Coaxial cable is the most commonly used transmission line for RF and microwave applications, because it provides reliable transmission with the benefits of wide bandwidth and low loss and high isolation. Major manufacturers of transmitting equipment, i.e. radio and TV, radar, GPS, emergency management systems, air and marine craft, use coaxial cables. The uses of coaxial cable apply to any system in which signal loss and attenuation must be minimized. Unlike waveguides, coaxial cable has no lower cutoff frequency but what about its upper frequency?

Max Frequency

With some exceptions, most coaxial cables do not have an actual cut-off terms of a specific stop-band frequency but instead use the term cutoff to refer to the highest frequency tested by the manufacturer, or when the frequency reaches a point where the coaxial cable becomes a waveguide and other modes, aside from the transverse-electromagnetic mode (TEM), occur. Hence, a coaxial cable cutoff frequency could be where the coaxial cable remains within specification, or within a reasonable margin to avoid transverse-magnetic (TM) or transverse-electric (TE) propagation modes. Though coaxial cables can still carry signals with frequencies above the TEM mode cutoff, TM or TE transmission modes are much less efficient not desirable for most applications.

Cutoff Frequency and Skin-Depth

Two important concepts of note when discussing frequency in coaxial cable are skin-depth and cutoff frequency. Coaxial cable is made up of two conductors, an inner pin, and an outer grounded shield. Skin depth occurs along the coaxial line when high frequencies cause electrons to migrate towards the surface of the conductors. This skin effect leads to increased attenuation and dielectric heating and causes greater resistive loss along the coaxial line. To reduce the losses from the skin affect, a larger diameter coaxial cable can be used but increasing the coaxial cables dimensions will reduce the maximum frequency the coaxial cable can transmit. The problem is that when the size of the wavelength of electromagnetic energy exceeds the transverse electromagnetic (TEM) mode and begins to “bounce” along the coaxial line as a transverse electric 11 mode (TE11), the coaxial cable cut-off frequency is created. Because the new frequency mode travels at a different velocity than the TEM mode, it creates reflections and interference to the TEM mode signals traveling through the coaxial cable. This is referred to as the upper frequency limit or cutoff frequency.

A cutoff frequency is a point at which energy flowing through the EM system begins to be reduced, by attenuation or reflected, rather than passing through the line. TE and TM modes are the lowest order mode propagating on a coaxial line. In TEM mode, both the electric field and the magnetic field are transverse to the direction of travel and the desired TEM mode is allowed to propagate at all frequencies. Higher modes are excited at frequencies above the cutoff frequency when the first higher-order mode, called TE11, is also allowed to propagate. To be sure that only one mode propagates for a clear signal, the signals need to be below the cutoff frequency. Reducing the size of the coaxial cable increases the cut-off frequency. Coaxial cables and coaxial connectors can reach into the millimeter wave frequencies but as the physical dimensions shrink, power handling capabilities are reduced and losses increase.

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Coaxial cables or COAX cables are the ideal solution for all applications requiring high speed data and video, flexibility, protection from extreme environment and high and low temperature resistance. Where you need a signal with minimum distortion and attenuation or no interferences, coaxial cables play a pivotal part. In fact, thanks to their twin conductor construction nature, they are able to ward off many issues produced by bifilary wires.

Coaxial cables consist of two copper or aluminium coaxially oriented conductors surrounded by a tubular insulating layer, enclosed by a conducting shield. The quality of coaxial cables affects the signal interference; higher quality ensures a minimum signal interference. Moreover, also the density of the shielding affects the quality of the COAX. The result is that high quality materials and manufacturing process are essential to manufacture a reliable coaxial cable.

Generally, coaxial cables end with connectors that are usually manufactured with high conductivity metals. There are many different kinds of coaxial cables, which are generally defined by electrical capabilities, outer diameter or materials. However, most coaxial cables are manufactured in line with RF industry standards, the American norm MIL-DTL-17 and the French norm NF-C-93550.

An example is represented by RG cables, where the copper and aluminium copper clad wire quality, as well as the foaming extrusion, are part of the efficiency of signal.

Although the solutions for a high quality coaxial cable manufacturing look like straightforward, the design of these lines requires expertise and a deep understanding of the final product. Here, we do not see our intervention as a mere product supplier. Our solid expertise in the field as manufacturer, designer and supplier of these remanufacturing solutions, gives us an holistic understanding and all the know how to optimize the solution based on customer needs and specific industry requirements from the drawing (single wire & multi-wire) to the extrusion (insulation & sheathing), passing through the wire assembling (bunching).

This article comes from sampsistemi edit released