Basics of transmission line selection

Everyone seems to spend their time picking out just the right transmitter and antenna for that new system. The transmission line often becomes the weak sister that is given little thought, but its selection and design are decisions that will have to be lived with for many years.

Rigid transmission line (shown above) is often the choice in North America, while semi-flexible line has traditionally been used in Europe and Asia. Photo courtesy Dielectric.

The simplest of systems include such applications as low-power television or translators at mountaintop sites. The power is low, usually 1kw or less, and the length is short. In such cases, just about anything will do that can handle the power. A line such as a 7/8-inch semi-flexible cable with foam dielectric comprises an inexpensive and low maintenance solution. Such a cable is rugged and resistant to damage and requires zero maintenance other than to check the hangers occasionally. While the loss in db per 100 feet might seem a bit high at the upper UHF channels, there will be little difference in the ERP for short cable runs.

In dealing with the smaller cables such as 7/8 inches, up to 1 5/8-inch foam dielectric types, the only real problem is in the hangers. Simply fastening the line to the towers with wraplock is totally unacceptable and may lead to significant cable damage over the years. The use of wraplock forces the cable to be pressed up against the tower, including over flanges and bolt heads. No matter how well the cable may be installed initially, it will move due to wind, thermal expansion, tower movement, etc. That will cause abrasion of the outer jacket and, eventually, the outer conductor. An opening in the outer conductor exposes the dielectric to the elements in a manner beyond the design expectations. The cable should still be resistant to moisture, but should never be forced to maintain its integrity in such a situation.

With larger power levels and greater lengths, the choice of cables becomes more difficult. The primary considerations are usually the efficiency of the cable run and the power handling capability. Another consideration is the maximum usable frequency for the larger cable sizes at the higher UHF channels. Then there is the old problem of whether to use semi-flexible cable, rigid transmission line or waveguide. Old habits may tend to sway the user one way or the other on this.

Traditionally, rigid transmission lines or waveguide have been the choice in the United States, while semi-flexible cables have been popular in Europe and Asia. There is no really big difference in power handling capability or system efficiency between semi-flexible cable and standard rigid line. The semi-flexible line has to be installed carefully, as does the rigid line, and can be damaged badly if hoisting grips and load lines are not used properly. Hoisting grips MUST be used, no matter who tells you differently. Also, the right hangers MUST be used — don't even consider wraplock or straps. Otherwise, the user is doomed to replacement of a very expensive piece of line. Finally, leave the hoisting grips on the cable to support it on the tower. Without their support, the line may work its way down until all of the weight is hanging on the top connector.

To touch quickly on the installation of rigid line sections, they are not wind chimes. That is, they should NOT be lifted in bunches and allowed to bang together on the way up. If several sections are to be lifted at one time, they must be tied off and held away from the tower to prevent denting of the outer conductors. Every dent will show up when the line is tested and will serve only to increase the system VSWR.

Semi-flexible line can be repaired in the case of mechanical damage. One of the more common problems is a bullet hole. It seems that frustrated hunters must shoot at something, and those big lights must seem to be mocking them. In any case, localized mechanical damage can be repaired by simply cutting out the bad part and replacing it with either a “splice” from the manufacturer or with a pair of flanges.

Problems in air dielectric semi-flexible lines that cause burning are more of a problem. Trash that falls down into the line will cause burnouts in the future. Owners often want to cut out the burnt piece to some distance below the burn and replace only that part. Dumb idea. While it might seem to be a cost-saving procedure, you can count on coming back to replace the rest of the cable later. Just do it all the first time — it's cheaper that way.

For long runs of semi-flexible line, one solution to the burnout problem is to insert a pair of flanges into the lines to break it up into three or four sections. Then it will only be necessary to replace the bad section, rather than the whole line. It makes the line somewhat easier to install as well.

Semi-flexible line looks great when tested in both the time domain and the frequency domain. It is not sensitive to the channel in use, has no bullets to cause reflections, needs few if any elbows and should be essentially transparent to the RF signal. The attenuation numbers are slightly different than comparable rigid line, and it may be a little less tolerant to physical damage. If the power handling capability is reached, some additional margin can be gained by using one of the more exotic gases to pressurize the system. This can be a solution to the power limit when a bigger transmitter is installed but should not be used in a new installation. Remember, if you lose pressure you won't have to worry about moisture, you will have to worry about the line burning up.

Rigid transmission line offers the advantage of being able to replace a single section in case of mechanical damage. If there is a burnout, it is possible to remove the center conductor from several sections of line to mop out all the ashes and dirt. There is a disadvantage in that line lengths have to be chosen based on frequency. At certain combinations of line length and frequency, the reflections from all of the bullets and flanges combine in phase to present a very poor VSWR. This can be a problem if the lengths aren't proper for multiple channels diplexed on a common transmission line. Fortunately, the major manufacturers have developed lines with varying section lengths to eliminate that problem.

For high-power operation, the power handling capability of coaxial lines may become a problem, especially if more than one station is combined into a single line. The primary problem here is the temperature of the center conductor and the ability of the line to dissipate the heat from the center conductor to the outer conductor. Again, this can be helped by using some of the more exotic gases for pressurization. That should not be used in the design of a new line, as it is not the preferred avenue to gain more power handling ability. At least one manufacturer has developed a transmission line that will operate at higher temperature than standard copper. Therefore, if one reaches the limit of power handling for a given line, it may be possible to change to a coated line with higher power rating rather than go up to a larger line.

Efficiency of rigid lines varies primarily as a function of line size. The loss involved in a line is a function of the cross-sectional area available for current flow. The skin depth for current is a function of frequency, making the lines seem to have less cross section for current flow as the frequency increases. To counteract that problem, it is possible to go to a larger line, offering greater area for current flow and a resulting higher efficiency.

For very long runs of cable, efficiency may become the deciding factor. For example, on a 2000-foot tower, the use of three-inch rigid line at high UHF frequencies may cause the system to lose half or more of the power before reaching the antenna. Remember, you are not burning up 60Hz power, you are burning up RF, which has a much higher rate per kilowatt-hour. A simple analysis can be performed that can determine the real cost of a transmission line over a reasonable lifetime.

You won't make many points with the front office suits by saving some money on smaller transmission line if the power bill goes up 50 percent or if a larger transmitter has to be purchased to make up for the smaller coax. Then, of course, there is waveguide. Great power handling, unlimited lifetime, very high efficiency coupled with higher tower loading and increased cost. Let's save that for next month.

Don Markley is president of D.L. Markley and Associates, Peoria, IL.

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