Servicing microwave links in two-way radio systems
Microwave communications systems are found within many mobile radio systems. They are used to connect repeater locations to dispatch points and to connect repeaters with satellite receivers and voting units.
Microwave systems normally are well-engineered. Microwave communications first came into widespread use to connect TV broadcast stations with network studios and to connect parts of the nationwide long-distance telephone network. Both applications demanded extremely high reliability. Between a network headquarters on the East Coast and a TV broadcast station in the Midwest there may be 30 or 40 microwave repeaters. If any of the microwave repeaters fails, the network TV feed can be lost by the local television station. The rarity of such failures is evidence of microwave communications system reliability.
What does all this have to do with troubleshooting a microwave link that forms a part of a two-way radio system? Microwave systems are normally so reliable, it is only necessary to troubleshoot one when almost all other possible causes for a malfunction have been eliminated. Before going to work on the microwave system, be sure the malfunction does not lie in the repeater, the repeater control system or the dispatch console.
External causes of malfunctions Microwave sites are often unattended and in isolated locations where they may be subject to accidents, weather damage or vandalism.
One microwave system in a water tower failed when the water control system malfunctioned and let several inches of water into the equipment building. Even so, the RF equipment was undamaged. The water had damaged the power supply in the bottom of the microwave system cabinet.
Nitrogen under pressure is widely used in waveguides to exclude moisture. In waveguides pressurized by nitrogen gas, waveguide malfunctions often are accompanied by loss of gas pressure. When properly built, a correctly pressurized waveguide system generally is not subject to internal ice or moisture buildup. Waveguides also may be damaged by vandals using firearms. Unfortunately, operators of large microwave systems consider such waveguide damage to be common.
Preparations for service calls Most of the radio circuits in microwave equipment use fairly low frequencies that can be measured with ordinary instruments found in most two-way radio shops. Servicing microwave systems may require special test instruments, such as a microwave wattmeter, a microwave frequency counter and a spectrum analyzer that covers microwave frequencies. Much of the signal processing takes place at an intermediate frequency that eventually mixes with the microwave oscillator front end. The part of the receiving and transmitting system actually operating a microwave frequency may consist of one or a few modules.
The equipment manual is essential for troubleshooting microwave systems. It explains what to check and outlines a logical troubleshooting procedure.
Keep track of important readings from the indicator instruments that are part of the microwave system. Signal strength and output power should be logged. Logging this information whenever you service the microwave site may reveal problems as they develop.
If a backup or standby circuit is available, switch communications to it before you start servicing equipment in the malfunctioning circuit. Service work then can be done with minimal inconvenience to the microwave circuit users.
Microwaves can be a health hazard. Most microwave systems used with mobile radio systems are low-power systems, but take precautions. The eyes are the most vulnerable part of the body. Never look into a microwave waveguide or feedhorn. Also avoid looking into other parts of the microwave transmitting system when the power is on. Many equipment shelters will have stationary radiation monitors positioned around the room. When leaks are a potential hazard, initial attention should be given to these monitors.
Protective clothing is available for use around non-ionizing radiation (see “Use protective clothing for safety in RF fields,” April 1994 issue). For a quick visual inspection to ascertain the location of a leak, a hood may suffice. For an extended service call where a ciruit is active and prolonged exposure to a leak is possible, full protective gear may be required.
Systems troubleshooting *Antenna alignment–One somewhat vulnerable part of the microwave communications link is the antenna aiming. A microwave system may require extremely precise aiming of the microwave antennas at each end of the link. If one antenna moves out of position slightly, perhaps only a few degrees of arc, a problem may result.
Most microwave systems can monitor received signal level (RSL), so one of the first steps is to check it to verify proper reception. With a bidirectional microwave system, RSL at each end can be compared. With a one-way system, such as one that would connect a satellite receiver location with a voting unit location, only the RSL at the receiving end can be checked.
If the transmitter power output is normal and the RSL differs greatly from normal, it may be necessary to have the antennas realigned by a tower crew. Small variations in RSL usually are unimportant because a typical microwave system is designed with as much as 40dB of safety margin over the limiter signal level. In strong and gusty winds, small received signal variations may be caused by the wind moving the antennas or towers at either end, although the microwave antenna mount should be made strong enough to prevent significant antenna movement.
