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MICROWAVE ENGINEERING
EKT 345
CHAPTER 2:
(Part 2)
MICROWAVE MEASUREMENTS
Outline:
1.3 Measurements : Power, insertion loss and
attenuation, VSWR, return loss, impedance,
frequency, cavity, dielectric constant,
scattering parameters of network, radar cross
section
1.3 Measurements
1.3.1 Power Measurements
� The quantity of energy dissipated or
stored per unit time.
� Low, medium and high power.
� A power sensor which converts the
microwave power to heat energy
� Schottky barrier diode, bolometer,
thermocouple.
Cont’d…
Cont’d…
� Power meter
Cont’d…
Cont’d…
� Thermocouple sensor
� A junction of two dissimilar metals or semiconductors that generate emf.
Cont’d…
� High power measurement
� Calorimetric method:
� Involves conversion of the
microwave energy into heat,
absorbing heat in fluid, measuring
the temperature rise of the fluid.
� Static calorimeters
� Circulating calorimeters
Cont’d…
Cont’d…
1.3.2 Insertion Loss and Attenuation
Measurements
� Insertion loss: the difference in the power
arriving at the terminating load, with or
without the network in the circuit.
Insertion loss = reflection loss + attenuation loss
Cont’d…
Cont’d…
1.3.3 VSWR Measurements
� VSWR & the magnitude of voltage
reflection coefficient
� to determine the degree of impedance
matching
� to measure the load impedance.
� By detecting Vmax and Vmin
� Low VSWR, high VSWR
Cont’d…
Basic experimental set up
Cont’d…
1.3.4 Return Loss Measurements
� Measure the magnitude of reflection
coefficient by reflectometer
� Two identical directional couplers are
connected opposite to each other.
� Forward wave
� Reverse wave
Cont’d…
Experimental set up
Cont’d…
1.3.5 Impedance Measurement
� Complex quantity – measure both
amplitude and phase:
� Slotted line method
� Reactive Discontinuity
� Reflectometer
Cont’d…
1.3.6 Frequency Measurement
� Commercially measured using
frequency counter and cavity
wavemeter.
� Wavemeter method
� Slotted line method
� Down conversion method
Cont’d…
1.3.7 Cavity Measurement
� Slotted line measurement of Q
� Q from transmitted power
� CW measurement
� Swept frequency measurement•Advantages
• Requires less frequency stability of the RF source
• Accurate measurement of BW
• Small measurement time
Cont’d…
� Reflectometer method
Cont’d…
1.3.8 Dielectric Constant Measurement
� Dielectric constant is defined by the
permittivity of material with the
respect to that ε0 of air or free space.
Cont’d…
Decrement method of measuring Q
Cont’d…
Transient response of cavity
Waveguide method
Cont’d…
•Material assumed to be lossless.
Cont’d…
Cavity perturbation method• Highly sensitive and accurate
• Advantageous in the determination of the dielectric
constant and small loss tangents.
Cont’d…
Cont’d…
1.3.9 Network Scattering Parameters Measurement
� S-parameter can be measured following Deschamps method – utilizes measured values of complex input reflection coefficient under a number of reactive terminations.
� S-parameters of a two-port network
� S-parameters of a magic-T
Cont’d…
Cont’d…
Cont’d…
1.3.10 Microwave Antenna Measurement
� Parameters to be measured to determine antenna performance
� Radiation amplitude patterns
� Radiation phase patterns
� Absolute gain
� Directivity
� Radiation efficiency
� Beam width
� Input impedance
� BW
� polarisations
Cont’d…
� Reflections from surrounding environment are reduced by:
� Selecting the directivity and side lobe level of Txantenna
� Making LOS between antennas obstacles free
� Absorbing the energy
For accuracy of the measurement, care must be taken so that
1. All antennas meet the far field criteria: R ≥ 2D2/λ.
2. The antennas are aligned for bore-sight radiation face-to-face.
3. The measuring system is frequency stable.
4. Impedance mismatched in the system components is minimum.
5. Polarisation mismatch is minimum.
6. Reflection from various background and support structure is
minimum.
Cont’d…
Cont’d…
1.3.11 Radar Cross Section Measurement
� RCS of a target
� Two basic – monostatic CS or bistatic CS
Cont’d…
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