coaxial cable and strip lines

7/30/2019 coaxial cable and strip lines

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COAXIAL AND STRIP LINE COMPONENTS

TERMINATIONS, CONNECTORS AND

TRANSITIONS

BY- JAI PRAKASH GURNANI

M.TECH (3RD SEM)


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TEMINATIONS

1.MATCHED LOAD

Resistor Loads

Tapered Loads

2. SHORT CIRCUIT AND OPEN CIRCUIT

3. STANDARD MISMATCH


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TERMINATIONS

Many microwave applications require that a transmission line be

terminated in a known impedance. This is particularly true in

measurement systems. Among the more widely used are the matched

load, the short circuit, the open circuit and the standard mismatch.


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1. MATCHED LOAD

A matched load is a one-port component that absorbs all the powerincident on it. This requires that its impedance equal the characteristic

impedance of the line to which it is connected.

Resistor-type loads. Since Zo is real for low-loss lines, a matched

load can be realized by terminating the line with a lumped-elementresistor of value R = Zo. At microwave frequencies, the resistor must be

specially designed to avoid the parasitic reactance normally associated

with low-frequency units.


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Configuration of microwaveresistor used in coaxial

systems.

Equivalent circuit of

microwave resistor for


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Resistive material is usually evaporated on hollow dielectric rod. In

order to minimize capacitive effects, the wall thickness of the rod is

made as small as possible, consistent with mechanical rigidity. When

used as a matched load its capacitive reactance should be at least ten

times greater than R in order to insure load SWR of less than 1.10. Thediameter of the two metal contacts is chosen to accommodate the

center conductor of the coaxial line without an abrupt change in

dimensions.


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Two forms of coaxial matched loads.


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Tapered loads. The restriction on resistor length makes it difficult to

achieve a low SWR above X band with resistor type loads. The tapered

load avoids this difficulty.

A tapered coaxial load


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The reasons for gradually tapering the absorbing material is to minimize

wave reflections. The taper length should be at least few wavelengths

long at the lowest frequency of interest. By satisfying this condition, the

input SWR is essentially unity and hence the input impedance of the load

equals Zo, the characteristic impedance of the coaxial line. The purpose of

the shorting plate at the end of the load section is to prevent radiation

leakage out of or into the unit. The length of the fully loaded lossy section is

chosen so that the total attenuation through and is greater than 20 dB.


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2.SHORT AND OPEN CIRCUITS

A coaxial short circuit may be realized by terminating the line with a

metal plate as shown below.

The plate creates a boundary at which the electric field

associated with the TEM mode is zero. Thus = -1, which is the

reflection coefficient of the short circuit.


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We can create a short circuit by just connecting a low-resistance wire

between the inner and outer conductors. This arrangement would

work at low frequencies but not at microwave s since the reactance

associated with the inductance of the wire would be appreciable and

also some of the field would radiate out of the line, thus adding a

resistive component of the terminating impedance.


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Coaxial and stripline versions of an open circuit are shown above. In

the coaxial unit, the outer conductor extends past the end of the inner

conductor to prevent radiation out of the coaxial line. The diameter of

the outer conductor D must be chosen so that the circular waveguide

section is below at the highest frequency of interest. Its length lshould be sufficient to attenuate the dominant mode by 20 db.

This insures that all modes will be attenuated by at least that amount,

resulting in negligible radiation.

COAXIAL AND STRIPLINE OPEN CIRCUITS


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3. STANDARD MISMATCH

Standard mismatch are used to calibrate SWR and impedance

measuring equipment. Typically, a standard mismatch consists ofa known impedance whose value differs from the characteristic

impedance of the connecting line. The resistor value can be either

greater than or less than Zo.


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CONNECTORS

1. DIELECTRIC BEAD SUPPORTS

2. STANDARD COAXIAL CONNECTORS

The type N connector

The SMA connector

The APC connector


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CONNECTORS

Connectors as the name suggests are use to make connection

between coaxial lines. For microwave applications, the most

common connectors are the Type-N, the SMA and the APC-7

precision connectors. These connectors are also used to provide

convenient input and output terminals for coaxial components andsystems. It is important that the SWR associated with the mating of

two connectors be quite low, typically 1.10 or less. Higher values of

SWR can cause a significant degradation in device performance.