*Waveguide–If high reflected power is indicated on the microwave wattmeter, the most likely cause may lie in the waveguide (or the coaxial cable) to the antenna. At the antenna itself, only malfunctions in the feed horn or feed antenna normally are capable of causing high reflected power. Even severe icing of the antenna rarely results in much reflected power.
When a wattmeter connected at the microwave transmitter indicates high reflected power, measure reflected power again at the point closest to where the waveguide exits the building. Low reflected power at this connection points to trouble within the building. High reflected power at this connection points to trouble outside, and it may be necessary to call in the tower crew. Remember to turn off the microwave transmitter before manipulating any couplings in the transmission line, as when inserting and removing a wattmeter.
Troubleshooting and replacing a long length of microwave waveguide or coaxial cable usually requires tools, instruments and capabilities that are beyond those of most two-way radio technicians.
*Power supply–Many microwave system malfunctions are traced to the power supply. If there is no microwave signal at all, first check to see whether the power supply is on; if it is not, check the fuse.
It is common for isolated microwave sites to lose utility power. Systems designed for high reliability usually have backup power in the form of batteries, a fuel-powered generator or both to supply power when utility power fails.
The backup power system should be checked and maintained during a microwave site visit. Check the fuel-powered generator if there is one, and check battery voltages. Clean the battery terminals and replace any lost electrolyte with distilled water.
At any site equipped with lead-acid batteries, ample water should be kept, not only for refilling the batteries, but to use for flushing acid spills from skin, clothing or other surfaces.
Check the various power supply output voltages, and trace them to see that they reach the various circuit cards and modules within the microwave equipment.
*Multiplexer cards–A single analog microwave link may be served by numerous multiplexer cards. Malfunctions often are caused by problems in a multiplexer card, as indicated when one channel malfunctions, but the other channels continue to work.
The various multiplexer cards are controlled by a system of oscillators at different frequencies. Most commonly, each multiplexer card delivers a single-sideband signal as its output. The multiplexer cards and their levels can be checked with instruments such as a frequency-selective voltmeter and a spectrum analyzer. Overmodulation on one of the multiplexer cards can cause interference with other channels.
Even when checking multiplexer cards, a word of caution is in order. Do not attempt to align or to adjust anything on multiplexer cards, unless you are fairly certain that you have found the cause for the problem. Otherwise, you may do more harm than good to the system.
Although the multiplexer cards use single-sideband modulation, the baseband signal (composed of all the single-sideband channels from the multiplexer cards) is frequency-modulated onto the microwave carrier. The microwave signal bandwidth can be huge, compared with narrowband FM signals used in mobile radio systems.
*Wave tubes–Most low-power microwave systems use solid-state microwave transmitters. When replacing a traveling wave tube in a high-power system, be extremely careful. Many traveling wave tubes are fragile (and expensive). If a tube breaks during removal or replacement, it will be difficult to remove the glass splinters inside the traveling wave tube mount. Another reason for extra caution when working on a traveling wave system is the high power-supply voltage.
Repair verification and prevention Some microwave systems have a special voice communications channel for use while troubleshooting. Otherwise, on-site communications may be conducted by telephone or portable radio. Once the malfunction is repaired, test the microwave link with the help of someone at the other end to verify proper operation. Network management systems are available.
While you are at the microwave site, it is a good idea to do some basic housekeeping. Keep the site clean, and keep parts, equipment and saftey supplies well organized. During the winter, you may want to shovel snow from the access path. During the summer, mowing and trimming may be necessary–both to keep access open and to eliminate concealment for trespassers. Security and safety measures (locks, chains, fences, fire alarms, remote monitors, etc.), particularly in a remote location, should be checked. Unmonitored tower lights can be checked during the daytime by covering the photocell that activates the lights or by using a switch that bypasses the photocell.
A well-designed microwave communications system is extremely rugged and durable. With these basic maintenance and service procedures outlined above, you will maintain the reliability of your system.