THE HEART OF THE MATTER: cable basics
Jul 1, 1999 12:00 PM,
It’s a fact of life; the active components in any professional A-Vinstallation get most of the attention, while wiring is often taken forgranted. Check out the booths at trade shows. The buzz is typically focusedon the latest projector/monitor technology, computer/video interface andcontrol interfaces. Things are comparatively sleepy around the cable andwire company booths.
Maybe that is not a bad thing. After all, we have had access tohigh-quality cables for much longer than we have had high-quality videoimages and interfaces. Advances in imaging technology are measured inquantum leaps from year to year; refinements in cable design andconstruction move in smaller increments.
That is, unless you believe all of the hype generated by resellers of cablefor the home theater and consumer electronics industry. Over the pastyears, we have seen Teflon-dielectric cables for both video and audio,loudspeaker cables with directional arrows and even audio cable with awater jacket, ostensibly for cooling. Claims of superior performance,reduced signal loss and constant energy flow abound as the unwitting handover hundreds of dollars for what essentially amounts to snake oil.
Do these wonder cables belong in your installation? Will they hold up overtime or require constant attention? High performance and high maintenanceusually 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 thatyou can install and not lose any sleep over?
Last May, I wrote about baluns and the part they play in the transmissionand matching of AC signals. Subsequent to that piece (and following a tripto this year’s NAB convention in Las Vegas), it became apparent that thereis a great lack of understanding when it comes to cable, specificallycables for carrying video signals from one point to another. This was madeall too clear as I thumbed through the latest copy of a major home theatermagazine-several advertisements and much of the editorial material revealeda 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 oldground. For those who do not, it is time for another session of Cable 101.
Back to basicsForget everything you have read about Teflon dielectrics, polarized cables,gold-plated RCA connectors and multiple shield strands. Most of thesegee-gaws are the result of marketing hype, not any scientific testing in alab. For most installations, there are garden-variety cables that will workjust as well, and they do not cost an arm and a leg.
There are several types of coaxial cables available from such manufacturersas Alpha and Belden. For our industry, we are primarily interested in the75 V types, a number we inherited from the CATV and broadcast industries.Readers of my May feature will recall the characteristic impedance ofopen-wire transmission line for TV antennas is 300 V, and a 4:1 baluntransformation results in a 75 V connection.
How does a manufacturer determine what makes a 75 V cable? Simple. Thecharacteristic impedance of a cable (Zo) is determined by using theformula: 138 log b/a, where b represents the inside diameter of the outerconductor (shield or braid), and a represents the outside diameter of theinner 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 whatcombination of wire sizes and diameters make up 50 V, 52 V, 72 V, 75 V, andeven 92 V coaxial cable. Because the impedance is strictly dependent onthis ratio-and not on the dielectric, outer jacket, or composition of wireused-it is a pretty simple matter to manufacture coaxial cables in sizesranging 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 propagatethrough air, coaxial cables also carry a velocity factor specification. Thevelocity factor is useful to know when working with RF energy; it isessential for the design and construction of antennas and coaxial baluns.For the A-V installation business, velocity factor is generally not aspecification we need to be concerned with, unless we are matching andtransforming RF energy-say, splitting a signal from a satellite dish, orconstructing power dividers for distributing cable TV signals.
The dielectric of a coaxial cable serves but one purpose-to maintainphysical support and a constant spacing between the inner conductor and theouter shield. In terms of efficiency, there is no better dielectricmaterial than air, but air does not offer us any structural integrity, socable companies use a variety of such hydrocarbon-based materials aspolystyrene, polypropylenes, polyolefins and other synthetics.
As AC signals increase in frequency, they have a tendency to be absorbed byrather than pass through insulating materials. Teflon, a synthetic plasticthat was invented more than 40 years ago, is often used as a dielectricmaterial in RF applications above 200 MHz. Microwave transmission lines andconnectors make extensive use of Teflon (when they are not using airdielectrics) because it is also an excellent high-voltage insulator.
