Module 2
Chapter 2: Microwave Passive Devices
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Topics
• Coaxial Connectors and Adapters
• Attenuators
• Phase Shifters
• Waveguide Tees
• Magic tees
(Text Book: Microwave Engineering – Annapurna Das, Sisir K Das, TMH
Publication, 2nd edition. >> 6.4.2, 6.4.14, 6.4.15, 6.4.16)
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Coaxial connectors and adapters
• Interconnection between co-axial cables and microwave components
is achieved with the help of shielded standard connectors.
• The average circumference of the co-axial cable, for high frequency
operation must be limited to about one wavelength.
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• Type-N (Type-Navy) connector: It is a 50 ohm or 75 components for
microwave sohm connector. This was developed during World War II
and extensively used as a microwave measurement connector up to a
frequency of 18 GHz.
• BNC (Bayonet Navy Connector): This connector was developed during
World War II and used for military applications. It has characteristic
impedance 50 or 75 ohm and is connected to flexible co-axial cable. It
is extensively used in electronic measuring equipments upto 1 GHz of
frequencies.
• TNC (Threaded Navy Connector): This connector is an improved
version of BNC and is threaded. This threading prevents radiation at
high frequencies so that it can be used upto about 12 GHz frequency.
• SMA (Sub-Miniature A type): SMA connectors can be used upto 24
GHz.
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• APC 3.5 (Amphenol Precision Connector - 3.5 mm):. This connector
can operate up to a frequency of 34GHz and has a very low voltage
standing wave ratio (VSWR). This connector provides repeatable
connections and has 50 ohm characteristic impedance.
• APC -7 (Amphenol Precision connector -7 mm): This connector are
used for very accurate 50 ohm measurement applications. This
connector provides a coupling mechanism without male or female
distinction (i.e., sexless) and its VSWR is extremely low, less than 1.02
in the frequency range upto 18 GHz.
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ATTENUATORS
• To control power levels in a microwave system by partially absorbing
the transmitted microwave signal, attenuators are used. Resistive
films are used in both fixed and variable attenuators.
• Coaxial line fixed attenuator: This uses the dielectric lossy material
inside the centre conductor of the co-axial line. The length of
dielectric rod decides the amount of attenuation introduced.
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Waveguide fixed attenuator:
In waveguides, the dielectric slab coated with aquadag is placed at the
center of the waveguide parallel to the maximum E-field.
• Induced current on the lossy material due to incoming microwave
signal, results in power dissipation, leading to attenuation of the
signal.
• The dielectric slab is tapered at both ends upto a length of more than
half wavelength to reduce reflections.
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• Waveguide variable attenuator: Here the resistive film is moved
using micrometer screw.
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Flap attenuator:
• This is a variable attenuator.
• When the flap is completely outside the slot, then the attenuation is
zero and when it is completely inside, the attenuation is maximum.
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Precision type variable attenuator:
• This consists of a rectangular to circular transition (RC), a piece of
circular waveguide (CW) and a circular-to-rectangular transition (CR).
• Resistive cards R1 , R2 and R3 are placed inside these sections.
• The center circular section containing the resistive card R2 can be
precisely rotated by 3600 with respect to the two fixed resistive cards.
• If the resistive card in the centre section is kept at an angle q relative
to the E-field direction of the TEll mode, the component Ecosq
parallel to the card get absorbed while the component Esinq is
transmitted without attenuation. This component finally comes out as
Esin2θ
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• Attenuation:
• The [s] matrix of an ideal attenuator:
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PHASE SHIFTERS
• A microwave phase shifter is a two port device which
produces a variable shift in phase of the incoming microwave
signal.
• A lossless dielectric slab can be used to produces phase
shifts.
• Two Types:
1. Dielectric phase shifter : They are reciprocal phase shifter
2. Ferrite phase shifter: a. Reciprocal , b. Nonreciprocal
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• Dielectric phase shifters: In dielectric phase shifter, a lossless
dielectric slab is placed within a waveguide parallel to and at the
position of maximum E field.
• Wave velocity:
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• The propagation constant through a dielectric slab of length l,
• The propagation constant through an empty waveguide,
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• The differential phase shift,
Df=(b1~b2)l
• [S] of an ideal phase shifter,
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