Transmission Line Pressure Monitoring
Antenna, transmission line, transmitter -- three key mission critical elements of any radio station. Without any one of those, the station is off the air. The transmission line, being totally passive, can sometimes be forgotten with so much other stuff going on. If it fails, though, suddenly it becomes the #1 priority. So, maintaining it - since it will last years and years of properly maintained -- is vitally important.
A big part of maintaining your transmission line is keeping moisture out. That's done in one of two ways - either by way of a dehydrator, or by way of nitrogen. Both are used to keep positive pressure inside the line itself.
If you happen to have a dehydrator installed at your transmitter facility, dutifully maintaining dry air and positive pressure inside your transmission line, you need to keep a few things in mind. (These all come from the Andrew user manual, by the way.) First, if the dehydrator is running more than 20 percent of the time, check the system for leaks. Second, the dehydrator itself needs an overhaul for every 6,000 hours of run time. There are two choices in getting the work done: Buy an overhaul kit from the manufacturer and do it yourself, or send the unit to the manufacturer. Most companies offer a loaner program if needed. ERI, SPX Communication Technology and others also provide dehydrators.
Because the dehydrator requires power and has moving parts there is certainly a possibility that it will fail at some point. Maintaining pressure inside the transmission line is so important it's wise to have some sort of backup pressure system available. That's where the nitrogen bottle comes in.
Nitrogen has advantages and disadvantages. The obvious advantage is that it can be stored on-site for long periods of time and have it available. It's inexpensive. There are no moving parts or power required to use it. The disadvantages are that it can be hard to get a bottle delivered (you know this if you've moved one yourself) and if some sort of substantial leak in the line develops, the bottle can empty quickly.
Whether you use nitrogen all the time or just as a back up for your dehydrator, there are a couple of things you want to know once you leave the site: What is the line pressure, or alternatively, is the line pressure too low, or too high?
To determine the line pressure, use a pressure transducer, such as the Broadcast Devices PSW-100, which provides an output voltage corresponding to the measured pressure (0 to 6 psi). The facility remote control can be configured to trigger an alarm should the pressure read too high or too low. Another company that makes pressure transducers is Omega Engineering.
Then again, maybe you don't really care what the exact pressure is; you just want to know if it is too high or too low. (If you already have a dehydrator likely you're using its low-pressure switch; however, if the unit is out of the rack, you may need an alternative.)
In this case McMaster-Carr has something to help. Page 569 of the online catalog has several choices in adjustable pressure switches. Using two of these devices, you could detect the too low as well as the too high conditions. You will need a manifold to connect all the sensors to line. Dehydrator suppliers sell these as well. You can also try the nitrogen gas supplier or a welding supply shop for the gas fittings.
Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at firstname.lastname@example.org.
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