dielectric and thus a lower operating frequency due to moding. SMA is traditionally considered to be an 18 GHz connector, but the first propagating mode is well above 20 GHz, depending upon the type of cable that is connected to the SMA connector. The chief advantage of SMA connectors is low cost, especially when mounted to semi‐rigid coaxial cables. The dimensions are such that the center wire of the coax can be used a connector pin for SMA, and only an outer conductor sleeve needs to be added to the coax outer conductor to form a male connector, shown in the lower‐right picture of Figure 1.23. But these cables are notoriously bad at maintaining the proper dimensions for the center pin, and often the center pins are poorly trimmed and improperly chamfered so that they cause mating problems with their female counterparts. This is particularly true when mating them to 3.5 mm female connectors, slotless ones in particular. Figure 1.23 shows examples of 3.5 mm and SMA connectors, with 3.5 mm on the left and SMA on the right.
Figure 1.23 3.5 mm (f) and (m) (upper left); SMA (f) and (m) connectors (upper right); 3.5 mm (lower left) and SMA adapters (lower right).
Figure 1.24 shows measurement plots of a mated pair of 3.5 mm male‐to‐male with a 3.5 mm female‐to‐female, as well as two SMA examples. The moding of the SMA connector is clearly seen above 25 GHz (Marker 2 on the SMA1 and SMA2 trace). The moding of the 3.5 mm connector is seen just above 30 GHz (Marker 2) and again at 34 and 38 GHz. There two typical construction types for SMA, one with a press‐fit of the Teflon and center conductor (SMA1 in the measured response) and one where the Teflon is held in with a small dot of epoxy through a hole in the outer conductor (SMA2 in the measured response). The second method usually gives a poorer match, and we can see that with the small dip in the S21 response of SMA2 near 12 GHz and the larger dip just above 20 GHz.
Figure 1.24 Performance of SMA and 3.5 mm mated‐pair connectors.
1.8.2.5 2.92 mm Connector
The 2.92 mm connector is scaled down from the 3.5 mm connector and can be mechanically mated to both the 3.5 mm and the SMA connectors. The smaller diameter outer conductor means that its mode‐free operation extends proportionally higher, to 40 GHz, and is usable to perhaps 46 GHz. The female connector has a two‐slot collet that provides sufficient compliance to mate with the center pin of the larger 3.5 mm and SMA connectors but that makes it less suitable for precision measurements due to increased uncertainty of the contact point on the center pin radius, which now depends upon the radius of the pin that is inserted. A further point is that the metal wall of the female collet on the 2.92 connector is quite thin and prone to damage if the mating pin is not well aligned or oversize. It's not uncommon to find 2.92 female adapters missing one of the collet fingers. The 2.92 mm connector was popularized by the Anristu company (formally Wiltron), which introduced it as the K connector, and it is common to hear any 2.92 mm connectors referred to by that name.
Figure 1.25 shows some examples of 2.92 connectors. The key difference is in the diameter of the inside of the outer conductor. Figure 1.26 shows the insertion loss of a mated pair of 2.92 mm female‐to‐female adapters with a 2.92 male‐to‐male adapter, along with an example of a 3.5 mm mated adapter pair. The moding of the 3.5 mm pair is clearly seen above 30 GHz, but the connector is generally usable up to 38 GHz as the first small modes are bead modes and are able to be calibrated out as they generally don't propagate through the cable.
Figure 1.25 A 3.5 mm connector compared with 2.92 mm female (upper) and male (lower).
Figure 1.26 Performance of a mated pair, 2.92 compared with 3.5 mm.
1.8.2.6 2.4 mm Connector
The 2.4 mm connector is essentially a scaled version of the 3.5 mm connector, with an associated scaling in maximum frequency. It is used extensively on 50 GHz applications, though it can be used up to 60 GHz. This connector cannot be mated to any of the SMA, 3.5 mm or 2.92, and in fact was designed to prevent damage if one tried to mate to these types. It comes with both slotted and slotless female center pins, much like the 3.5 mm connector.
1.8.2.7 1.85 mm Connectors
There are two variants of the 1.85 mm connectors, designed originally by Anritsu and Agilent. The Anritsu variety is called the V connector, and the Agilent variety is called the 1.85 mm connector. They are mechanically compatible and were originally designed for 67 GHz operation, usable to above 75 GHz. These connectors are mechanically compatible with the 2.4 mm connector. Figure 1.27 shows the wideband response to 85 GHz of the 1.85 mm connector in the upper plot and a zoomed‐in view of the first mode in the lower plot. This first mode is a “bead‐mode” and is caused by the increased dielectric constant of the bead, which holds the center pin, lowering the first mode of the 1.85 mm coaxial line. In general, this mode is non‐propagating (since it is contained in the bead) and may be calibrated out in some circumstances. For example, if this connector is used with an on‐wafer probe and the coax from the connector to the probe‐tip is mode free, then the bead mode will act like a small, stationary resonance that can be removed with a calibration. If the mode is propagating, then changes in the termination impedance change the effects of the mode, and it cannot be calibrated out (it is not stationary with respect to an external impedance); but if it is non‐propagating and there is a sufficient length of mode‐free line (such a cable) between this bead mode and the reference plane, the evanescent fields associated with the mode will die off before arriving at the reference plane and thus will not couple to the terminating impedance. As this first mode is less than 0.2 dB, in many cases it is not significant. The 1.85 mm connector has been used out past 75 GHz.
Figure 1.27 Response of mated pair of male‐to‐male and female‐to‐female 1.85 mm connectors (upper), with zoomed‐in view of the first mode (lower).
1.8.2.8 1 mm Connector
The 1 mm connector is essentially a scaled version of the 1.85 mm connector but cannot be mated to it. It is typically specified to 110 GHz performance but is usable to above 120 GHz, with some versions being specified up to 120 GHz and used up to
