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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.