THE HEART OF THE MATTER: cable basics
Jul 1, 1999 12:00 PM, Pete Putman
It's a fact of life; the active components in any professional A-V installation get most of the attention, while wiring is often taken for granted. Check out the booths at trade shows. The buzz is typically focused on the latest projector/monitor technology, computer/video interface and control interfaces. Things are comparatively sleepy around the cable and wire company booths.
Maybe that is not a bad thing. After all, we have had access to high-quality cables for much longer than we have had high-quality video images and interfaces. Advances in imaging technology are measured in quantum leaps from year to year; refinements in cable design and construction move in smaller increments.
That is, unless you believe all of the hype generated by resellers of cable for the home theater and consumer electronics industry. Over the past years, we have seen Teflon-dielectric cables for both video and audio, loudspeaker cables with directional arrows and even audio cable with a water jacket, ostensibly for cooling. Claims of superior performance, reduced signal loss and constant energy flow abound as the unwitting hand over hundreds of dollars for what essentially amounts to snake oil.
Do these wonder cables belong in your installation? Will they hold up over time or require constant attention? High performance and high maintenance usually go hand in hand; ask any professional race car driver or mechanic. Or, are you better off going with a simpler but tried-and-true product that you can install and not lose any sleep over?
Last May, I wrote about baluns and the part they play in the transmission and matching of AC signals. Subsequent to that piece (and following a trip to this year's NAB convention in Las Vegas), it became apparent that there is a great lack of understanding when it comes to cable, specifically cables for carrying video signals from one point to another. This was made all too clear as I thumbed through the latest copy of a major home theater magazine-several advertisements and much of the editorial material revealed a basic lack of cable smarts.
For those of you who already understand how AC signals (power, audio, video, RF) move over transmission lines, my apologies for covering old ground. For those who do not, it is time for another session of Cable 101.
Back to basics Forget everything you have read about Teflon dielectrics, polarized cables, gold-plated RCA connectors and multiple shield strands. Most of these gee-gaws are the result of marketing hype, not any scientific testing in a lab. For most installations, there are garden-variety cables that will work just as well, and they do not cost an arm and a leg.
There are several types of coaxial cables available from such manufacturers as Alpha and Belden. For our industry, we are primarily interested in the 75 V types, a number we inherited from the CATV and broadcast industries. Readers of my May feature will recall the characteristic impedance of open-wire transmission line for TV antennas is 300 V, and a 4:1 balun transformation results in a 75 V connection.
How does a manufacturer determine what makes a 75 V cable? Simple. The characteristic impedance of a cable (Zo) is determined by using the formula: 138 log b/a, where b represents the inside diameter of the outer conductor (shield or braid), and a represents the outside diameter of the inner conductor. Any unit of measure can be used (millimeters, inches, microns) as long as it is the same for both terms in the equation.
Coaxial cable companies have had more than 50 years to determine what combination of wire sizes and diameters make up 50 V, 52 V, 72 V, 75 V, and even 92 V coaxial cable. Because the impedance is strictly dependent on this ratio-and not on the dielectric, outer jacket, or composition of wire used-it is a pretty simple matter to manufacture coaxial cables in sizes ranging from 1/8 inch (3.2 mm) in diameter (RG-176/U) up to 2 inch (51 mm) waveguide.
Because AC signals travel at slower speeds through wire than they propagate through air, coaxial cables also carry a velocity factor specification. The velocity factor is useful to know when working with RF energy; it is essential for the design and construction of antennas and coaxial baluns. For the A-V installation business, velocity factor is generally not a specification we need to be concerned with, unless we are matching and transforming RF energy-say, splitting a signal from a satellite dish, or constructing power dividers for distributing cable TV signals.
The dielectric of a coaxial cable serves but one purpose-to maintain physical support and a constant spacing between the inner conductor and the outer shield. In terms of efficiency, there is no better dielectric material than air, but air does not offer us any structural integrity, so cable companies use a variety of such hydrocarbon-based materials as polystyrene, polypropylenes, polyolefins and other synthetics.
As AC signals increase in frequency, they have a tendency to be absorbed by rather than pass through insulating materials. Teflon, a synthetic plastic that was invented more than 40 years ago, is often used as a dielectric material in RF applications above 200 MHz. Microwave transmission lines and connectors make extensive use of Teflon (when they are not using air dielectrics) because it is also an excellent high-voltage insulator.