Accuracy of microwave measurements are affected by imperfect

connectors.


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1.Dielectric Bead Supports

Most coaxial connectors use a dielectric bead to mechanically

support the centre conductor. Four types of low-reflection beadsupports are shown below.


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The first one uses a half wavelength section of low-loss dielectric

material. Since the inner and outer conductor diameters are thesame throughout and the dielectric is nonmagnetic,

Where and are the characteristic impedances of the air-insulated

and the dielectric filled lines, respectively.

Since impedance repeats every half wavelength, a matched load

connected to the air-insulated line on the right results in at the

left edge of the dielectric. Thus the SWR along the line is unity, which

means that there is no reflection loss associated with the dielectric bead.

The disadvantage of this type bead support is that its SWR is very

frequency sensitive since the bead can only be a half-wavelength long at

the design frequency.


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The bead support shown above is useful at frequencies below 3 GHz. To

insure low reflections, its length is typically 0.02 or less at the highest

frequency of interest. Since the bead represents a very short length of

low-impedance line, it is equivalent to a shunt capacitance. Thus its SWR

increases as the operating frequency is increased. Due to the electrical

requirement that the bead length be less than 0.02 , a mechanically

rigid support becomes difficult to achieve at the higher microwavefrequencies.


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In the third type of bead support the center conductor of dielectric section is

reduced to maintain the same characteristic impedance through out the

structure. Therefore,


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With the characteristic impedance the same in both the air-insulated

and dielectric-filled lines, the SWR is practically unity at all

frequencies. As usual, it is assumed that the radius b is chosen to

avoid mode propagation.

Last type of bead supports are used in both Type-N and APC-7 connectors.

Teflon and rexolite are commonly used as the dielectric material. The

conductor diameters in the dielectric region are adjusted so that .

The capacitive effects of the step discontinuities are compensated for by

undercutting the dielectric on both ends as shown. This creates small

sections of high impedance lines which behave like series inductances.


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2.Standard Coaxial Connectors


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TRANSITIONS1. COAXIAL TO COAXIAL TRANSITIONS

2. COAXIAL TO STRIPLINE TRANSITIONS

3. BALUNS


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1. COAXIAL TO COAXIAL TRANSITIONS

Some times a lowSWR transitions is required between coaxial

lines having the same characteristic impedance but different

dimensions.

Assuming that the dielectric material is the same throughout, the ratio of

outer to inner diameters must be the same for both lines. Thediscontinuity de to change in diameters creates a shunt capacitive effect

at the junction which causes reflection. If is equal to or greater than

, a short circuit occurs at the junction and SWR becomes infinite.


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2. COAXIAL TO STRIPLINE TRANSITIONS

Coaxial equipment is often used to measure the electrical performance

of stripline components and systems. This requires the use of low-SWRtransitions between coaxial and strip-type transmission lines having the

same characteristic impedance.


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The strip width is so chosen that , the only reflection will be

due to the sudden change in the conductor configuration at the

junction. This discontinuity may be approximated by a shunt

capacitance. As before, a small section of the high impedance line

provides the required inductance. The high impedance line is created

by reducing the strip near the junction. This arrangement provides a

low-SWR transition over a wide frequency range.


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3.BALUNS

A balun is a device that provides a low-SWR transition between a

balanced and an unbalanced one. In some cases, a change in

impedance level is also involved. A balanced line is defined as one in

which the voltage to ground of the two conductors are equal and

opposite. In an unbalanced line, one of the two conductors is at the

ground potential. Coaxial, stripline and microstrip configurations are

examples of unbalanced lines.


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The turns ratio of the transformer is chosen to provide a low SWR at both

ports. This type of balun is fairly popular at TV and radio frequencies.

Excessive dissipations and parasitic reactance effects, however, make it

unsuitable for use at microwave frequencies.


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The operations of this type of balun is based on the fact that both the

voltage and current waves are phase delayed by the half-wave line

section. Since impedance repeats every half wavelength, the current I splits

equally at point A. Due to phase delay, the current direction at point B

is opposite that at point A. Similarly the voltage polarity at point B is

opposite that at point A. Thus the voltage and current of the unbalanced

line are transformed to those required for balanced line transmission. The

line length between A and B can only be a half wavelength at one

frequency, this balun is only effective over a narrow frequency range.

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