Industry News

A Push-on Connector Series

The development of the multi-coax/DC connector has been prolonged and demanding, taking over 25 years. At first there was the development of the Quick Connectors or ‘push-on’ type connectors, which were originally designed and introduced for applications where limited space requirements made tightening, torquing and loosening a threaded coupling nut difficult, impossible or time consuming. As an example, Figure 1 shows Spectrum’s push-on connectors that mate with standard SMA, N, TNC, 7/16 and F series connectors.

The next issue to be addressed was the demand for high power blind mates, which are needed for high power components, to be designed on a modular basis. Hence, the SBX, SBY (shown in Figure 2) and SBZ connector series were developed and introduced for a wide range of microwave interconnect problems and mating applications for modularized packages, racks and panels, and dense packaging. Their design means that in most cases they are not visible after the modules have been connected, as they can be integrated completely into the package.

Push-ons and blind mates serve applications where a number of coaxial lines are to be connected and disconnected in seconds. However, there are other fields, particularly for airborne and shipborne applications, or even in test centers, where a higher number of RF lines need to be packaged densely. As well as possessing the facility to be connected and disconnected easily, securely fastened in seconds and locked safely, they also need to be rugged and able to withstand the stringent requirements of military programs and be capable of operating in harsh environments.

Eight-line Connection

The SQ series has been designed to meet these requirements. The first to consider is the SQ-8, which uses the standard size 21 MIL-C-38999 shell. This type of shell offers five keyed connector versions, eliminating connection errors, ensuring that only the correct harness can be connected with the proper mating connector at the equipment. They are available with different finishes—aluminum nickel-plated or cadmium-plated. As its name suggests, the SQ-8 is designed for connecting eight RF lines at once. It is illustrated in Figure 3. It has eight floating and spring-loaded inserts in both the male and the female unit, ensuring good connection at the interfaces for the best electrical performance and enabling operation for frequencies up to 25 GHz.

Accurate mating and spring-loaded interfaces are required even at lower frequencies when the cable assemblies of a harness have to be phase matched. Replaceability is guaranteed for any damaged or defective coaxial line of the harness, which can be replaced in seconds by just turning the bayonet catch of the coaxial insert at the SQ-8. Two different flexible cables are available for the SQ-series, Type 11 and Type 43, offering a trade-off between insertion loss and flexibility, but both ensuring operation to 25 GHz.

High Frequency Operation

The internal dimensions of a coaxial insert or connector and cable, and the constants of their dielectrics are the only limiting factors of the operating frequency range, assuming that state-of-the-art design criteria is used. Therefore, there is no reason that any SMA should not operate properly at least to 18 GHz, or even 26 GHz, depending on certain limiting criteria of the cable. Spectrum’s push-on SMAs operate to 26 GHz, the N and TNCs to 18 GHz, the 7/16 to 7.5 GHz, and the complete SQ and SM families to 25 GHz.

The operating frequency range of the SQ series is ensured by instrument grade design and spring-loaded interfaces, ensuring that the male and female mate exactly, with no uncontrolled gap at the interface. Also important is the fact that the electrical performance of connectors is not just dependent on the design, but also on the tolerances of the different components of the coaxial connector or the inserts—namely the center contact, the dielectric and the outer conductor. Precision parts are critical, so for the electrical performance of the connector or insert Spectrum allows a tolerance of 5/1000 of a millimeter (2/10000 of an inch).

The SQ-8F version of the connector has been developed for certain applications where the spring-loaded design of the male unit was not required (for example, when an angled configuration of the male connector, which cannot be handled with spring-loaded inserts, is to be used). The SQ-8F utilizes eight coaxial inserts and the male inserts terminate the coaxial cable by being securely mounted in the housing of the size 21 MIL-C-38999 shell.

The bulkhead or four-hole panel mount female inserts remain floating, ensuring excellent connection for good electrical performance up to 25 GHz. And any damaged and defective coaxial line of the female part of the harness can be replaced in seconds, using the bayonet catch. Replacing a coaxial line in the male SQ-8F is also possible by unthreading the back body of the coaxial insert.

