The most common coaxial connector in the microwave market, the SMA (the acronym for Sub Miniature A Connector), was first introduced by Bendix Corporation in the late 1950’s as a connector specially designed for .141 diameter semi-rigid cable. At the SMA connector’s inception, the operating frequency didn’t exceed 12 GHz. As the design matured, the maximum operating frequency was pushed up to 18 GHz and eventually higher. As higher operating frequencies have become common over the past several decades, other interfaces, most of which are considered precision interfaces designed for laboratory use, have been introduced to handle signal transmission at frequencies far exceeding 18 GHz. However, the SMA connector design can be optimized to extend the maximum operating frequency of the interface to offer a reliable and cost effective solution to the higher frequency requirements of modern systems.

The traditional limitation in operating frequency of the SMA connector has been set at 18 GHz. This theoretical limit of the operating frequency of the SMA connector is based upon the presence of a signal resonance that exists at the transition from the first mode (TEM) to the second (TE11). The connector will operate at higher frequencies than that of the second mode resonance, operating in dual mode transmission. However, this is undesirable for most applications, particularly in high power applications, as second mode resonances will overheat the SMA connector and cause a voltage breakdown.

Theoretically, the second mode can be excited from the “cut-off frequency”, which depends on the transverse dimensions of the line and the effective dielectric constant. The second mode resonance will typically appear at a frequency greater than that of the “cut-off”. Typical connectors require a combination of coaxial lines with different geometries behind the established interface dimensions to adapt to various diameters of coaxial cable. Therefore, the connector, irrespective of its interface designation, will have its operating frequency derated by the internal coaxial line with the lowest “cut-off frequency”. The exact value of the resonance frequency for a connector highly depends on the length of the coaxial line with the lowest “cut-off frequency”. The shorter the length of this limiting coaxial line, the higher the resonance frequency is. Consequently, the higher the resonance frequency, the higher the connector operating frequency is.