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…

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� 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…

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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…

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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

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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.

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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…

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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…

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1.3.11 Radar Cross Section Measurement

� RCS of a target

� Two basic – monostatic CS or bistatic CS

Cont’d…