There are at least two good reasons why it is important to properly pressurize coaxial cables. First, it keeps moisture out and therefore prevents arcs inside the cable. Secondly, it improves the power-handling capacity of the coax by increasing the breakdown voltage between the inner conductor and the shield. Using nitrogen tanks, dehydrators or a combination of these devices can ensure that dry air at the proper pressure is present in the cables at all times, regardless of weather changes. The pressurization system should be capable of handling the load requirements, no matter the conditions.
When designing a cable-pressurization system, the first issue to consider is the behavior of air, which naturally expands when heated and contracts when cooled. For example, coax air pressure that is at 1psi during the cool of the evening can rise to 5psi in the heat of the afternoon, depending on the volume of the air inside the coax. Also, large coax cables, such as 4- to 6-inch diameter cables, can contain a significant volume of air. A large volume of air is good because, if there's a leak in the cable, it takes longer for the air to leak out. But, such a large volume of air also can experience drastic changes in air pressure between a hot day and a cold night. The important thing is not to exceed the maximum air pressure specified by the cable's manufacturer. This is especially critical for microwave waveguides because overpressure can actually expand the diameter of the cable, resulting in an impedance change and a corresponding increase in VSWR. Overpressure can also pop the pressure window on a microwave dish, necessitating a tower climb to repair the damage. Leaks often begin as small openings that let in moisture. They usually go unnoticed until the facility detects a decrease in transmitted or received signal or an increase in VSWR.
There are different pressurization-system configurations that a facility can employ. The final choice depends on the size of the transmission-line system and the amount of manpower available to maintain it.
Figure 1. This diagram shows the simplest and most cost-effective arrangement for a single-coax system. Click here to see an enlarged diagram.
A configuration using a nitrogen tank is the simplest pressurization arrangement, as well as the most cost-effective, for a single-coax system. (See Figure 1.) The nitrogen tank directly feeds the gas barrier input of the coax. When the system reaches the desired pressure, you simply close a valve. The nitrogen tank should have a dual gauge; one gauge shows the tank pressure (and, consequently, capacity) and another displays the regulated pressure to the coax. The latter typically ranges from 0- to 15psi, which is suitable for most broadcast applications. When setting the pressure in such a system, make sure you account for changes in temperature. This configuration is adequate if you can check it daily and detect any minor leaks right away. Adding a pressure monitor connected to the transmitter's remote telemetry can alert the master-control operator if a leak occurs.
As mentioned above, the main advantages of this setup are simplicity and cost-effectiveness. Nitrogen gas is dry and is readily available for less than $20 per tank. Unless there is a leak in the system, you shouldn't need more than two tanks per year. The main disadvantage of the system is that you have to add air manually as needed. It's not recommended for remote sites because, if a leak develops, the pressure may fall to zero before someone can come and do something about it.
Dehydrator and nitrogen tank
Figure 2 illustrates a system that uses a dehydrator as the main supply of air and a nitrogen tank as backup. Dehydrators come in different sizes to accommodate the differences in air volume found in various cable systems. A typical application might consist of just one dehydrator for the cables that serve a pair of 8-VSB and NTSC transmitters as well as the waveguides for STLs or RPUs. The antenna and cable manufacturer can advise you as to the pressure required to protect the feedlines. Such a configuration is automatic; all you have to do is set the dehydrator's cut-in and cut-out pressure for the best operation. The cut-in setting defines the pressure at which the dehydrator starts pumping air; the cut-out setting defines when it stops. Most systems allow a range of settings. A typical application uses a cut-in setting of 1psi and a cut-out of 4psi. Usually, there is an adjustable screw for each adjustment located at the dehydrator's pressure switch. It's a good idea to inspect the setting on a regular basis because the adjustment is mechanical and the unit vibrates when it operates. A once-a-week inspection of the dehydrator (and the moisture desiccant/filter) may be enough to ensure proper pressurization on the system. As a rule of thumb, the dehydrator should not run more often than once every 15 minutes. If it runs more often, either the system was not sized properly or there is a leak.
