variation, every 30 cm, and another −0.1 Ω variation every 2.7 m, each on the same 300 m coaxial cable with an insertion loss typical for main‐line CATV cables. In the figure, two SRL effects are shown; a smaller effect every 50 MHz or so, due to the 2.7 m periodic variation, and a much higher effect every 500 MHz or so due to the 30 cm impedance variation. The higher impedance variation occurs more often, and so the periodic error will have a greater cumulative effect resulting in a nearly full reflection, as shown in the figure.
Figure 1.15 A model of a coax line with periodic impedance disturbances.
Figure 1.16 The return loss of a line with structural return loss.
1.8.2 Connectors
Connectors provide the means to transition from one transmission media to another. They are often not considered as part of the device or measurement system, but their effects can dominate the results of a measurement, particularly for low‐loss devices. Connectors can be distinguished by the quality and application. One remarkable aspect of connectors is the great difficulty in measuring them with any kind of accuracy. This is because most connectors provide a transition between different media, such as from a coaxial cable to a connector interface or from a PC board to a connector interface. While the connector interface is often well defined, the “back‐end” of the connector is poorly defined.
Connectors that are “in series” provide transitions from male to female and provide interconnections between components. These are easiest to characterize as the ports are well defined and typically calibration kits are available and calibration methods are well understood. Connectors that are “between series” are equally well defined, but until recently they have been difficult to characterize as there were not well‐defined standards for between‐series adapters. Recent improvements in calibration algorithms have essentially eliminated any difficulty with characterizing these between‐series adapters. Figure 1.17 shows some examples of in‐series and between‐series connectors.
Figure 1.17 In‐series and between‐series connectors.
For microwave work, there are some commonly utilized connector types that are found on the majority of components and equipment. Table 1.1 lists these common connectors along with their normal operating frequency range. These are divided into three broad categories: precision sexless connectors, precision male/female connectors, and general‐purpose or utility connectors. These connectors are typically 50 Ω, but a few can be found as 75 Ω versions as well.
Table 1.1 Test connectors used for RF and microwave components
| Name | Outer Conductor Diameter (mm) | Rated Frequency (GHz) | First Mode (GHz) | Maximum Usable Frequency (GHz) |
| Type‐N (50 Ω) precision | 7 | 18 | 18.6 | 26.5a |
| Type‐N (50 Ω) commercial | 7 | 12 | 12.5 | 15 |
| Type‐N (75 Ω) precision | 7 | 18 | 18.6 | 18 |
| Type‐N (75 Ω) commercial | 7 | 12 | 12.5 | 15 |
| 7 mm | 7 | 18 | 18.6 | 18 |
| SMA | 3.5 | 18 | 19 | 22 |
| 3.5 mm | 3.5 | 26.5 | 28 | 33 |
| 2.92 mm (“K”) | 2.92 | 40 | 44 | 44 |
| 2.4 mm | 2.4 | 50 | 52.5 | 55 |
| 1.85 mm (“V”) | 1.85 | 67 | 68.5 | 70 |
| 1 mm | 1 | 110 | 120 | 130 |
a Some instrument manufacturers place this connector on 26.5 GHz instruments because it is rugged; it has the same first modes as Type‐N and 7 mm.
From Table 1.1 one can see that there are three frequencies associated with connectors: the generally understood operating frequency (often dictated by the calibration kit's maximum certified frequency), the frequency of the first mode, and the maximum frequency determined by the waveguide propagating mode of the outer conductor. The operating frequency is always below the first mode and usually by several percent. The first mode in many connectors is due to the support structure for the center pin. It is often made of some plastic material and thus has higher dielectric constant and a lower frequency to support a mode. In connectors and cables, modes is the term used to refer to non‐transverse‐electromagnetic (non‐TEM) propagation that can occur in a circular waveguide mode defined by the inside dimension of the outer conductor. Adding dielectric in the bead that supports the center pin can theoretically lower the mode frequency, but if the bead is short, the mode will be evanescent (non‐propagating) and may not affect the quality of the measurement. At a somewhat higher frequency, there will be a propagating mode in air for the diameter of the center conductor, but if the cable attached to the connector is sufficiently small, this mode may not propagate as well. It is the propagating modes that cause the significant dips in the response, and more importantly, these dips cannot be removed with calibration because they are not localized and because reflections in the mode of transmission far removed for the connector interface can interact with these connector modes, causing the frequency response of the mode effect to change when different devices