Understanding coaxial cable specifications

Not all coaxial cables are the same, even if they have the same mechanical specifications. Structure, materials, and therefore electrical characteristics, vary widely. This can make similar looking cables suitable for very different uses, so it’s important to properly understand the implications of the cable’s properties.

In this article we’ll be comparing an RG178 cable from RS Components with a diameter of less than 2mm to a popular 10.278mm-diameter RG8U cable from Belden (both pictured below) in order to illustrate the effect of various cable properties on performance. In general, most performance metrics improve as the cable diameter gets bigger, but of course for electronics applications space is often at a premium, so thinner cables are more desirable. This leads to a trade off between thickness and performance.

 

Structure

Coax has come a long way since it was patented in 1880. Modern coax comprises an inner conducting core, surrounded by an insulating layer and a shielding layer, and finally a plastic jacket. The dimensions of these layers must be tightly controlled to ensure the cable’s properties are uniform.

The conducting core is where the signal travels.  This is usually copper, though cables for high frequency signals may have silver-plated copper conductors. This conductor might be single strand, or it might be made of several thinner strands for the purpose of mechanical flexibility.  

Surrounding the inner core is an insulating dielectric layer, which is usually plastic. The thicker the layer the better, but how good an insulator it is also depends on the exact plastic used. Some cables use plastic foam, which includes air – air being a good insulator.

The dielectric isolates the conductor from the shield layer, which is typically braided conductive wire kept at ground voltage. The shield protects the signal from any electrical and magnetic fields in the environment, and vice versa. Like the central conductor, the shield layer is typically made of copper or silver coated copper wires, but it can be complemented by a thin layer of metal foil to help cover the dielectric layer completely.

Generally speaking, loosely braided shield layers offer less coverage and are therefore lower performance, while a tightly braided shield layer with smaller gaps offers better performance. High performance cables may also have a thin layer of metal foil inside the shield layer to improve coverage, and some specialist cables even have two layers of foil and braid, but this is at the expense of the cable’s mechanical flexibility.

The whole thing is surrounded by a plastic jacket for protection against the environment.

(Above) RG178 cable from RS Components

(Below) RG8U cable from Belden

 

Let’s compare the structure of our thin and thick example cables.

The thin RS RG178 2-mm cable’s core is made of 7 strands of silver-plated copper clad steel, each with a diameter of 0.1mm. Its dielectric layer is PTFE, and outside that is 16 silver-plated copper wires braided into a shield with 96% coverage. Finally, there’s an FEP jacket.

Meanwhile, Belden’s RG8U 10-mm cable has a 2.74mm bare copper conductor, and a semi-solid polyethylene dielectric layer. The shield is Duobond II – a high spec option from Belden which combines an aluminium foil layer followed by a 90% coverage tinned copper braid layer, which improves performance and increases tensile strength compared to foil alone. The Belden cable’s jacket is PVC.

 

Properties

A key property describing coax is its characteristic impedance. This is the ratio of the amplitudes of the voltage and current of a single wave propagating down the cable (without any reflection), and it depends on the ratio of the diameter of the core and the inner diameter of the shield, as well as the dielectric constant of the dielectric layer. The most important thing when choosing a cable is that the impedances match at the transmitting and receiving ends to minimise any reflection of the signal. Coax typically comes in 50Ω and 75Ω types, with the 50Ω type almost always used for ordinary signal and data transmission applications since it offers a good balance between power rating and attenuation (75Ω is used only for VHF and UHF signals). Both the thin cable and thick cable we are considering here have 50Ω characteristic impedance.

Attenuation, or loss per unit length of cable, is a good measure of performance, since it measures the reduction in signal strength as it travels down the cable. Losses in coaxial cables are down to the losses in both the core and the dielectric layer.

Losses in the core are dominated by the skin effect at high frequency, whereby the signal tends to propagate along the surface of the core. Though the resistance of the core is still inversely proportional to its diameter, as a result of the skin effect it also increases with the square root of the signal frequency. Losses in the core dominate the overall losses at lower frequencies.

Losses also occur in the dielectric layer as a result of its exposure to a changing electric field. These losses also increase with signal frequency and decrease with the thickness of the layer, but they dominate over the core losses at very high signal frequencies.

Since both types of loss are thickness dependent, we’d expect the thicker cable to have better attenuation performance. To compare cables we need to be sure that we are comparing apples with apples, that is, considering attenuation for the same frequency of signal and the same length of cable. Our thin RS RG178 cable offers attenuation of 151 dB per 100 metres for a 1000 MHz signal, while the thicker Belden RG8U cable offers 4.4 dB per 100ft for the same frequency. This works out as 1.51 dB/m for the thin cable, and 0.14 dB/m for the thick cable, a factor of ten better.

 

Performance

Another performance metric often quoted is the velocity of propagation, which is the transmission speed of electrical energy in a set length of cable. For ease of comparison, it’s usually expressed as a percentage called velocity factor, which is the ratio of the velocity of propagation in the cable to the speed of light in free space. Signal velocity through coax depends mainly on the dielectric constant of the insulation layer surrounding it. For our thin RG178 cable with its PTFE dielectric, velocity factor is 69%. For the thicker RG8U, with its high dielectric constant polyethylene foam dielectric, it’s 84%.

Other cable properties for consideration include power rating, which depends on the cable’s rated voltage and the permitted temperature rise above ambient before the cable gets damaged by the heat. Generally, the lower the losses in the cable, the lower the temperature rise, so the higher the power rating of the cable – as discussed above, thicker cables typically have lower losses, so they have higher power ratings. Power rating needs to be derated for use in higher ambient temperatures, and in situations where less heat can be dissipated, such as if the cable is buried or installed in a confined space.

Overall, performance metrics such as attenuation are generally better for thicker coax cables. Specifying a coaxial cable for an RF application will depend on performance to an extent, but this will be traded off against the allowable thickness permitted by the application, as well as, of course, cost.