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. 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.
Slotted lines are no longer widely used, but can still be found in budget applications. Their main drawback is that they are labour-intensive to use and require calculation, tables, or plotting to make use of the results. They need to be made with mechanical precision and the probe and its detector need to be adjusted with care, but they can give very accurate results.
Description
The slotted line is one of the basic instruments used in radio frequency test and measurement at microwave frequencies. It consists of a precision transmission line, usually co-axial but waveguide implementations are also used, with a movable insulated probe inserted into a longitudinal slot cut into the line. In a co-axial slotted line, the slot is cut into the outer conductor of the line. The probe is inserted past the outer conductor, but not so far that it touches the inner conductor. In a rectangular waveguide, the slot is usually cut along the centre of the broad wall of the waveguide. Circular waveguide slotted lines are also possible.
Slotted lines are relatively cheap and can perform many of the measurements done by more expensive equipment such as network analysers. 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. In particular, they can only carry out a measurement at one spot frequency at a time so producing a plot of a parameter versus frequency is very time consuming. This is to be compared to modern instruments like network and spectrum analysers which are intrinsically frequency swept and produce a plot instantly. Slotted lines have now largely been superseded, but are still found where capital costs are an issue. 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. They are also useful as a teaching aid as the user is more directly exposed to basic line phenomena than with more sophisticated instruments.
Operation
The slotted line works by sampling the electric field inside the transmission line with the probe. For accuracy, it is important that the probe disturbs the field as little as possible. For this reason the probe diameter and slot width are kept small (usually around ) and the probe is inserted in no further than necessary. It is also necessary in waveguide slotted lines to place the slot at a position where the current in the waveguide walls is parallel to the slot. The current will then not be disturbed by the presence of the slot as long as it is not too wide. For the dominant mode this is on the centre-line of the broad face of the waveguide, but for some other modes it may need to be off-centre. 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 slot may be tapered at its ends to avoid discontinuities causing reflections.