page_title: Slotted line
section_title: Test setup
hierarchical_section_title: Slotted line / Test setup
caption_attribution_description: Deutsch: Schnittbild eines Stehwellenmessgerätes, mit dem sich die Spannungsänderungen am Innenleiter eines Koaxkabels bestimmen lassen. Die kapazitiv ausgekoppelte Spannung regt den orange gezeichneten λ/4-Schwingkreis an, an dessen Anzapfung die HF-Spannung gleichgerichtet wird. Mit einem verschiebbaren Kurzschluss am linken Ende des Schwingkreises wird Resonanz eingestellt. Zur genauen Messung der Abstände von Maxima und Minima kann der Messkopf parallel zum Innenleiter verschoben werden.
context_page_description: Slotted lines are used for microwave measurements and consist of a movable probe inserted into a slot in a transmission line. They are used in conjunction with a microwave power source and usually, in keeping with their low-cost application, a low cost Schottky diode detector and VSWR meter rather than an expensive microwave power meter.
  Slotted lines can measure standing waves, wavelength, and, with some calculation or plotting on Smith charts, a number of other parameters including reflection coefficient and electrical impedance. A precision variable attenuator is often incorporated in the test setup to improve accuracy. This is used to make level measurements, while the detector and VSWR meter are retained only to mark a reference point for the attenuator to be set to, thus eliminating entirely the detector and meter measurement errors. The parameter most commonly measured by a slotted line is SWR. This serves as a measure of the accuracy of the impedance match to the item under test.
context_section_description: A typical test setup with a waveguide slotted line is shown in figure 2.  Referring to this figure, power from a test equipment source (not shown)  enters the apparatus through the co-axial cable on the left and is converted to waveguide format by means of a launcher (1).   This is followed by a section of waveguide (2) providing a transition to a smaller size of guide.  An important component in the setup is the isolator (3) which prevents power being reflected back into the source.  Depending on the test conditions, such reflections can be large and a high-power source may be damaged by the returning wave.  The power entering the slotted line is controlled by a rotary variable attenuator (4).  This is followed by the slotted line itself (5) above which is the probe mounted on a movable carriage.  The carriage also carries the probe adjustments: (6) is the probe depth adjustment, (7) is a length of co-axial section with tuning adjustments, and (8) is a detector, which uses either a point contact crystal rectifier or a Schottky barrier diode. The right-hand end of the slotted line is terminated in a matched load (9) which absorbs all the power exiting the end of the waveguide.  The load can be replaced by the component or system that it is desired to test.  It can also be replaced with a reference short-circuit (10) which is used to calibrate the slotted line.  The carriage can be moved along the slotted line by means of a rotary knob (11) which simultaneously moves a vernier gauge (12) for accurate measurement of the probes position along the line.
  The probe is connected to a detector and a display meter (not shown in figure 2).  These can be, respectively, a thermistor and power meter, or an envelope detector and VSWR meter.  The detector can be a crystal detector or a Schottky barrier diode.  The detector is mounted on the probe assembly, usually a distance λ/4 from the probe tip as shown in figure 3.  This is because the detector looks almost like a short circuit to the transmission line, and this distance will convert it to an open circuit through the quarter-wave impedance transformer effect.  Thus, the detector has minimal effect on loading the line.  The probe tuning stub can be seen on figure 3 branching from the line linking the probe to the detector.  Figure 2 has a slightly different arrangement; the main probe into the waveguide leads to a vertical co-axial tuning and adjustment section but the detector is on a horizontal side-section with a secondary probe into the upright co-axial section.
caption: Figure 3. Co-axial slotted line
exif: {}