What’s the Difference Between Shielded and Unshielded Category Cable for AV?
Category cabling is made of twisted pairs, which—when carrying a balanced signal (signals that are equal in magnitude and opposite in phase)—creates its own electromagnetic field within and around the twisted pair. When the signal is balanced, it provides its own immunity to outside noise interference. (Watch for a future Belden blog about how we manufacture the best balanced twisted-pair cable possible using expertise we’ve developed over several years.)
Shielding can be another good way to keep outside electrical interference from impeding a signal. It can also keep the signals inside a cable from impacting other cables, but more on that later. In noisy environments or for sensitive signals—which are often found in AV installations—a shielded Category cable might offer some benefits if chosen and installed correctly.
So how do you decide whether a shielded version is the better option—or a required solution—for your situation? The best place to start is with the equipment manufacturer. What type of cabling does it recommend? The manufacturer should be designing its system around the requirements. (This isn’t to say you won’t run into an issue. Maybe you want to design a system to support multiple applications and build futureproofing into the project.)
What do we mean by “shielded cable” as it applies to Category cabling? To start, there are two ways to provide a shield for a cable:
- Metal foil
- Metal wire braid
You can use one or the other—or a combination of both. Foil is most common, provides 100% coverage and is good for high-frequency signals; however, it doesn’t flex well. As a result, you’ll occasionally see braid used in specialty applications where cable is often moved around. A braid consists of multiple small strands of wire woven around a cable. Due to spacing between the strands, the braid is good only for low-frequency signals. For this reason, a braid is usually used along with a foil.
Category cabling typically consists of four twisted pairs. Shielding can be placed on the outside of all pairs, on each pair individually or both. Do you want to shield from outside noise or from noise between each pair within a cable? Either way, the continuity of the shield throughout the cabling system must be maintained, including within the connectors (which is often overlooked). Later, we’ll discuss why outside noise is usually the biggest interference factor in AV projects.
For most common cables—Category 5e, Category 6 and Category 6A—there are two choices:
- Unshielded twisted pair (UTP)
- Aluminum foil tape around all twisted pairs (foiled/unshielded twisted pair, or F/UTP), which is commonly referred to as shielded Category cable
Some specialty shields exist, but those are generally for specific harsh environments, such as those requiring the high flexibility we mentioned earlier.
A shield works by transferring induced currents from external noise signals to ground and away from the twisted pair. This requires not only the cable shield to be bonded to ground, but also the connectors. In addition to shielded cable, Belden offers shielded jacks, plugs, patch panels and patch cords.
Our recommendation is that the shield be bonded to ground at both ends of the cabling system per TIA standards; however, this isn’t followed by everyone and could result in additional issues. Systems are often installed with telescoping ground, which is only bonded on one end—and this is not ideal, either. If you don’t ground both ends, then you could be creating an antenna (an invitation to external noise), making things even worse.
Alien Crosstalk: A High-Speed Signal Killer
One of the biggest sources of noise for a twisted pair are the twisted pairs next to it. They can be internal and external to the cable. Through predictive processing, chip makers figured out how to filter noise from other pairs within the cable. For signals outside the cable, however, they don’t know the signal and can’t filter it out. This type of crosstalk is referred to as “alien crosstalk” or “crosstalk alien to the cable.”
Alien crosstalk is what separates Category 5e and 6 from Category 6A cabling. Category 6A cabling features special twists and isolators to dramatically reduce crosstalk from adjacent cables. This is another great reason to consider upgrading to Category 6A cabling. You might not need 10 Gb/s speed, but you will benefit from the cable’s enhanced design to improve immunity to external noise within your network.
Doing Bonding and Grounding Right
Earlier, we mentioned that the shield needs to be continuous and bonded to ground at both ends for best results.
Belden recently completed testing with 10 Gb/s signals and Category 6A F/UTP and UTP cables. (More about that in a future blog.) For this testing, we measured the integrity of a 10 Gb/s signal when subjected to external noise signals of between 0 MHz and 500 MHz.
Not surprisingly, we found that the most effective solution was the shielded F/UTP cable with the shield bonded to ground at both ends. What was surprising was the UTP cable outperforming the F/UTP cable when the shield bond to ground was only at one end. This helps reinforce our stance that shielded cable needs to be bonded to ground on both ends to be most efficient.
Avoiding a Grounding Loop
Of course, one possible downside of connecting a cable’s shield at two points is the fact that, under certain conditions, it can create a grounding loop (current on the shield due to two ground points having different potentials).
This is especially true in very long runs or runs between buildings. It’s important to note: Grounding is a critical safety concern within a building. Please rely on a qualified professional to address grounding issues. Ground loops can cause a large amount of noise and hum on cables and may be a good reason to avoid shielding and use a UTP cable instead.
Shielded cables also come with an increased cost. In general, shielded cables, connectors, patch panels and patch cords have a higher cost than UTP versions. In addition to increased material costs, extra labor is required to properly bond the shielding to ground. Combined, these additional expenses could increase the installation cost of a shielded system by as much as 25% per drop when compared to a UTP system.
If a UTP system isn’t an option for you, then the REVConnect® shielded system is a good option. It minimizes additional labor, reducing added costs from 25% per drop to less than 15% when compared to a UTP system. In addition, shielded cables have larger diameters and require more conduit cable tray space; some shielded solutions also have a large bend radius.
In the end, the real question comes down to this: Do the additional costs and steps warrant the use of an F/UTP solution? Can you be assured that the installation will be done correctly? How much electrical interference is at the site? Has noise been a problem in the past? Like most situations, there isn’t one answer for all. When answering this question, you’ll want to consider:
- What’s recommended by the equipment manufacturer?
- How much electrical noise is in your environment?
- Do you have access to a common ground at both ends?
- Are system requirements going to change in the future?
Belden offers both options, and we would be more than happy to help you address whatever your job needs. To learn more, watch a recent Shielded vs Unshielded Cabling webinar we led with PSNI Global Alliance.
P.S. Don’t miss our upcoming blog, where we take a look at well-balanced cables, what they are and how they compare to shielded cables. We’re here to help you make the right decision when it comes to mitigating noise.