- Buyers Guide
A Waterproof QMA Coaxial Connector
Development of a waterproof QMA coaxial connector for use in a multitude of radio frequency applications.
Five years ago Huber + Suhner developed a range of QMA coaxial connectors to replace the SMA connectors that had been traditionally used. Although they have been primarily used in the telecom market for radio base stations they have also been recognized widely in other market sectors. This wide acceptance instigated an investigation into the development of the QMA waterproof RF coaxial connector, prompted by the fact that there is no snap-on RF connector on the market that is water sealed.
For such a product to be developed the main factor that needed to be addressed was the fact that normally these connectors incorporate a spring member, which makes it difficult to make it waterproof. The challenge was not only to overcome this inherent problem but also to develop a waterproof plug that is backwards compatible with a standard QMA female. However, it was not possible to just put an O-ring inside the existing QMA connector. This is because it has a spring mechanism that has spring fingers, which is a slotted part. This restricts the space so that there is no room to accommodate an O-ring seal.
A Complete Redesign
To address this limitation Huber + Suhner came up with a new concept whereby they took the existing female and endeavored to design a new plug around it. However, this meant a complete redesign of the connector interface so that the spring fingers were repositioned differently inside the connector, while maintaining the same outer dimensions of the connector. It needed to be the same size to achieve the same centre line to centre line as the standard QMA. Several concepts were considered before finally settling on one that met all the design and applications criteria to produce a connector plug that is water sealed according to IP68.
The challenge in redesigning the QMA interface was to maintain or even exceed the proven performance parameters of the Quick Lock concept. A critical issue is the fact that the diameter of the male spring part, which snaps onto the female body, will enlarge a certain amount under mechanical and/or thermal stress. This is the normal behavior of any spring material, and is related to material relaxation. This behaviour will have an impact on the contact force, which determines the transmission characteristics. Careful material selection and spring part layout ensures optimum connector performance under stress.
During the development phase the QMA connector underwent verification testing. Particularly important was environmental testing, which, in order to widen the applications, was conducted to military standards. Hence, a higher level of vibration tolerance was required.
The result of this development and testing is a waterproof version of the high density Quick-Lock connector that can be used for a multitude of RF applications. The waterproof QMA connector terminated on a flexible cable, featuring a crimped cable entry with heat shrink tube, is shown in Figure 1. With its outstanding features like IP68 ingress protection rating, its ability to be intermateable with a QLF® certified standard QMA jack (see Figure 2), having the same outline as a standard QMA, an enhanced temperature rating of +105°C and MIL vibration level, it will not only be applicable for the telecom market but also the defense and industrial sectors.
From a specifications standpoint, the QMA interface exhibits its best transmission characteristics in the DC to 6 GHz range. Suitable applications include radio base stations and other radio equipment, especially where high density connector population is required, as shown in Figure 3. The interface’s snap-lock mechanism makes it very fast and easy to mate and de-mate, with little force needed. The interface retention force is 60 N.
Advantages of fast mating and de-mating include reducing the user’s total operating costs without loss of quality or functionality, while the replacement of the threaded coupling mechanism by the snap-lock mechanism increases the packing density as no torque wrench clearance is necessary. The QMA connector also allows 360° rotation in the mated position, which improves the flexibility during the installation of terminated cables.
Electrically the waterproof QMA connector has an impedance of 50 Ω, an interface frequency range of DC to 18 GHz, a minimum return loss of –32 dB at DC to 3 GHz and –25 dB at 3 to 6 GHz, and a minimum RF leakage of –80 dB at DC to 3 GHz and –70 dB at 3 to 6 GHz. Mechanically it has a minimum durability of 100 cycles and a typical engagement/disengagement force of 25 N/20 N.
Environmentally the connector is RoHS compliant, has an operating temperature of –40° to +105°C, and is moisture resistance to MIL-STD-202, method 106 F, thermal shock to MIL-STD-202, method 107, condition B, vibration to MIL-STD-202, method 204, condition D, shock to MIL-STD-202, method 213, condition I and corrosion to MIL-STD-202, method 101, condition B.
The watertight QMA product line currently includes straight and angular connectors for flexible and semi-rigid cables as well as terminations. However, as Huber + Suhner is the only company with such a design on the market, it is likely that it will talk to other partners in the QLF® Alliance and issue licenses for the product in order to ensure second sourcing possibility to its customers.
The new waterproof QMA coaxial connector may not look any different to a standard connector on the outside but has to be very different on the inside in order for it to be waterproof. It is not just a standard QMA connector with an additional O-ring seal either, as the interface has been completely redesigned. Consequently, the interface complies with the existing standards and is backwards compatible with an existing female QMA connector, so that customers with female connectors on their equipment can mate with the new waterproof QMA plug.
It has also been designed with users in mind who need fast and space saving connections in outdoor applications such as wireless communications and transmission systems, portable radio systems, air traffic control and monitoring systems, and assemblies for mobile radio base stations. The new QMA connector essentially supports all radio frequency connections.
Huber + Suhner AG,
Herisau, Switzerland+41 71 353 41 11,
RS No. 302