Watch that line
A station's transmission line is an important, yet often neglected, part of the RF system. Most stations take it for granted until a catastrophe happens requiring a significant capital expense for its replacement. The lack of proactive preventive maintenance for transmission lines will almost always result in expensive replacements in due time. The quality and efficiency of the RF transmission can be adversely affected when the transmission line system is not at its best. This becomes more critical as stations move into digital operation, requiring better average power handling and uniform bandwidth response. Therefore, it is important to include transmission lines in the preventive maintenance program.
A transmission line serves as the conduit for RF power flow from the transmitter to the antenna. It is supposed to deliver, without any alterations, all the RF power from the transmitter to the antenna. However, real-world installations are not perfect. Some of the power is lost and the RF signal can be altered even before it reaches the antenna. The amount of power loss and RF signal alteration depends on a number of factors. The important factors that characterize its effectiveness in accomplishing this function are VSWR that shows the impedance mismatch along the line; frequency response that may alter the desired bandwidth of transmission; and losses due to cable attenuation and the lack of mechanical (hence electrical, too) integrity along the line.
Temperature and weather
Coaxial lines--or any metal for that matter--expand when the weather is hot and contract when the weather is cold. To make matters worse, a differential in expansion occurs between the inner and the outer conductors. An adequate provision for this expansion and contraction should be implemented in a rigid coaxial cable installation to prevent undue mechanical stress on its components. This is one reason why a minimum horizontal run for a rigid coaxial line is recommended by the cable manufacturer to accommodate changes in vertical length due to temperature variations. Suspension hangers and connectors allow small movements on the horizontal line while keeping good electrical contact on connectors and flanges. The bullets on flanges keep these movements from causing undue mechanical stress while maintaining the required impedance. Routine maintenance involving visual inspections of the flanges, connectors and hangers needs to be done at regular intervals depending on the age of the system. Replace inner conductor components or even line sections as needed.
Cable connections must be weatherproofed when they are exposed to the weather. Use rubber tape for type N and smaller connectors if it will be exposed to the sun and rain. Moisture inside the connectors can cause corrosion and drastically degrade their performance. The problem manifests itself as excessive loss of power on the line or even a burn-out for high-power applications.
Air pressurization of air dielectric cables increases the breakdown voltage between the inner conductor and the shield. This breakdown voltage is directly proportional to the power handling of the cable. This means that the better the air pressurization, the better the power handling of the cable. The air that is pumped into the coax cable should be dry. Inspect the dehydration or pressurization equipment and its ability to remove moisture as part of the preventive maintenance program.
Installation and use
Grounding the coaxial shield redirects lightning energy to the ground. Lightning during a thunderstorm can cause a breakdown of insulation between the shield and the inner conductor aside from damaging sensitive electronic equipment at the sending end of the coax line. The shield of the coax line should be grounded to the tower at least at the top, middle and bottom parts of the run.
The transmission line should be essentially flat along its length. This flatness refers to the impedance matching along the line that should be consistently 50Ω or 75Ω, depending on the system. This is determined with the use of a time domain reflectometer (TDR) where a pulse is sent through the line and the reflected pulse is monitored on a display. Impedance bumps indicate a mismatch. The location of the mismatch can be computed, and a physical inspection can often locate the problem. When reviewing a TDR read-out, there will be small bumps from flange connections, tuning sections and elbows. A better way to measure the line is with a network analyzer (Figure 3) because we can do other electrical testing and tuning to flatten the line. A plot, as shown in Figure 4, can yield valuable information as to the changes in the impedance along the coax run. VSWR changes at specific points need to be determined annually to identify abnormal deterioration of connections and other components of the transmission from the output of the transmitter's PA, through the filter and patch panel and then to the antenna. Fine matchers after the elbow, as shown in Figure 2, may need to be adjusted to compensate for aging of the system.
Frequency response sweeps
Test the coaxial line for frequency response sweeps at least once a year. The test needs to include the filter and patch panel, and the signatures should be compared from previous years. This is more important now with digital due to the wider actual bandwidth being used. Compare signature plots to determine problems that are brewing and therefore be able to rectify the situation prior to any signal field tests.
Keep coax healthy with this checklist
During a tower inspection or maintenance
1) Check for dents or bending on rigid line sections. Replace deformed sections.
2) Inspect all flange connections for warping and tightness. Tighten bolts and use lock-washers with all nuts. Use high-strength stainless steel bolts and nuts only.
3) Inspect that all line hangers are properly installed and free from corrosion.
4) Check that gas barriers and tuning slugs are not leaky.
5) Inspect that grounding kits are present and properly installed. Replace defective or install additional grounding kits as needed. Improve weatherproofing as applicable.
6) Check for signs of corrosion on all line support hardware. Replace or clean and cold galvanize as needed.
On a regular basis
7) Check that the dehydrator is working properly. Replace desiccant that indicates excessive moisture.
8) Check the air pressure on the coaxial cable. There should be enough air pressure at all times especially during humid weather. Fix leaks for good.
At least once a year
9) Conduct a transmission sweep from the PA output of the transmitter to the antenna. Tune coaxial fine matchers and sections as needed. You may need a tower crew (at least two) to do the hands-on work along the tower.
10) Open sections with excessive impedance bumps for internal physical inspections. Replace bullets and any suspicious inner hardware as needed and clean thoroughly before re-assembly.
11) It is not a bad idea to check the coupling value of the coaxial directional coupler, if the system has one, for accurate RF power measurements for analog transmitters. This ensures that power measurements are calibrated at least once a year.
Lintag is an RF engineer for Victory TV Network in Little Rock, AR.
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