The Difference Between Dual Shield vs Quad Shield Coaxial Cable
Written by Don Schultz, trueCABLE Network Expert, BICSI INST1, INSTC, Fluke Networks CCTT
Coaxial cable has been with us for a long time. In fact, you have used it at some point or another, even if you did not know what it was called! Have you ever screwed in a cable to a threaded connector sticking out of a wall plate or on the back of a television? Perhaps that threaded connector you screwed the cable onto was at the back of a cable modem. In all cases, you just worked with coaxial cable.
RG stands for Radio Grade. Usually this is followed by a number like 58, 59, 6, 11 and more. Additional variations come in outer jacket types and the amount of shielding the cable has. The two most often seen variations are dual shield (DS) and quad shield (QS). This article focuses on the difference between the shielding types. Interestingly, the amount of shielding has nothing to do with what kind of signal you are passing through the coax cable. It is all about the environment. More about that in a moment. First, let’s talk about history
Where Did It All Begin A Short History Lesson.
Like much of our technology we take for granted, coaxial cable started out as a US military project. The military has always been at the forefront of developing communications cable for defense needs. When you have to launch a missile or something, you need the right cable to do it! When you need to launch a whole lot of missiles at once in a world-ending scenario, you get the basis for the modern Internet. So, what was once the province of the military has now made it into the hands of the general public. But, I digress….
Coaxial cable in general has been used, and is still likely in use for the following:
Cable TV television signals (CATV) programming
Satellite dish systems
Cabling to connect up a two-way antenna (like for radio applications)
Networking computer systems in a LAN
Audio and visual (A/V) signals
As the needs evolved for faster signals to carry ever more data, the “RG” numbers started incrementing. We had RG58 a very long time ago, and now you see RG6. Want to run longer with your cable and RG6 just won’t cut it? Well, that is what RG11 is for. There are different RG coaxial cables for different applications. Get the idea?
Dual Shield Coaxial vs Quad Shield Coaxial
When coaxial cable started being used in many different applications, it was quickly discovered that sometimes you need extra shielding due to environmental issues like electromagnetic interference and radio frequency interference (EMI/RFI). This is where the different types of coaxial cable also started getting greater or lesser amounts of shielding. Residential coaxial cable might not need any additional shielding, where coaxial cable installed in a business might have issues with interference and require more shielding. Coaxial cable used for antennas, on the other hand, benefit greatly from additional shielding. The less loss the better when it comes to antennas!
The similarities between dual shield and quad shield coaxial cable:
Coaxial cable is constructed with center conductor, and this conductor can be solid or stranded for different applications
Wrapped around the center conductor is a piece of hard foam insulation known as the dielectric
Wrapped around the hard foam dielectric is foil tape shielding (sticky side down, of course)
Above the foil tape shielding is a wire braid shield that might be aluminum wire braiding or copper, again depending on the application
The entire cable is then wrapped in a sheath called a jacket made out of different types of plastic for different applications (like indoor or outdoor)
Quad shield coaxial cable (often referred to as QS and seen as RG/6QS as an example) has similar construction, but above the first layer of wire braid shielding will be another wrap of foil tape
Immediately above the additional wrap of foil tape will be a second wire braid shield
So, in effect, quad shield coax is simply doubling up on the shielding. An illustration will help out a great deal:
Things You Need To Know
So, more shielding must be better? Sometimes yes, and sometimes no. In fact, unless you KNOW you need quad shield coaxial cable you should probably use dual shield. The following items apply to coaxial cable used in Broadband applications such as cable Internet, digital cable, CATV, satellite, and the like.
Quad shield coaxial cable does not automatically provide higher performance, the signal loss across the maximum run length is the same as dual shield
Dual shield coaxial cable weighs less than quad shield
Quad shield coaxial cable cannot be bent as tightly during installation as dual shield, and there are rules for that! The rules as specified by ANSI/TIA 568.0-E indicate that the maximum bend allowable for coaxial cable is 10X the diameter of the outer jacket. Since quad shield cable is thicker, the bends you make cannot be as tight.
Quad shield and dual shield coaxial cable have the same maximum run length, so therefore using quad shield coax does NOT give you more distance
If you find that using quad shield coaxial gives you better performance on any one run, it was due to the EMI/RFI found in the environment and you needed quad shield coaxial cable from the beginning
Quad shield coaxial cable is typically more difficult to terminate due to the additional shielding adding to the cable thickness. Finding an appropriate F connector can be a challenge if working with RG6/U QS for example. It just gets infinitely worse if you are trying to terminate RG11/U QS (Series 11 Quad Shield).
Cable preparation of dual shield coaxial cable is straightforward. Quad shield coaxial cable requires removal of the additional second foil tape shield prior to termination. This adds time and complexity to the installation. In fact, quad shield coaxial cable has a bit of a legendary reputation as not for the faint of heart. It is not quite that terrible, but it is a bit more challenging.
Don’t worry, trueCABLE will be releasing universal fit F Connectors that will make terminating quad shield coaxial cable much more pleasant.
Ok, so now that quad shield coaxial cable has been “called out” as a “PIA” what is it good for? Well, there are situations where you can get closer to sources of EMI/RFI with quad shield coaxial cable. In some other situations, you may be forced into using quad shield coaxial cable out of necessity. For example, the building structure won’t allow sufficient separation between electrical and your communications cable. Another great use case for quad shield coaxial cable is when antennas (like for over the air digital HDTV) come into play and the absolute lowest loss performance is critical. Take a look at Top 2 Things to Consider When Running Ethernet and Power Cable for a handy chart that will show where quad shielded can be of great benefit.
So, there you have it. The differences and similarities spelled out and myths dispelled. With so many coaxial installations and installers out there, much (well intentioned) misinformation has made it online. trueCABLE and I are here to sort it all out for you!
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