FM Systems: Open Your Eyes and Take a Look
This is the last in a series of articles on the fundamentals of FM transmission systems.
In most cases, once a transmission line and antenna have been installed on a tower, little thought is given them until something goes wrong. That is unfortunate, because they do require a certain amount of maintenance to keep them reliable and operating at peak efficiency.
Regular mechanical maintenance of an FM antenna system should include a complete visual inspection at least annually. This can be done in conjunction with relamping or other tower maintenance to minimize costs. Excitation must be removed from the antenna and other steps taken to ensure that the tower is safe for workers to climb and work in the vicinity of the antenna.
The tower worker making the visual inspection should look at each element in the array for proper installation; for burns or pitting on the element ends; for cracks, carbon traces or defects in insulators; and for any evidence of abnormal mechanical stresses. Weep holes in the antenna elements, if any, must be oriented so that they face down.
If the antenna is equipped with deicers, they should be turned on during the inspection. The tower worker should then verify that the deicer in each element is working by feeling of the element.
One of the most dangerous icing situations for an FM antenna occurs when all the elements' deicers are working except one. All of the elements except that one remain clear of ice, and a serious mismatch occurs in the interbay line.
Because the mismatch is limited to that one element, the transmitter's VSWR foldback circuit may not see sufficient reflected power to cause it to activate. With full power being fed to the antenna, an arc can occur and sustain itself until serious damage has been done to the antenna. From a protection standpoint, it is better to operate with no deicers than with a partially operating system.
A good deicer maintenance tool is a permanently-installed AC ammeter on the deicer circuit. With the deicers operating normally (and after a 10-minute warm-up period), mark on the meter face the nominal current. From time to time, the deicers should be turned on and allowed to warm up. The current then can be checked against the mark. If there is a deviation, a tower worker should be dispatched to investigate.
Transmission lines likewise should be inspected visually every year. Inspect the connections at the top and bottom for security, and investigate any evidence of movement. Ground connections should be checked and tightened and any corrosion removed.
In the case of rigid transmission lines, the length of each hanger spring should be checked against the manufacturer's recommendation. If necessary, adjust the springs. Check nylon buttons or bushings in the spring hanger collars. Should these fall out or become damaged, the copper transmission line will chafe against the metal of the hanger. This eventually will wear through the copper of the line, leading to line failure.
Rigid and semi-flexible lines should be checked for "hot spots" near flanges and connectors. In the case of rigid lines, this is every 20 feet. An infrared thermometer can be used for this, or the tower worker can simply feel of the joint and compare it to the adjacent portions of the line. Localized discoloration in rigid lines is a good indicator of abnormal heating.
The cause of any significant heating should be investigated, as heating at a joint is a precursor to a line failure. Such heating often is caused by a "split bullet" or chafing in the inner conductor where it makes contact with the "bullet." The heating is evidence of I2R loss in the joint, which is power not being radiated. In many cases, heating in the joints can quickly produce a thermal runaway situation leading to catastrophic failure of the line.
One tool available today that is of great help in locating hot spots in a transmission line or antenna is infrared photography. Hot spots will show up as a lighter color. The advantages of this method are that it can be done from the ground and it can detect subtler temperature rises. While IR photography has several advantages, it is not a substitute for a good visual inspection.
In some situations where an FM antenna is mounted on an AM tower, the transmission line will be mounted using insulated hangers. Inspect each insulator for cracks and carbon traces. Check the shorting stub, usually located somewhere close to 90 electrical degrees (at the AM frequency) above the tower base, for a good electrical connection at the line and the tower.
Other FM on AM situations employ an isocoupler to cross the base insulator with the FM transmission line. In these cases, it is important that the transmission line be electrically bonded to the tower frequently along its length. Check each bond for good electrical connection.
In addition, check the entire outer jacket for evidence of arc-through to the tower. If such evidence is present, it likely indicates that more frequent bonding is needed. Such arc-through, if permitted to continue, eventually can penetrate the outer conductor of the line and open the line to the elements.
Grounded-base AM towers, such as folded unipoles and shunt-fed antennas, likewise require the FM transmission line to be bonded frequently to the tower steel along its length. The same checks of the bonding and outer jacket should be made on grounded-base AM towers.
It's in the air
Proper pressurization of air-dielectric lines is critical to their performance. Pressurization with dry air or nitrogen has a direct effect on the breakdown voltage (and thus the power handling capability) of the line.
If moisture is allowed to enter the line, corrosion will result, producing higher losses, hot-spots and eventual line failure. The pressure integrity of the line and antenna should be checked by closing the air valve feeding the line at the bottom of the run and observing the line pressure over a period of several hours. If the pressure bleeds off, there is a leak and it should be located and repaired.
Maintain an adequate nitrogen supply at the site to ensure that the supply is not exhausted before another delivery can be made. If the pressure integrity of the line is maintained, very little nitrogen should be used. Inspect pressurization equipment regularly, at least weekly, and maintain it in good working order. Desiccant should be rotated out and dried or replaced whenever it begins to turn from blue to pink. A dehydrator with a saturated desiccant pumping moist air into a line is almost as bad as having no pressurization at all. Use a self-recharging dehydrator to prevent this from ever happening.
Finally, make a check of the system reflected power during every site visit. A significant change, up or down from the nominal value, should be investigated. A more sensitive indicator of load impedance in some power amplifier designs is the screen current. Higher screen current generally indicates a lighter (higher Z) load. If PA tuning is otherwise correct, an increase in screen current may be an indicator of a developing antenna or line problem.
In addition to checking the reflected power, the VSWR foldback and trip circuits in the transmitter or external protection device should be checked from time to time to ensure that they are working. Transmitter manufacturers usually provide a procedure for checking and adjusting the internal protective circuits.
If the directional coupler(s) use removable slugs to detect forward and reflected power, check regularly for proper operation and good connections. A reflected power meter that reads zero all the time may be an indication that the connection, either between the slug and the coupler or between the coupler and the transmitter, is bad. If that is the case, chances are there is no VSWR protection in place, which can lead to a catastrophic failure of the line/antenna and damage to the transmitter.