To set up any wireless system, you need coaxial cables and assemblies. The dual conductor electrical cable consisting of an outer conductor and a central conductor with insulating dielectric material between them find a wide range of applications in different kinds of environment. Since long, RG coaxial cables have been the staple choice for setting up wireless networks and have set the standard in the industry. However, RG cables do not suit all conditions, especially those that require low loss transmission. This is when the low loss coaxial cables are the only choice.
Although invented in 1929, coaxial cables made its commercial appearance only in 1941. The advent of televisions contributed to the popularity of coaxial cables because the cable is just perfect for carrying television signals and connecting two video equipment. Coaxial cables are special because unlike the electrical cables and RG style cables it uses solid center conductors instead of strands. Not only in its use but the construction of LMR-400, which is a low loss coaxial cable is also different from the standard RG cables in the type of shielding of the cable. Low loss cables have superior shielding that results in low attenuation or loss over a distance. In order to understand low loss coaxial cables, you must first understand how it differs from RG cables in its construction and features.
In this article, we will touch upon the constructional features of low loss coaxial cables and some applications where these cables are the only choice.
RG coaxial cable
It has been many decades since the military used wireless connections across wide areas for which they used RG coaxial cables that was the best for the purpose. It will not be wrong to say that RG coaxial cables are military-born cables. They named it Radio Guide coaxial, cables of RF Government cables, both resembled by the acronym RG. In order to utilize various military applications, the military had a pre-specified dimension for various coaxial lines that helped to generate a system of interconnects that were quite predictable. With time, the commercial variants of RG coaxial cables also came into the market.
For setting up WLAN, cellular, GPS, etc., RG cables with 50 ohms impedance are suitable, and for audio/ video applications like in security systems and CATV, 70 ohms impedance is most suited. RG cables with a higher impedance of 92 ohms, is the third cable variant available. RG cables are susceptible to losses, especially in wireless applications over a long distance.
Wireless applications like SCADA, WLAN, ISM, PCS, etc. require cables with minimal losses and it became necessary to look for some alternative to RG cables that led to the discovery of low loss coaxial cables.
Low loss coaxial cable
Low loss coaxial cables deliver lower attenuation as compared to RG cable of the same diameter when used in the same environment for wireless applications. The lower attenuation is the result of several improved features in the low loss coaxial cables like superior dielectric material, solid inner conductor, more shielding of superior quality and jacketing materials suitable for specific applications.
The heavy emission losses of stranded conductors used in RG cables are greatly reduced by using solid inner conductor in low loss cables. The proximity effect is the main reason for losses. The proximity effect imparts a tendency for EM energy in a conductor that compels it to stay farthest away from the conductors in its close vicinity through which current flows in the same direction. This is typically a multi-conductor version of skin effect that results in current concentrating at the periphery of a conductor at high frequencies. Although carefully stranded cables can minimize the losses due to skin effect, it is unable to offset the loss of proximity effect.
The impedance of coaxial cables has a direct correlation to the uniformity of the inner dimensions. Since stranded center conductors are less uniform than the solid inner conductor, the former is prone to more losses. However, the flexibility of stranded cables is better than cables with solid inner conductors. Some solid conductor coaxial cables offer higher flexibility even though the central solid conductor alone does not contribute to the flexibility because the shielding and dielectric too impact flexibility.
The dielectric in coaxial cable separates the outer conductor from the inner conductor while helping to maintain uniform cross-sectional dimensions throughout the transmission medium. The speed of travel of a signal through dielectric is much slower than traveling in free space, and this is a factor to consider when choosing a dielectric medium that must have a lower dielectric constant. There is a number of ways to lower the dielectric constant by introducing air by helically wrapping the dielectric around the conductor, by foaming the dielectric material and using dielectric spacers. Foaming is part of the extrusion process and achieved by introducing bubbles into the polyurethane, PTFE (Polytetrafluoroethylene) and FEP (Fluorinated Ethylene Propylene) that reduces the attenuation of coaxial cables.
Shielding effects attenuation because for high-frequency signals if there is a lack of coverage, it causes signal degradation. The outer conductor acts as the return path and carries current in the opposite direction to the current flowing in the inner conductor. Effectively, this forms a shield that carries equal and opposite signals to those flowing through the inner conductor. The skin effect is evident in the shielding as braided shields perform well at low frequencies but as the frequency increases and pushes the signal towards the shielding surface it can cause EMI as fields can emerge through the holes in the coverage. Adding an aluminum foil shielding (bonded or non-bonded) can prevent transmission losses.
The jacketing material helps coaxial cables adapt to different environmental conditions. Some jacketing materials for outdoor applications have moisture resistant and vibrational strain resistant properties besides being UV resistant and in some case may be resistant to oils and chemicals too. Mixing plasticizers with the jacketing material makes it UV resistant. For underground applications, cable covered in water-resistant gel would be most suitable to make it completely moisture proof.
Low loss coaxial cables are quickly replacing RG cables to achieve better signal transmission, especially over a longer distance.