e.

y For a copper transmission line, the propagation constant can be calculated from the primary linecoefficients by means of the relationship;

= ZY y The propagation constant itself measures change per meter but is otherwise dimensionless.

line parameter for group2

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