How to Measure VSWR Using a Directional Coupler
A directional coupler is used in RF systems as a power divider.
Directional couplers can be designed to sample power from a microwave circuit and measure it with an inductive probe, microwave ADC, or receiver.
A directional coupler can be used to sample some power in a standing wave, which can be used for a voltage standing wave ratio measurement.
In order to ensure high transmission of radiation between a source and a downstream device over the air, antennas need feedlines to be carefully matched to the antenna impedance. Impedance matching for antennas is a fundamental subject in RF design, but new designs also need to be evaluated to ensure the matching technique provides desired power transfer. The goal is to ensure the antenna has low return loss and insertion loss at the interface between the feedline and the radiating element.
Impedance matching is also important in RF devices beyond antennas, and there is a metric that can be used to evaluate antennas on a finished PCB. The voltage standing wave ratio (VSWR) is one convenient metric that is linked to impedance matching within the desired antenna bandwidth. One useful way to evaluate antenna impedance matching is to measure VSWR using a directional coupler. If you plan to use a directional coupler to measure VSWR, here’s how to analyze your coupler design and the measurement results for your system.
Measuring VSWR Using a Directional Coupler
Whenever a traveling wave encounters an interface between two media with different dielectric properties, a reflection can occur if the impedances are mismatched. If there is a reflection, the reflected wave can interfere with the incident wave, and the two waves can superimpose to create a standing wave pattern. The intensity of the standing wave pattern depends on the phase difference between the two waves and their intensities.
VSWR is defined in terms of the voltages of the input and reflected waves, and it can be used to quantify the level of reflection at the interface between the two impedances. In terms of a reflection coefficient, VSWR is defined as:
VSWR definition in terms of the reflection coefficient.
Here, the reflection coefficient can be used to write the above equation in terms of measured voltage ratios, if desired. For an antenna/feedline system, the reflection coefficient is the value taken at the interface between the feedline and the antenna. In other words, this is defined by the load (antenna impedance) and the feedline impedance only. Any standing wave that arises on the feedline will be a function of reflection at the end of the feedline.
To see where a directional coupler can be used to measure VSWR, take a look at the diagram below. The directional coupler receives some power via capacitive and inductive coupling, which allows propagating waves to flow out of the two coupler ports (isolated and coupled signals). This type of structure is the basis of commercial VSWR meters. An antenna has been shown in the system below, as this is typical for RF devices, but in principle, any component could be used in place of the antenna.
You can measure VSWR for an RF component like an antenna, feedline, or waveguide using a directional coupler.
In the above system, we have a 4-port network that can be described using a 4x4 S-parameter matrix. Direct signal measurements can be used to get the S-parameter matrix for this system, which normally involves an average input power measurement within the standard S-parameter theory. Note that 50 Ohms is normally used as the reference impedance for this type of measurement. Be sure to de-embed the S-parameters for any test fixtures in the design to ensure the measurements only reflect the S-parameters of the feedline/antenna/coupler composite structure. More advanced techniques will use a high sample rate oscilloscope with live de-embedding for real-time S-parameter and VSWR measurements.
In the above system with a directional coupler, a vector network analyzer (VNA) can be used to measure the S-parameters of the system and determine the VSWR value from the reflection coefficient and input impedance. If you’re not a fan of S-parameters, there is a simpler measurement that can be done with a spectrum analyzer as long as you know the directivity of your antenna (D):
Connect the spectrum analyzer inputs to the isolated and coupled ports on the directional coupler.
Set the low-end frequency on your source and input this into the feedline.
Measure the magnitude of the wave voltages at the coupled and isolated ports.
Use these voltage values and D in the following equation to get the reflection coefficient:
Solve this equation for Γ to get the reflection coefficient and VSWR.
With this equation, you can plug in the value of D and the measured voltages, and finally solve the above equation for the reflection coefficient. You can then use the result to calculate VSWR.
If you do use a VNA, the test fixture S-parameters need to be de-embedded before taking measurements with the VNA. S-parameter data for test fixtures can be determined by direct measurement, using a reference line for comparison, or it may be provided by the manufacturer if a connector is used. This alternative method with a spectrum analyzer allows VSWR to be calculated from a direct reflection coefficient measurement.
Use S-Parameter Extraction to Aid Measurements
The most advanced techniques involve the use of field solvers to extract the S-parameters of a feedline/antenna structure. When these components are used in the PCB layout, PCB design software with an integrated field solver utility can be used to extract S-parameters without using an external application. The same idea applies to the feedline/antenna/coupler composite structure.
Once you know the VSWR and reflection coefficient, either from measurements or simulations, you can determine how to modify the antenna or feedline (or both) to get maximum power transfer into the antenna. By adjusting the feedline geometry, impedance matching circuit, or stub impedance, you’ll have a simple method for adjusting the input impedance of the antenna to provide maximum power transfer into the antenna.
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