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Home ›Technology and Equipment› Technical Tips
Sean Edwards explains that almost anything can cause an RF system to malfunction
Author: Sean Edwardsâ‹… Published: October 20, 2021
The author is the head of RF engineering at Shively Labs.
We all want to get the most value from the money spent.
RF system repairs can be expensive. However, the implementation of a good maintenance plan can greatly reduce the need for repairs-when the system is damaged or degraded, the plan can detect and solve the problem early with lower repair costs.
A very useful tool in RF system maintenance is baseline measurement.
Taken during acquisition or system debugging, these provide a snapshot of the status of the RF system. They can then be compared with future measurements and reveal performance trends.
These measurements may include transmission line scans, transmitter operating parameters, forward and reflected power samples, thermal readings of filters, transmission lines and connections, current and resistance measurements of deicing systems, RF spectrum measurements, pressurization, and signal coverage.
The hardware between the transmitter output and the antenna radiator is the final stage of the FM transmission system. This part of the transmission system can include radio frequency switches; directional couplers; elbow joint complexes; band pass, band stop and notch filters; transmission line length; power splitter; "T"; matching network; feeder cable, etc.
It is crucial to deliver the signal to your coverage area, take the time to get this part right, and then keep it right.
During the antenna installation process, the tower staff and site managers rely on the manufacturer to clearly show the location and direction of the antenna on the tower.
I have seen failures in this exchange, causing the antenna parasitic elements to be installed in the wrong location, resulting in high VSWR and poor coverage. Fortunately, this is a quick solution to correct VSWR and signal coverage. Unfortunately, it requires a tower worker and two engineers to visit the site.
If you pay attention to details and optimize the system during installation, it is in its best condition and should provide years of service.
(I intentionally left the "non-faulty" part. RF system maintenance...what can be damaged? It's just a bent piece of metal, right?)
Almost anything can cause the RF system to malfunction.
Antenna damage due to wind, falling ice, lightning, tower work, vandalism, loose connections, and aging components are just a few of them. When engineers need to deal with multiple systems, they always seem to need to pay attention to certain things.
One way we have some control over such failures is through regular system maintenance.
Have you ever checked the site parameters after a major weather event and found that some parameters have changed-not to the point of failure, but enough to prompt an investigation? Then take a closer look at the damage you find that needs to be repaired?
Or, during a routine site visit, you discover that one or more components are overheating, and upon further investigation you find an elbow that is almost unusable-it will fail catastrophically within weeks or days.
This is proactive maintenance and repair. If these almost happen to you, they are likely to happen.
If you cannot check these readings after that storm, and therefore cannot notice an increase in VSWR, or if you do not visit the site and notice a hot elbow, this condition will persist, worsen, and eventually fail, causing your site to stop. broadcast.
That call usually comes at midnight on Super Bowl weekends.
It is prudent to inspect sites affected by extreme weather events. The same is true for regular visits, even for sites that may be considered trouble-free. The cycle will be different-more frequent for problematic sites, and maybe quarterly or even semi-annually for more reliable sites.
Annual tower climbs are great if they are within budget, but when they are not possible, we will return to an in-depth understanding of system performance and those baselines, as well as operational history logs, allowing us to see any signs that the problem has started , And what is the rate of change in it.
This can be useful information when determining whether you need a maintenance visit immediately or can be scheduled at a later date.
Some things to pay attention to when checking whether the antenna is damaged:
In more complex systems, the installation of power splitters and coaxial cables should not impose excessive mechanical stress on the components.
The coaxial cable should have appropriate hangers and fasteners where it crosses the tower member or other antenna feeder components. Please consult the manufacturer for specific recommendations and best practices.
Antennas with deicer systems usually have an external wiring harness that distributes AC power to each heating element in each radiator. The manufacturer will provide the resistance value of each component and the expected current consumption.
The ammeter measurements on each branch of the circuit, including the neutral point, will provide the first clue to the condition of the deicing system.
If the wiring harness is incorrectly installed or the fasteners fall off over time, the wiring harness may hang in a high radio frequency environment. This can cause problems with reflected power of the transmitter and changes in coverage; it can cause current to be induced into the harness, and the voltage is large enough to cause an arc between the conductors of the harness and the tower or other nearby cables.
Finally, take a lot of photos, photos, photos.
This is a good way to record how the antenna is installed and its current status. When you share photos with antenna manufacturers, they will provide very useful information to inform their suggestions.
For a single radiator antenna, it is obvious when there is a problem. On panel antennas, multiple brackets, multiple radiators per bracket, and a large number of feeder cables can indeed cover up the problem.
This is reminiscent of a recent incident when a station engineer noticed that VSWR had slightly increased from 1.05 to 1.15. This happened after the tower crew worked above the antenna and removed the old TV equipment.
The engineer noticed this change and hired a staff member to climb up and check the antenna. They found that during the rigging process, the cable got stuck at some point and pulled aside until it caused a severe kinking at the point where it was connected to the input of the radiator.
If you do not pay attention to the instructions and replace the cable, the next instruction will be the pressure loss caused by the burned coaxial cable. We know what happens when we release magic smoke.
Finally, an in-depth understanding of the performance of your system in the RF system and coverage area will provide you with good services. This knowledge will enable you to prioritize maintenance plans based on your needs, so that small problems can be maintained to correct them before they become expensive, big ones.
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Label â‹… Antenna â‹… Shively Lab â‹… Technical Tips
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