12-line Connection

For certain applications eight coaxial RF lines in a compact housing are not sufficient. Hence, the SQ-12F has been developed, accommodating 12 coaxial RF lines in a standard size 25 MIL-C-38999 shell. The SQ-12 is illustrated in Figure 4. This connector uses the same technology as the SQ-8F—five keyed shells are available, aluminum nickel- or cadmium-plated, together with coaxial inserts for Type 11 and Type 43 cables. Effectively, it is an enlarged version of the SQ-8F with securely mounted inserts in the male unit and spring-loaded inserts in the female unit.

Often in airborne applications bulkhead feed through connectors are required to be mounted in walls between chambers of different pressure. To meet these requirements the SQ-8P and SQ-12P series are available, which offer this type of sealed Bulkhead Feedthrough Jack (BFJ). All coaxial lines in this female connector are mounted and sealed firmly in the MIL-C-38999 shell (sizes 21 and 25, respectively), while the male connector uses inserts, which are floating and spring-loaded, ensuring proper mating interfaces. The SQ-8P and SQ-12P series are color-coded and have special markings to make it clear that it is not possible to replace a coaxial line in a sealed BFJ connector without damaging the seals. Replacement can only be carried out by the manufacturer.


This pressurized condition apart, the exchangeability of the coaxial inserts within the SQ-series is guaranteed. The spring-loaded male inserts of the SQ-8 male, SQ-8P male and SQ-12P male are the same, as are the fixed male inserts of the SQ-8F male and SQ-12F male. Identical female inserts are used in the SQ-8 female, SQ-8F female and SQ-12F female. Also identical are the pressurized female inserts of the SQ-8P and SQ-12P. This means that, with the exception of the cable assemblies used in the pressurized connectors, cable assemblies can be exchanged from the SQ-8F to the SQ-12F, and vice versa.

23-line Connection

For certain applications, even 12 RF coaxial connections are not enough. There is also the need for DC and driver signals. Hence, the development of the SM23-DC26 multi-coax/DC connector, which connects and disconnects 23 coaxial RF lines (shown in Figure 5) and 26 signal lines in seconds. For this design, a minor modification at the spring finger system of the outer conductor of the SMA push-on was necessary in order to meet the insertion and withdrawal force requirement of a maximum of 150 N (for all 23 coax lines, including the 26 DC and driver signal paths).

The female coax inserts terminating the cable use the standard SMA female interface, mating with any standard SMA male connector, while for the male part SMA push-ons incorporating a lower insertion force design are employed. This ensures that any standard SMA female connector can mate with any of these coax lines by just being pushed on, instead of threading and torquing.

With 23 RF lines in a connector there is the possibility that one, or even several may be damaged at some time and will need to be replaced. The cable assemblies are mounted in groups of four or eight that are securely fastened by mounting bolts in the supporting structure. This ensures that any coaxial line can be replaced quickly. The maximum operating frequency is guaranteed to 25 GHz and the connector uses Type 11 or 43 cables.

Phase Matching

Modern systems often require phase matching of harness cable assemblies. Hence, Spectrum uses the latest cable manufacturing and interface cutting techniques, together with advanced adjustable connector designs that can meet almost any phase matching. Selecting the right materials and aging techniques as part of well defined processes is also an important parameter in order to produce cable assemblies and harnesses that are capable of operating in temperature ranges of –54° to +115°C as standard. Extended temperature ranges from –72° to +200°C can be accommodated and for some sophisticated receivers connectors are available that operate at –200°C.

All connectors are RoHS-compliant and meet the condition and corrosion requirement to MIL-STD-202, method 101, condition B. The SQ connector series is also compliant to thermal shock to MIL-STD-202, method 107, condition B, vibration to MIL-STD-202, method 204, condition D, and shock to MIL-STD-202, method 213. Additional information may be obtained via e-mail at

Spectrum Elektrotechnik GmbH
Munich, Germany

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