Slotted line measurements

Updated October 14, 2008

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Note to readers... this discussion of slotted line measurements is here mostly for historic value. It will help you understand why microwave engineers use voltage standing wave ratio (VSWR) as a requirement, even though a "more modern" way to measure impedance mismatches is to measure S-parameters using a network analyzer.

Not that long ago, in a time before network analyzers, engineers of yore used something called a slotted line to measure voltage standing wave ratio. You might turn up such an instrument if you work in a lab that is more than 25 years old. Basically it is a coax line with a slot down one side where a probe can be moved longitudinally to measure varying electric field strength. The probe has a detector that converts RF energy to DC voltage, so you can measure peaks and valleys using an voltmeter. For circuits that were extremely mismatched (or open or short circuited), the peaks and valleys are the most noticeable. The ratio of the peak voltage to the valley voltage was the most directly calculated piece of data you can get with a slotted line... hence "voltage standing wave ratio".

Using a slotted line, you could also measure an unknown frequency by measuring the distance between the voltage peaks and noting that the distance is 1/2 wavelength.

Here's some slotted line equipment that we found recently on Ebay (none of which sold!)

Hewlett Packard C17540 HP 816A

Alford 3300

C17452 General Radio 900-LB

Let's look at a slotted line measurement. Suppose you recorded the detected voltage along a 25 centimeter slotted line. The data might look like this:

In this case, the peak voltages are about 1.3 volts and the nulls are about 0.7 volts. That's a standing wave ratio of 1.85:1.

What else can you tell from this measurement? You can measure the frequency of the source (if it was unknown), if you know the dielectric constant of the slotted line (1 if it is air dielectric). The distance between nulls is a half wavelength. You should always measure between the nulls, not the peaks, because they are much sharper and easier to discriminate in distance (in spite of the above graph where they look the same!) From the plot, if the X-axis is in centimeters, we could estimate a wavelength of 6.25 cm (four wavelengths in 25 centimeters). Just divide that into the speed of light (30,000,000,000 cm/s) and you will get an answer of 4.8 GHz. Thanks for the correction, Renato!

If anyone has further interest in the topic of microwave measurements as they were done in the 1960s, we recommend this book : "MICROWAVE THEORY AND APPLICATIONS" by Stephen F. Adam of Hewlett Packard, you can probably find a used copy somewhere. Or just borrow a copy from an old dude out in your lab!