An Overview of the Slotted Line Impedance Measurement Technique

Key Takeaways

• Slotted line impedance measurements use the reflection phenomenon in a slotted line at microwave frequencies to calculate complex impedance. 

• In impedance measurements using the slotted line technique, the VSWR value is calculated before impedance. 

• If the minimum value of voltage (Vmin) shifts to the left of that recorded in the loaded condition, then the load connected is inductive. If the shift is to the right of the already recorded Vmin, the load is capacitive.

The slotted line impedance measurement technique can be used to measure impedance in microwave circuits 

In microwave applications, quantities such as power, attenuation, quality factor, guided wavelength, voltage standing wave ratio (VSWR), and complex impedance must be measured. Impedance is an especially critical measurement in certain microwave applications and, luckily, there are several methods used to measure this important factor. 

Methods of rotating loops and resonance lines can be used to determine impedance. Measuring impedance in a microwave circuit can be done using either reflectometer techniques or the slotted line technique. 

In this article, we will focus on the slotted line impedance measurement technique. This technique utilizes the reflection phenomenon in a slotted line at microwave frequencies to calculate complex impedance. 

Let’s delve into the structure of a slotted line and the procedure to follow for slotted line impedance measurements.

Slotted Line Impedance Measurements From the VSWR Value

As in any circuit, the impedance in microwave circuits is the ratio of voltage to current. The direct impedance calculation method requires one to measure the voltage and current at any instant, and take the ratio of voltage and current at that instant. However, at microwave frequencies, it is difficult to measure the voltage and current in a circuit. The slotted line impedance measurement technique is an indirect method that measures the VSWR created by reflections from the device under test for impedance calculations. 

The impedance (Z) is equivalent to the reflection coefficient (), which is the ratio of the reflected wave amplitude to the incident wave amplitude. The reflection coefficient can be determined from the VSWR value. Once the reflection coefficient is obtained, the impedance can be calculated.  The relationship between the VSWR and impedance can be given by the following equations:

The Structure of a Slotted Line

The basic structure of a slotted line is a coaxial, microstrip, or waveguide section of minimum length, equal to half of the guided wavelength with a slot across its length. A movable probe with a tunable detector is inserted into the slot. The probe is inserted so that it is parallel to the electric field and the transmission line. Usually, a microwave source is connected to one end of the line and the device or load under test is placed at the other end. The probe acts like a receiving antenna and enables sampling of the electric field amplitude.

The Procedure for Measuring Impedance With the Slotted Line Technique

In a slotted line impedance measurement, the VSWR value is calculated before impedance. According to equation (1), the VSWR value is calculated using the Vmax and V min. 

The V_max and V_min are recorded at two different conditions:

1. With the load or device under test connected, whose impedance is unknown. 

2. With the load removed and the terminals short-circuited. 

According to the type of load, the Vmin value calculated in the short-circuited condition varies from that of the loaded condition. If the minimum value of voltage shifts to the left of that recorded in the loaded condition, then the load connected is inductive. If it shifts to the right of the already recorded Vmin, the load is capacitive.

The slotted line impedance measurement offers a less precise measurement of microwave impedance. The usual system imperfections are due to probe effects, line attenuation, and residual VSWR. For an accurate measurement of microwave impedance, the slotted-line data needs to be corrected for system imperfections. If you are looking for high precision slotted line impedance measurements, computer simulation can be used for this data correction.