Intermateability of Connectors

Will Different Connectors Mate?

There is discussion about intermateability of connectors. We know that a 2-inch water pipe will not fit a 1 inch port. So should connectors of different types mate and work properly. Yes, but only if they are electrically as well as mechanically compatible. The types SMA, 3.5 mm and 2.92 mm connectors are an example. Their names suggest they should not mate mechanically at all (look at the numbers which one would normally assume to be related to size). And if they did mate, would the  intermateability of connectors perform well electrically when interconnected? The good news is that they actually do, mechanically as well as electrically. What is the reason? The reason lies in history and evolution over nearly four decades.

The SMA

The Sub Miniature A connector popularly known as the SMA connector, entered the market in the late ’50s with the name “BRM”. It was manufactured by Bendix (Scintilla Division). In the next decade Omni Spectra popularized their own production of the same as the “OSM,” connector. In 1968 it was formally received the designation “SMA” (Sub-miniature A). The SMA design uses a solid dielectric interface and connectors with air interface cannot be designated as SMA.It is rated for frequencies up to 18 GHz, although products with better specifications are now available. higher frequency variants are available. Originally designed as a miniature and economical connector for system applications not requiring frequent connection and disconnection. It is rated for up to 500 connect / disconnect operations. Certainly, it was not intended to be a precision connector for laboratory work or for systems requiring very frequent connect /disconnect procedures. MIL-STD-348 (SMA) specifications apply.

The 3.5 mm or APC3

The 3.5 mm connector was born in the early seventies as a result of a joint effort between Agilent Technologies then known as Hewlette Packard or HP, and Amphenol. The development work was primarily carried out by HP while production was done by Amphenol. It was named as Amphenol Precision connector APC3.5 3.5 mm, or in brief as APC3.5.  It was meant to become a precision interface for calibration measurements of devices equipped with the SMA connector. The design targets pursued by HP aimed at the following improvements over the SMA:

1. Ability to mate with the SMA
2. A more durable interface than the SMA that could stand thousands of interconnect / disconnect operations. The IEEE P287/D3 standard (provisional) rates 3.5 mm connector at 3000 connect /disconnect operations.
3. A higher upper frequency limit. It is rated for 26.5 GHz whereas the theoretical upper frequency is 34 GHz.

An SMA-to-3.5 mm interface actually results in better electrical connection than an SMA-SMA connection. The reason for this unexpected improvement is that the SMA uses a solid dielectric. Connection between SMA male and SMA female leaves a tiny air gap in between. There is an impedance discontinuity between solid and air dielectrics and then another between air and solid dielectrics. This limits the performance. The 3.5 mm connector is designed on air dielectric. Thus, the SMA to 3.5 mm connection eliminates one of the two discontinuities improving the performance limits.

The 2.92 mm

The 2.92 mm connector is an improvement even on the 3.5 mm connector raising the performance limit to 40 GHz. Introduced by Maury Microwaves in the mid-seventies the new connector was called MPC3. Obviously, it's designed to the same geometry as the predecessors ie, SMA and the 3.5 mm connectors. It was not very popular at that time because there was not much equipment in the field above 40 GHz. Wiltron (now Anritsu Corp) started producing 40 GHz equipment and reintroduced the connector as K-connector (K for the K-band of frequencies). IEEE 287 applies to this connector also.

 Intermateability of Connectors

So, what is the deduction about intermateability of connectors? It is pretty clear. They are all designed to intermate mechanically.  Electrically also they intermate successfully but results are slightly different depending on the pair of types intermating.  Obviously, using a higher spec connector allows for better electrical matching.

A Word of Caution

These deductions about intermateability of connectors are valid when the connectors are in good condition. Repeated connection and disconnection will cause wear and hence performance will deteriorate.

Laboratory Verification

The co-performance of various combinations of these three types of connectors has been verified through rigorous lab tests. For further details of the tests the reader is advised to refer to MICROWAVE JOURNAL® March 2007 by Horizon House Publications, Inc.