Coaxial assemblies are a fundamental component of wireless systems, henceforth finding their application in diverse environments and circumstances. For beginners, the military-style RG coaxial cables have been the go-to standard and easy answer when figuring out which cable is most suitable for a particular installation. But there are exceptions where these cables of choice will not be an appropriate choice and a low loss alternative is recommended. This article will explain the difference between a low loss cable and standard RG assemblies in terms of design and application for these coaxial cables.
The RG coaxial cables were basically created for military applications and later, commercial variants were introduced with similar design and performance. The impedances range for RG cables can vary between 50 ohms, 75 ohms, and sometimes 92 ohms. Mostly the 50 ohms impedance coaxial cables are used for data transmission in applications such as WLAN, GPS, and cellular. On the other hand, the 75 ohms cables are utilized for audio/video applications (e.g.: security system, CATV). Traditional RG cables are quite lossy and this is especially apparent at long distances for wireless applications. Therefore in wireless applications datasheets used for cables do not specify average insertion loss, or attenuation, in its decibel (dB) value, but the only attenuation over a specified distance such as dB/100ft, dB/100m. The nominal attenuation value is listed at a number of frequency points within the cable’s operational bandwidth.
There is a high demand for several wireless applications (e.g.: WLAN, SCADA, PCS, ISM, etc.) to have an alternative to typical RG cable constructions especially for medium to long-distance runs. Low loss cables are an alternative to such cabling that roughly holds up to the standards of the RG cables.
What is a “Low Loss” Coaxial Cable?
Low loss coax deliver a lower attenuation when used as a replacement for similar diameter RG cables used in very same wireless applications. They offer a lower overall attenuation due to several key factors:
- Solid inner conductor
- Superior dielectric material
- Superior shielding and more shielding
- Application-specific jacketing materials
Several factors contribute to a low loss solid inner conductor in comparison to the stranded conductors that are often used in RG cables.
The proximity effect is the tendency for EM energy in a conductor to gather farthest away from nearby conductors carrying current in the same direction. This lack of uniform current distribution causes the AC resistance of a conductor to increase rapidly thereby increasing transmission loss with frequency.
The conformity in the cross-sectional area of the coax. The characteristic impedance of a coaxial cable is directly correlated to the uniformity of its inner dimensions. Stranded center conductors are inherently less uniform than a solid inner conductor and can therefore cause more reflections and ultimately loss of signal.
The dielectric material in a coaxial cable fundamentally separates the inner conductor from the outer conductor while holding uniform cross-sectional dimensions across the transmission medium. A signal travels significantly slower in a dielectric in comparison to the one traveling in free-space. Therefore a lower dielectric constant is preferred to reduce the delay in the line. This can be achieved by introducing air into the dielectric lowers the dielectric constant employing various method such as helically wrapping dielectric around the inner conductor, using dielectric spacers, or by foaming the dielectric material. The foaming minimizes attenuation in the coaxial assembly for two main reasons:
- Smaller loss tangent
- Larger center conductor
The shielding impacts the attenuation, especially for high-frequency signals. In the case of high-frequency signals, a lack of coverage will result in signal degradation. The outer conductor serves as the return path for the inner conductor and transmits current in the opposite direction, acting like a shield by carrying equal and opposite signals to those in the center conductor.
The skin effect – as the braided shields perform well at low frequencies (hundreds of kHz) but as the frequency increases and the signal is pushed towards the surface of the shielding, fields can emerge through the holes in coverage creating EMI. This is dealt with by adding a bonded or non-bonded aluminum foil shielding. It significantly reduces the losses in the transmission line but also reduces flexibility.
Jacketing materials are a significant deciding factor based on the environmental conditions of the intended application. Outdoor applications demand resistance to vibrational strain from wind, resistance to moisture from rain and humidity, resistance to UV, and, in some cases, resistance to chemicals/oils. These conditions can easily crack or damage the typical PE or PVC cable jackets. Inserting plasticizers into the jacketing material can generate UV resistant properties. A flooded cable or a cable coated with a water-resistant gel may be needed for an underground burial application.