Comparison and Selection of Microwave and RF Coaxial Adapters
When one looks to select a microwave/RF coaxial adapter, at first glance there’s a wide variation between pricing for apparently similar parts. As an example, a glance at what’s available on the web shows N/M-SMA/f coaxial adapters ranging from $3 to $110. This article will address the differences between coaxial adapters & help in the selection of the right adapter for a particular application.
The first issue to be aware of is that different specifications exist for many classes of adapters. There are 6 different standards for the tnc connector, though the industry has largely settled on 3 standards; All seem to mate, the upper frequency limit can be 11, 16, or 18 GHz. The SMP standard is in a constant state of revision, and the tolerances specified facilitate parts to be built that will not mate properly. N adapters come in 11/12 and 18 GHz versions, depending upon the interface specification being utilized; SMA’s come in 18 and 27 GHz versions. Multiple standards exist for the F connector, not all of which are interchangeable. For most Microwave connectors, either a Military specification or Industry specification exists, though for large volume low cost applications these specifications are sometimes ignored.
A second issue is the material used. N, SMA, and rf connectors come in both Brass and Stainless-steel versions. The original reason to offer brass SMA parts was a large differential that existed at that time between the cost of brass and stainless-steel, a differential that has became different quite a bit over time. The different materials have different torque requirements, some connectors such as the brass sma can be damaged by applying too much torque. The military and industrial specifications for sma’s say virgin Teflon might be utilised; to save money, lower cost applications will substitute formed teflon, or rexolite. BNC female connector contacts in the Mil Specs are BeCu; almost all BNC’s at consumer outlets are brass.
When one looks at the options available, as in, N, SMA, and TNC adapters, one must keep in mind that these options are available from a single supplier, & be aware of the tradeoffs involved.
Thirdly, plating varies commonly. An SMA built to the Mil Specs will have 50 micro inches of Gold on the male and female contacts; a widely employed European standard specifies 30 micro inches; it’s not uncommon to find low cost parts intended for high volume applications to have either 10 microinches of gold, or even “flash” gold, which is roughly 3 microinches. Some coaxial microwave adapters now are intended for low passive intermodulation (PIM) applications, and the plating and material selection are critical to parts with good, low PIM.
So which part is suitable for your application? In the example of the $3 vs. $110 N to SMA adapter, the $3 adapter is fine for applications in benign environments, where the part will only be connected once or twice, the temperature doesn’t change radically or quickly, the frequency range is not too high, & no severe vibration exists; ham radio and home applications. On the other hand, for an 18GHZ part requiring rather low VSWR, will be used in military aircraft or space applications, will be mated/demated often, then the $110 part is the right choice.
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