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Voltage Waveforms

How to Measure VSWR with an Inline Power Meter

Posted 28 August 2014 by Tom Kuklo

5 Guidelines for the Highest Quality Results

INTRODUCTION
The measurement of Voltage Standing Wave Ratio (VSWR) is considered to be the most universal indicator of the health of transmission systems. Continuous monitoring of the transmission system VSWR from the output of the transmitter to the input of the antenna is an important part of system maintenance.

While a vector network analyzer can make high quality measurements on inactive systems, inline directional measurement devices usually perform the monitoring of system VSWR under operating conditions.

VSWR DEFINITION
VSWR is the ratio of the amplitude of a standing wave on a transmission line at its maximum voltage to the amplitude at an adjacent node’s minimum voltage. The larger the difference is, the larger the standing wave.
The standing wave pattern is developed by the interaction of the forward traveling wave with the reflected traveling wave, where the magnitude of the reflected traveling wave is related to the degree to which the load is mismatched to the characteristic impedance of the transmission system. (see Figure 1)

IN-LINE MEASUREMENTS
Direct measurement of the forward and reflected traveling wave (forward and reflected power) using in-line directional power meters is the most common technique used today for the ongoing maintenance of broadcast communication networks for routine VSWR measurements.

There are several important considerations when making these measurements that will help to insure high quality measurements, including:
 
1. Power Meter Accuracy - What is the basic accuracy of the directional power meter to be used?
2. Power Meter Dynamic Range - Over what range of power measurements will the instrument maintain rated accuracy?
3. Power Meter Directivity - What is the directivity of the directional coupler which forms the heart of the power meter system?
4. Signal Type - What are the characteristics of the signals that are being measured?
5. Location - Where is the power meter located within the transmission system?

1. Power Meter Accuracy
When using a directional power meter (reflectometer) for VSWR measurement, the VSWR value is based upon the values of forward and reflected power according to the following formula:
VSWR calculation 

Where, Pr and Pf are values of reflected and forward power as determined by the power meter.

The accuracy of the power meter will have a direct effect upon the VSWR measurement. In addition, the power meter accuracy must be considered over the entire dynamic range of the instrument, rather than simply the accuracy of the power meter at a single point.

2. Power Meter Dynamic Range
The dynamic range of a directional power meter is defined as the range of power values over which the instrument is capable of resolving measurements, while maintaining full rated accuracy.
It is important to note that in most cases, power meters used for broadcast applications are configured as full two channel instruments, with separate channels for incident and reflected measurements. Often, the full scale power ranges of each channel are tailored to specific measurement conditions.

However, there is a limitation to this approach based upon the maximum power handling capability of the directional coupler/voltage detector circuit.

3. Directional Power Meter Directivity
A directional coupler is a key component of every in-line, directional power meter for sampling the transmission line voltage while distinguishing between the forward and reflected traveling wave.
The directivity parameter for the couplers is a measure of how well the coupler is capable of distinguishing between the energy traveling toward the load, and the energy that is being reflected due to mismatch of the characteristic impedance of the transmission system with the impedance of the load.

Directivity is optimized by balancing the contributions from the electric and magnetic field components.

Knowing the directivity specification of your measurement device can assist in setting measurement expectations. Look for test and monitoring equipment with the highest level of directivity available to optimize accuracy.

4. Signal Type
In-Line RF power meters can be affected by the signals that they measure depending upon the power meter type and the modulation characteristics of the signals being measured.
Conventional in-line instruments used since the 1950’s use point contact diode detectors, configured as peak detectors. These are often connected to analog meters calibrated to read average power and result in greater inaccuracy as the crest value of the signal is increased.

Later generation diode type power meters use detectors operated in the diode square law region and will provide detected voltages proportional to true power, as long as the signal applied to the detector is within the diode square law region.

5. In-Line RF Power Meter Location
The location of the directional power meter within the transmission system may have a significant effect upon the ability of the instrument to resolve antenna VSWR.
The prime consideration related to the location of the power meter is that the losses associated with the transmission line, as well as the insertion loss values of any other system components will have the effect of isolating (masking) the power meter from the measurement point.

The power meter will be further isolated if the path between the power meter and the antenna includes components such as switches, adapters, or other items because of the insertion losses of these components.

Conclusion
VSWR measurements using an inline power meter are important components in the operation of RF and microwave transmission systems of all types. Errors due to directivity can be significant and may influence the decisions made based upon the test results.
Following these guidelines will help to insure that the highest quality measurements are obtained.

To get detailed information of our examples, calculations, results and implications, download our whitepaper “VSWR Measurements Using In-Line Power Meters”.

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