Coax

Sealing Concepts
for Coaxial Connectors

Requirements and Seal shapes

The electrical connecting points in coaxial connectors must be protected against ambient influences. Dust and water or moisture are a constant hazard especially in outdoor applications, e.g. mobile radio masts. Moisture, particularly, which penetrates a coaxial system, changes the HF transmission enormously and leads to impedances or poor return loss values. Seals must be fitted at all connecting points to protect the sensitive and expensive electronics. This two-part newsletter presents the requirements for correct sealing design and different sealing concepts.

Requirements for seals

Seals are a class of construction elements which differ widely and which are used in almost every technical object. The task of a seal is to separate two functionally different spaces so that no transfer of substances can take place between them. Seals can be classified in many different ways. The first classification distinguishes moving seals (dynamic sealing points) and seals at rest (static sealing points). Only the static sealing points are relevant for coaxial connectors. A distinction is made here between contacting seals (O-rings, profile seal, flat seal) and non-contact seals (venting). Static seals can also be subdivided into sealing compounds, irremovable, removable and diaphragms. The removable seals are what we colloquially refer to as gaskets. The frequently used O-rings are therefore static, contacting and removable seals. The influences on the static, contacting sealing function in coaxial connectors can be divided into liquid and sealing body influence. Pressure, density, temperature, viscosity and the pH value play a role on the fluid side. The sealing function is additionally determined by elasticity, surface roughness, chemical resistance, wear resistance and porosity of the sealing body. There is no such thing as absolute tightness in the physical sense. It has to be clearly defined what is to be understood as "tight" (atoms, molecules, moisture, drips...). This tightness is known as technical tightness. In coaxial connectors a technical tightness on the level of the molecules is defined. This means that tightness against molecules, i.e. multi-atom particles, such as water is guaranteed but not against single atoms.

IP protection classes

In order to classify the tightness of coaxial connectors clearly, their degree of protection must always be specified. Degrees of protection are identified in accordance with IEC 60529. The used identification system is the IP-Code or International Protection Code, e.g. IP-67. Here, the first figure indicates the degree of protection against solids, the second figure the degree of protection against water. The meanings of the individual degrees of protection are shown in the figure below.

Protection against solid foreign bodies*
1st code number Description
0 No particular protection
1 Protection against ingress of solid foreign bodies with a diameter of 50 mm or more
2 Protection against ingress of solid foreign bodies with a diameter of 12.5 mm or more
3 Protection against ingress of solid foreign bodies with a diameter of 2.5 mm or more
4 Protection against ingress of solid foreign bodies with a diameter of 1.0 mm or more
5 Dust protected
6 Dust-proof

 * Definitions see IEC 60529

Protection against water*
2nd code number Description
0 No particular protection
1 Protection against dripping water
2 Protection against vertically dripping water. There must be no harmful effect on materials tipped (in a container) up to 15° from it's normal position)
3 Protection against fine water spray
4 Protection against water spray
5 Protection against water jet
6 Protection against strong water jet
7 Protection against water, when the material is immersed in water
8 The material is suitabel for continuous submersion in water. Must be agreed between customer and supplier.