Module - 4: Microwave Measurements

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On completion of this module, you will be able to:

Introduction to Microwave Measurement

At low frequency, it is convenient to measure voltage, current and frequency and use them to calculate the power.
However, at microwave frequencies, the amplitude of the voltage and current on a transmission line are function of distance and are not easily measurable.
Hence, at microwave frequency it is more desirable and simpler to measure power directly. Therefore, it is necessary to measure power instead of voltage and current.

SWR is defined as the ratio of the maximum radio frequency (RF) voltage to the minimum RF voltage along the line.
In radio engineering and telecommunications, SWR is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide.
Impedance mismatches result in standing waves along the transmission line and SWR is defined as the ratio of the partial standing wave's amplitude at an antinode (maximum) to the amplitude at a node (minimum) along the line.

VSWR meters are widely used to measure VSWR. VSWR is defined as ratio between the maximum voltage to minimum voltage.
VSWR stand for Voltage Standing Wave Ratio and it is also referred to as Standing Wave Ratio (SWR).
VSWR is a function of the reflection coefficient, which describes the power reflected from antenna.
When a signal is transmitted through a transmission line, some signal power is always reflected or returned to the source due to discontinuities in the transmission line.
The measure of this reflected power is called Return Loss. The Return Loss is expressed in dB .
If the reflection coefficient is given by τ, then the VSWR is defined by the following formula:

The typical VSWR meter is a high gain, low noise amplifier. It is tuned to the modulating frequency of the microwave signal (1 KHz).
The display panel of the meter consists of three scales :
1. Normal Scale
2. Expanded Scale
3. dB Scale
Normal Scale: If the reading of VSWR meter is between 1 and 4, then the top normal SWR scale can be used. If the VSWR reading is between 3 and 10, the lower SWR scale is used.
Expanded Scale: If the VSWR is smaller than 1.3, a more accurate reading can be taken by selecting the Expanded SWR scale, graduated from 1 to 1.3. Expanded .
dB Scale: Third scale at the bottom is graduated from in dB.
To measure VSWR, the meter needle is initially adjusted to 1 after placing the probe in Vmax position within the guide.
The gain control panel is used for making this adjustment. For any input voltage, the amplifier output voltage is measured directly in terms of VSWR.
The input of the VSWR meter is the detected output voltage of the tuned detector that is fed by a coaxial cable.

VSWR meters are widely used to determine the amount of radio frequency that is being reflected back to the transmitter / source compared to the amount that is being sent out during transmission.
If the ratio is high the source will surely damage.
The maximum standard rating for a VSWR meter where a transmission can withstand is 1.5:1.
The ideal rating is 1:1 (i.e., the power reaches the destination and without reflection).