![]() ![]() This capacitance can be cancelled out with an inductance of equal and opposite impedance. 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. 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 first part is due to the power the probe has extracted from the line and manifests as a lumped equivalent circuit of a resistor. The disturbance to the field inside the line caused by the insertion of the probe is minimised as far as possible. The slot may be tapered at its ends to avoid discontinuities causing reflections. 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. 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. The current will then not be disturbed by the presence of the slot as long as it is not too wide. 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. For this reason the probe diameter and slot width are kept small (usually around 1 mm) and the probe is inserted in no further than necessary. For accuracy, it is important that the probe disturbs the field as little as possible. ![]() The slotted line works by sampling the electric field inside the transmission line with the probe. They are also useful as a teaching aid as the user is more directly exposed to basic line phenomena than with more sophisticated instruments. ![]() 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. Slotted lines have now largely been superseded, but are still found where capital costs are an issue. This is to be compared to modern instruments like network and spectrum analysers which are intrinsically frequency swept and produce a plot instantly. 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. 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. Slotted lines are relatively cheap and can perform many of the measurements done by more expensive equipment such as network analysers. Circular waveguide slotted lines are also possible. In a rectangular waveguide, the slot is usually cut along the centre of the broad wall of the waveguide. The probe is inserted past the outer conductor, but not so far that it touches the inner conductor. In a co-axial slotted line, the slot is cut into the outer conductor of the line. 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. The slotted line is one of the basic instruments used in radio frequency test and measurement at microwave frequencies. 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. Their main drawback is that they are labour intensive to use and require calculation, tables, or plotting to make use of the results. Slotted lines are no longer widely used, but can still be found in budget applications. This is especially important for transmitter antennas and their feed lines reflected power is wasted and can damage high-power transmitters. This serves as a measure of the accuracy of the impedance match to the item under test. The parameter most commonly measured by a slotted line is SWR. 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. A precision variable attenuator is often incorporated in the test setup to improve accuracy. 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. 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 are used for microwave measurements and consist of a movable probe inserted into a slot in a transmission line. ![]()
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