SMA | 3.5mm

 

SMA-type
The precursor of the SMA connector was developed around 1960 at the Bendix Research Laboratories Division by mechanic Val Colossi under the supervision of James Ceal. The required a small connector for a pulse doppler radar system – the N-type which was typically used at the time was too large. The group was working on components which were far ahead of their time and did not think it to be necessary to deploy an HF-engineer for a part as trivial as a plug. From 1961, semi-rigid lines were offered increasingly, and the SMA-type was also developed in consideration of these special lines.

Ceal gave the diameter relationships and Clous was supposed to develop a connector as small, as short, and as mechanically stable as possible, with a standard thread. The result features 50Ω consistently, and a cut point between plug and socket in one plane for both inner and outer conductor which is at the same time the reference plane.The concept of the connector was kept and after some further developmental steps was distributed by Omni Spectra Inc. as OSM from 1962. Other companies entered the market with their own developments and „SMA“ established itself as designation for this kind of connector.

 

 

The SMA is mode-free up to TE11 cut-off (i.e. 25.22 GHz), dependent on the given tolerances. Designed with suitable dimensions and tolerances, some available versions are mode-free up to 26.5 GHz. One disadvantage is the danger that - as the plug is not plugged in - the pin next to the spring body is pressed into the PTFE if the plug is not in use. If this happens, it leads to the socket most often being damaged to such an extent that it should be discarded.

 

 

3.5mm-type
The 3.5mm connector was developed at Hewlett Packard and was later manufactured
by Amphenol. Larry Renihan reported on this connector at the IEEE MTT-S International Microwave Symposium 1976. It is a logical evolution of the SMA-type but using air as dielectric. While the SMA was intended to only work for a few plug-cycles and not for instrumentation, the developers of the 3.5mm-type received the specification for a connector holding up to many plug cycles, being a match to the SMAS interface and working up to 26.5GHz.

During the design the problem of the SMA plug was considered, and the connector was constructed such that the spring body is not damaged in case the plug is inserted at an angle. This was accomplished by an early lead via the outer conductor which aligns the pin before it reaches the spring body. Both the 3.5mm and the 2.92mm are compatible to the SMA. All three can be used in combination without causing any damage.

Moreover, it has been shown that better matching values at higher frequencies can be obtained. This is due to the fact that with the SMA being plugged in, a small air gap remains around the inner conductor. This region of course fails to maintain the 50-Ω-condition and deteriorates the matching. The 3.5mm, on the other hand, has air as dielectric to begin with and thus this effect is smaller. With the nominal dimensions, a mode-free use is calculated to reach up to 38.8 GHz. Some sources, however, give 34 GHz and it is recommended to limit the suet 34 GHz, unless the manufacturer gives other specifications.