Flip open a consumer electronics magazine, and you will see ads for boutique loudspeaker cables and coaxial cables with Teflon dielectrics and (in some cases) Teflon outer jackets. Do not waste your money or your client's-the improvement in performance is negligible at these frequencies, often measuring less than tenths of 1 dB. That improvement is hardly worth hundreds of dollars, particularly when there are industrial-grade cables available that are economical and adequately suited to the job.
These traveling medicine show ads play off fears of signal attenuation, and they often attempt to convince would-be purchasers by boasting of 99% shield coverage and multiple interwoven strands for greater shield coverage. The truth is, attenuation has absolutely nothing to do with shield coverage-it is strictly a function of the electrical resistance of the center conductor and the efficiencies of the dielectric material.
When AC signals propagate through wire, some of the energy is lost as heat. (Remember Ohm's Law?) This effect can be overcome by using larger-diameter wire, which is the reason we specify RG-59/U for cable interconnects and RG-6/U or RG-11/U for longer cable runs. There will also be some absorption by the dielectric material, resulting in energy being dissipated as heat. Certain organic plastics (polyolefins, polypropylene and polyethylene) exhibit less absorption than others and are consequently better-suited for carrying AC signals.
At VHF radio frequencies and higher, certain cables are sold with a combination of air and solid dielectrics. Special pressure fittings allow the user to fill the line with nitrogen, which prevents moisture build-up. Water should never be used as a dielectric or insulator in any cable. The nitrogen approach is appropriate for cable installations in harsh environments, such as cellular phone repeaters, microwave towers and broadcast stations.
You may be surprised to learn that open-wire line has considerably less attenuation than coaxial cable. RG-59/U coaxial cable with a foam dielectric has a loss specification of 2.4 dB per 100 feet (30.5 m) @ 50 MHz, while RG-11/U (1/2 inch, 75 V coax) clocks in at just over 1 dB at that same frequency. What about generic 300 V TV antenna wire? How about .27 dB per 100 feet-almost ten times better than RG-59/U.
Shielding is another aspect of cables that is widely misunderstood. When AC signals travel in a balanced transmission line, which is where AC signals propagate most efficiently, there is little or no radiation of the signal. Assuming the source and load impedance transformations are optimized, all of the energy goes where it is supposed to-a loudspeaker, monitor or antenna.
Coaxial cables exist because we cannot run open-wire line near metallic objects (such as ducting) or bury it. We trade signal loss for convenience and flexibility. Unfortunately, we also complicate matters when making transitions between balanced and unbalanced wiring; if we do not have a perfect match (and we rarely do), there will be a certain amount of energy reflected back down the cable as standing waves.
Even with a very good match, there is always some signal radiating from coaxial cable. Hence, the outer conductor also functions as a shield to reduce coupling of the signal into adjacent wiring. More shield coverage means less radiation of energy, but it does not mean less attenuation. A Teflon-insulated cable that is improperly matched to a load will radiate more signal than a generic Radio Shack coaxial cable that is properly matched.
Many of our cable difficulties were foisted on us by manufacturers of consumer video equipment. For one reason or another (usually simplicity), manufacturers began making specialty cables that were convenient to use but exhibited poor performance. You need look no further than the ubiquitous S-Video cable, typically two strands of very small 75 V coax terminated at either end with a connector that is anything but constant impedance.
Typical S-Video cables are lossy, and why not-their counterpart would be RG-176/U, a miniature 75 V coax that swallows up 2.5 dB of signal for every 100 feet at 4 MHz. Even Belden's 1807A and 1808A S-Video dual-coax cables are rated at 1.5 dB attenuation per 100 feet @ 5MHz. Again, we pay a price in signal strength for convenience-both of these cables are small and easy to bundle.
A better approach would be to use BNC connectors on either end of standard 75 V coax for moving S-Video (and, for that matter, YUV component or YPbPr decoded analog HDTV) from point to point. As long as all cables are made from the same type of coax, phasing and delay problems should not arise unless there are huge differences in the cut cable lengths. That is where velocity factors and consistency in manufacturing come into play.
So before you buy any cable, why not get a copy of a reputable cable manufacturer's catalog and use it as your Bible. Be careful of claims that cannot be substantiated, such as improved performance from unusual outer jacket construction, crazy weaving patterns in shields and braids, combinations of copper and other metals in the center conductor and excessive use of Teflon for dielectrics and connectors.
Look instead for specifications on attenuation, temperature ratings, voltage and current capacity, and the type of outer jacket used. Many cables come in both contaminating (cannot be buried) or non-contaminating (go ahead and bury it) versions. Some can even be submerged in water without serious problems-something to think about when specifying cable for a humid environment.
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