Flip open a consumer electronics magazine, and you will see ads forboutique loudspeaker cables and coaxial cables with Teflon dielectrics and(in some cases) Teflon outer jackets. Do not waste your money or yourclient’s-the improvement in performance is negligible at these frequencies,often measuring less than tenths of 1 dB. That improvement is hardly worthhundreds of dollars, particularly when there are industrial-grade cablesavailable that are economical and adequately suited to the job.
These traveling medicine show ads play off fears of signal attenuation, andthey often attempt to convince would-be purchasers by boasting of 99%shield coverage and multiple interwoven strands for greater shieldcoverage. The truth is, attenuation has absolutely nothing to do withshield coverage-it is strictly a function of the electrical resistance ofthe 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-diameterwire, which is the reason we specify RG-59/U for cable interconnects andRG-6/U or RG-11/U for longer cable runs. There will also be some absorptionby 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 forcarrying AC signals.
At VHF radio frequencies and higher, certain cables are sold with acombination of air and solid dielectrics. Special pressure fittings allowthe 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. Thenitrogen approach is appropriate for cable installations in harshenvironments, such as cellular phone repeaters, microwave towers andbroadcast stations.
You may be surprised to learn that open-wire line has considerably lessattenuation than coaxial cable. RG-59/U coaxial cable with a foamdielectric has a loss specification of 2.4 dB per 100 feet (30.5 m) @ 50MHz, while RG-11/U (1/2 inch, 75 V coax) clocks in at just over 1 dB atthat 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 ACsignals travel in a balanced transmission line, which is where AC signalspropagate most efficiently, there is little or no radiation of the signal.Assuming the source and load impedance transformations are optimized, allof the energy goes where it is supposed to-a loudspeaker, monitor orantenna.
Coaxial cables exist because we cannot run open-wire line near metallicobjects (such as ducting) or bury it. We trade signal loss for convenienceand flexibility. Unfortunately, we also complicate matters when makingtransitions between balanced and unbalanced wiring; if we do not have aperfect match (and we rarely do), there will be a certain amount of energyreflected back down the cable as standing waves.
Even with a very good match, there is always some signal radiating fromcoaxial cable. Hence, the outer conductor also functions as a shield toreduce coupling of the signal into adjacent wiring. More shield coveragemeans less radiation of energy, but it does not mean less attenuation. ATeflon-insulated cable that is improperly matched to a load will radiatemore signal than a generic Radio Shack coaxial cable that is properlymatched.
Many of our cable difficulties were foisted on us by manufacturers ofconsumer video equipment. For one reason or another (usually simplicity),manufacturers began making specialty cables that were convenient to use butexhibited poor performance. You need look no further than the ubiquitousS-Video cable, typically two strands of very small 75 V coax terminated ateither end with a connector that is anything but constant impedance.
Typical S-Video cables are lossy, and why not-their counterpart would beRG-176/U, a miniature 75 V coax that swallows up 2.5 dB of signal for every100 feet at 4 MHz. Even Belden’s 1807A and 1808A S-Video dual-coax cablesare rated at 1.5 dB attenuation per 100 feet @ 5MHz. Again, we pay a pricein signal strength for convenience-both of these cables are small and easyto bundle.
A better approach would be to use BNC connectors on either end of standard75 V coax for moving S-Video (and, for that matter, YUV component or YPbPrdecoded analog HDTV) from point to point. As long as all cables are madefrom the same type of coax, phasing and delay problems should not ariseunless there are huge differences in the cut cable lengths. That is wherevelocity factors and consistency in manufacturing come into play.
So before you buy any cable, why not get a copy of a reputable cablemanufacturer’s catalog and use it as your Bible. Be careful of claims thatcannot be substantiated, such as improved performance from unusual outerjacket construction, crazy weaving patterns in shields and braids,combinations of copper and other metals in the center conductor andexcessive 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. Manycables come in both contaminating (cannot be buried) or non-contaminating(go ahead and bury it) versions. Some can even be submerged in waterwithout serious problems-something to think about when specifying cable fora humid environment.