6.1 The Microwave Bench and a Fundamental Measurement Setup

The design and analysis of microwave systems cannot rely on theory alone; it must be validated by practice. Accurate measurement is therefore a critical discipline in microwave engineering. A standard set of laboratory equipment, collectively known as a microwave bench, is used to generate, control, and measure microwave signals, allowing engineers to characterize the performance of individual components and entire systems.

A general-purpose microwave bench setup for making fundamental measurements typically includes the following components arranged in a logical signal-flow order:

• Signal Generator: The source of the microwave signal, often a Gunn Oscillator or a Reflex Klystron, producing a low-power continuous wave.
• Isolator: Placed immediately after the source, this crucial device allows signals to pass in the forward direction but absorbs any reflected signals, preventing them from returning to the source and causing instability.
• Precision Attenuator: A calibrated, variable attenuator used to precisely control the power level of the signal for accurate relative measurements.
• Frequency Meter: A resonant cavity device used to accurately measure the frequency of the microwave signal.
• Variable Attenuator: This component provides additional, often uncalibrated, control over the signal level, used for adjusting power to an appropriate level for a detector or device under test.
• Crystal Detector: A semiconductor diode used to rectify or demodulate the microwave signal, converting it into a DC or low-frequency voltage that can be measured.
• Standing Wave Indicator (VSWR Meter): A voltmeter specifically designed to measure the output of the crystal detector, often calibrated to directly display the Voltage Standing Wave Ratio (VSWR).

The Slotted Line

The slotted line is a fundamental instrument used to directly observe the standing wave pattern inside a waveguide. From this pattern, several key parameters can be derived, including VSWR, wavelength, and the impedance of a load.

• Purpose: Its primary function is to measure the electric field strength at various points along the length of a transmission line. By finding the locations and magnitudes of the voltage maxima (V_{max}) and minima (V_{min}), one can directly calculate the VSWR.
• Construction: A slotted line consists of a section of waveguide with a narrow, non-radiating slot cut axially along its top wall. A movable carriage rides along this slot. Attached to the carriage is a probe that extends a small distance into the waveguide to sample the internal electric field. This probe is connected to a crystal detector.
• Operation: As the probe carriage is moved along the slot, it samples the electric field strength of the standing wave at each position. The crystal detector converts this RF field into a proportional DC voltage. By moving the carriage and observing the output on a VSWR meter, an engineer can locate the physical positions of V_{max} and V_{min} and measure their relative magnitudes.

This fundamental setup can be configured to perform a variety of specific measurements, which will be detailed in the following section.

6.2 Measurement of Key Microwave Parameters

This section details the practical procedures for measuring the most common and critical parameters in microwave systems: Power, Attenuation, Phase Shift, VSWR, and Impedance. Each measurement requires a specific configuration of the microwave bench and a distinct methodological approach.

Power Measurement

Microwave power is measured in watts, and the appropriate technique depends heavily on the power level being measured.

• Low Power (0.01mW to 10mW): For low power levels, the bolometric technique is used. A bolometer is a device whose resistance changes with temperature. It contains a temperature-sensitive element, such as a barratter (a thin wire) or a thermistor (a semiconductor bead). The element is placed in one arm of a balanced bridge circuit. When microwave power is applied, the element heats up, its resistance changes, and the bridge becomes unbalanced. The amount of DC power required to re-balance the bridge is equal to the absorbed microwave power.
• Medium Power (10mW to 1W): For medium power, the calorimeter technique is employed. A special load absorbs the microwave power, causing its temperature to rise. The change in temperature is directly proportional to the energy absorbed, allowing for a calculation of the average power.
• High Power (>10W): High power levels are measured using calorimeter watt meters. These can be flow types, where a fluid like water flows through a load to absorb the heat, or dry types, which use a section of coaxial cable filled with a high-loss dielectric to absorb the power.

Attenuation Measurement

Attenuation is the reduction in power of a signal as it propagates through a component. It is measured in decibels (dB). Attenuation \: (dB) = 10 \: log_{10} \left(\frac{P_{in}}{P_{out}}\right)

• Power Ratio Method: First, the output power of the test setup is measured without the device under test (DUT). Second, the DUT is inserted, and the output power is measured again. The ratio of the two power levels gives the attenuation. The main drawback is its potential for inaccuracy if the detector does not have a perfectly square-law response.
• RF Substitution Method: This is a more accurate technique. First, the DUT is placed in the line and the output power level is noted. Second, the DUT is removed and replaced with a precision calibrated variable attenuator. Third, this calibrated attenuator is adjusted until the output power is identical to the level noted in the first step. The attenuation of the DUT is then read directly from the calibrated dial. This method is more accurate because the detector is only used as a reference indicator.

VSWR Measurement

• Low VSWR (S<10): For low VSWR values, the measurement is made directly using a slotted line. The probe is moved to find the maximum voltage (V_{max}) and the minimum voltage (V_{min}). The ratio of these two readings on the VSWR meter gives the VSWR value.
• High VSWR (S>10): When VSWR is high, the voltage minimum is very sharp and deep, making an accurate measurement of V_{max} difficult. In this case, the double minimum method is used. The probe is moved to either side of a voltage minimum to find the two positions (d_1 and d_2) where the power is double the minimum value (the 3dB points). The VSWR is then calculated using the formula: VSWR = \frac{\lambda_{g}}{\pi(d_2-d_1)} where \lambda_g is the guide wavelength.

Impedance Measurement

• Using a Slotted Line: This method determines both the magnitude and phase of an unknown impedance. First, the unknown load is connected, and the VSWR and the position of a voltage minimum are measured. Second, the load is replaced by a perfect short circuit, and the new position of the voltage minimum is recorded. The shift in the position of the minimum, along with the measured VSWR, allows for the calculation of the complex reflection coefficient and thus the unknown impedance.
• Using a Reflectometer: A reflectometer provides a simpler way to find the magnitude of the impedance. The setup uses two directional couplers oriented in opposite directions. One coupler samples a portion of the incident power (P_i), and the other samples a portion of the reflected power (P_r). The magnitude of the reflection coefficient (\rho) is then found directly: \rho = \sqrt{\frac{P_r}{P_i}} From this value, the VSWR and the magnitude of the unknown impedance can be calculated.

Understanding these measurement techniques is fundamental to practical microwave engineering. To solidify this knowledge, the final module will walk through several worked examples.