Slotted line
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.
This is especially important for transmitting antennas and their feed lines; high standing wave ratio on a radio or TV antenna can distort the signal, increase transmission line loss and potentially damage components in the transmission path, possibly even the transmitter.
However, slotted line measurement techniques are more labour-intensive and often do not directly output the desired parameter; some calculation or plotting is frequently required.
This is to be compared to modern instruments like network and spectrum analysers which are intrinsically frequency swept and produce a plot instantly.
Their remaining uses are mostly in the millimetre band, where modern test apparatus is either prohibitively expensive or not available at all, and with academic laboratories and hobbyists.
This is not an issue for the co-axial line because this operates in the TEM (transverse electromagnetic) mode and hence the current is everywhere parallel to the slot.
The first part is due to the power the probe has extracted from the line and manifests as a lumped equivalent circuit of a resistor.
This is minimised by limiting the distance the probe is inserted into the line so that only enough power is extracted for the detector to operate effectively.
The second part of the disturbance is due to energy stored in the field around the probe and manifests as a lumped equivalent of a capacitor.
Lumped inductors are not practical at microwave frequencies; instead, an adjustable stub with an inductive equivalent circuit is used to "tune out" the probe capacitance.
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).
The microwave power source is amplitude modulated with, typically, a 1 kHz signal which is recovered by the envelope detector in the probe and sent to the VSWR meter.
A large amplification is required in the VSWR meter because the limit of the square law range[note 3] of the detector diode is no more than 10 μW.
However, when this is replaced by a device under test (DUT) which is not perfectly matched to the line there will be a reflection back towards the source.
This causes a standing wave to be set up on the line with periodic maxima and minima (collectively, extrema) due to alternating constructive and destructive interference.
[11] Standing wave ratio (SWR or VSWR) is a basic parameter and the one most commonly measured on a slotted line.
A high SWR indicates a poor match between the feed line and the antenna, which increases wasted power, can cause damage to components in the transmission path, possibly including the transmitter, and cause distortion to TV, FM stereo and digital signals.
With the input power set so that the maxima are at 0 dBm, a measurement of a minimum in decibels will directly give SWR (after discarding the minus sign).
The magnitude and phase representation of ρ can, if required, be expressed as real and imaginary parts instead by the usual manipulation of complex numbers.
[16] The detected voltage signal output of the Schottky barrier diodes typically used in microwave detectors have a square law relationship to the power being measured and meters are calibrated accordingly.
However, as the power increases, the diode deviates significantly from a square law and remains accurate up to an output voltage of only around 5-10 mV.
[17] It is possible to completely eliminate errors in the detector and meter if a precision variable attenuator is used in the test setup.