Dehydrator, nitrogen tank and reservoir tank
The cable-pressurization system in Figure 3 is an improvement on the previous two designs. The air-reservoir tank serves as a buffer, increasing the system's capacity without the expense of a large dehydrator. The additional cost depends on the size of the tank and the air-pressure regulator attached to it. A 20-gallon tank may cost from $60 to $100, and the regulator may cost up to $600. The tank and regulator are quite heavy. So, to save on shipping and handling costs, it's best to shop around. Connect the lines you want to pressurize to the regulated output of the tank. Connect the reservoir tank to both the nitrogen tank and the dehydrator. You can set the nitrogen tank's pressure two ways: just above the dehydrator's cut-in pressure setting so that the dehydrator pumps only when the nitrogen depletes, or just below the dehydrator's cut-in pressure so the dehydrator still pumps at regular intervals. In the latter case, the nitrogen tank can continue to supply the needed air even if the dehydrator suffers a power failure. A one-way valve at the output of the nitrogen tank allows air to flow only when the pressure drops lower than the regulated setting of the tank. Be sure to set the nitrogen tank's regulated setting as if it were feeding the coax lines directly.
Figure 2. This configuration uses a dehydrator as the main supply of air and a nitrogen tank as backup. Click here to see an enlarged diagram.
To avoid overpressurizing the coax lines, use pressure-relief valves. They come in different relief pressures and should be rated below the maximum pressure that the coax system can handle. For example, KVTN-TV in Arkansas uses an Andrew UHX10 dish with EW62 cable and can handle a maximum of 10psi. The station chose to pressurize it at 3psi and use a 5psi relief valve to prevent overpressure during Arkansas' hot summer afternoons. When evening comes, the dehydrator starts pumping to compensate for the drop in pressure. With these settings, the waveguide pressure will never increase more than the pop-out pressure of the relief valve (5psi), thereby preventing damage to the pressure window of the dish, which is located on top of the tower.
Sizing the system
To calculate the appropriate dehydrator size, list the cables and waveguides you want to pressurize with their corresponding lengths. Then check the manufacturer's data and determine the total cubic feet volume per length (in feet) of each cable. Then, total the cable air volumes to arrive at the total volume of air that the pressurization system must handle. Considering the pumping capacity of the dehydrator and the volume of the air tank, estimate the time intervals the system should run for a given percentage of leakage. It's better to oversize the capacity (for a higher cost) than to operate the dehydrator at a higher duty cycle because the latter would incur increased maintenance cost and reduce system life. Armed with the total coax air volume, consult the cable manufacturer for help selecting the properly sized dehydrator.
Increasing the air pressurization in the cable improves its power-handling capacity. It's possible to almost double the peak power rating of a coax cable by increasing the pressure to 11psi, as on page 633 of Andrew's catalog 38 and page 6 of Dielectric's Transmission Line and Components. But, if you do decide to go this route, don't exceed 20psi. Make sure that you install 15psi pressure-relief valves.
Figure 3. In this configuration, the air reservoir tank serves as a buffer, increasing the system’s capacity without the expense of a large dehydrator. Click here to see an enlarged diagram.
Regularly inspect the color of the desiccant on the dehydrator. Usually, is it blue when dry and slowly turns to pink or red as it absorbs moisture from the compressed air. Ensuring that the desiccant is dry can prevent pumping moist air into a cable system. Remember, the goal is to keep moisture out of the line. For safety reasons, don't try to dry the desiccant and re-use it; just replace it, being careful not to inhale the dust.
Observe how often the dehydrator pumps. If it starts to pump more often than once every 15 minutes, use soapy water to check for leaks at all valves and couplings up to the gas barrier. If you can't locate the leak at ground level, it's time to climb.
After each tower or antenna work or inspection, observe the running time of the dehydrator. In one case, the antenna-match tuning slugs were adjusted too far out and it took another day before a serious leak became apparent. Chasing down the tower crew after they have left the site and arguing about who will foot the bill for the extra tower work is not the way to go.
Finally, it's always a good idea to talk to the manufacturers of the antenna and the coax. They can provide you with information that can help you plan for the pressurization configuration that best suits your installation.
Rolin Lintag is an RF engineer for a TV network in Little Rock, AR.