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Measuring voltage standing wave ratio with DSA815

10 Nov 2015  | Kenneth Wyatt

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In the example of figure 1, I used a small square of aluminium foil to serve as a counterpoise for the monopole antenna being measured. I extended the telescoping antenna to its full height of 27 inches. By using the Marker, you can measure both the resonant frequency and return loss. Note that classical "return loss" is a positive number, but is represented on the analyser as a negative number. Just ignore the "minus" sign for purposes of this measurement. Note, also, the primary (double) resonance about 108 and 130MHz, as well as the odd-order resonances about 338, 561, 780, and 1,000MHz. By moving the marker to 130MHz, we can read the return loss of 18.07 dB. From the Mini-Circuits chart, we see this is equivalent to a VSWR of 1.29:1.

Figure 2: Screen capture showing the return loss of a resonant antenna. Note the additional resonances at odd-order (x3, x5, x7, x9) frequencies.

VSWR method
The VSWR method requires the activation of the VSWR option. This may be added at any time, as it's merely a software licence activated by the user. There's also an optional VSWR coupler available that attaches directly to the Gen Output and RF Input ports.

Figure 3: Measurement setup using the VSWR option is really the same as for the return loss measurement. It's just that the analyser will read out VSWR directly.

To use the VSWR measurement option, set the frequency limits as desired, then set the resolution bandwidth at 30kHz, or so, for a better dynamic range. Be sure the device to be measured is disconnected from the IN coupler port. Press the Meas button on the main keyboard, then VSWR On (soft key). Then press Meas Setup on the main keyboard and Cal Open. Connect the device to be measured and press VSWR (soft key).

There will be three traces displayed. The bottom yellow trace is the XXX, the violet trace is the reference trace, and the green trace is return loss. This is the one used to calculate Reflection Coefficient and VSWR, which will be displayed in the lower half of the screen. Many times, I'll use the Ref Lvl to adjust the lower two traces off the screen, so just the green return loss trace is showing. By placing markers at each resonant point, the table will display up to four VSWR measurements. For example, you could set one marker at resonance and an upper and lower frequency limit for a VSWR of 1.5:1 in order to determine the usable bandwidth of an antenna.

Figure 4: Screen capture of the VSWR measurement showing several markers with a table of return loss and associated VSWR calculations.

The Rigol DSA815TG is an amazing value and with the built in (optional) tracking generator, it is possible to make several additional measurements. While the base model with tracking generator may be used to calculate VSWR, purchasing the VSWR option makes this measurement even easier with the ability to directly read out VSWR.

About the author
Kenneth Wyatt is an independent consultant and specialist in electromagnetic compatibility (EMC) design, test and troubleshooting. One speciality is his use of do-it-yourself probes and use of low-cost test equipment and measurement techniques for evaluating EMC issues on the workbench.

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