Elliptical waveguide is a hollow, corrugated metal tube with an elliptical cross section used to guide microwaves.
Unlike ordinary cables, waveguide has no interior conductor—merely air or nitrogen as a dielectric. To maintain the efficiency of waveguide as a transmission medium, it must be kept free of moisture, since water inside waveguide inflicts the same attenuating effects as rainfall does when microwaves are transmitted through atmosphere. Any moisture can attenuate a signal and increase VSWR.
A recommended method of keeping waveguide clear of moisture is pressurization, wherein the interior of the waveguide is hooked into a dehydrator that provides a pumped source of dry air or nitrogen. Because this creates positive pressure inside, nothing can infiltrate from the outside—including unwanted moisture. Without pressurization, the system tends to “breathe” as temperatures change, allowing moisture in as humidity in the air, which then condenses into water when temperatures drop.
The choice of waveguide pressurization equipment requires as much careful consideration of the specific site as the choice of microwave antennas. Apart from logistical factors such as the availability of reliable power and maintenance access to the equipment, the most important factor is the volume of dry air required by the waveguide. This is a simple matter of calculating the total volume inside the waveguide using its length and cross section area; a large microwave system may have hundreds of feet of waveguide comprising many cubic feet of volume. The system must be able to accommodate this volume plus an additional 1 percent to account for leakage, and provide sufficient pressure during a 19°C (35°F) temperature drop over 60 minutes.
Another consideration is the power source available for the pressurization equipment. To prevent voltage drops between the site’s main power and the dehydrator, it is vital that the correct electrical wire size be used on an appropriately-sized circuit breaker.
Pressurization systems are available in two basic types: static and dynamic. Determining which is best for a given application depends on which best aligns with the specific requirements.
1. Static systems
In a nonpressurized system, the use of a breathing static desiccator in one option. As breathing leads to increased pressure, air is forced out through the desiccator. Later, as breathing decreases the pressure, new air enters through the desiccator, which absorbs moisture before it enters the waveguide. The desiccator has a limited lifespan and must be periodically replaced, but it is very effective and may last many months in a small, tight system of 57 liters (2 cubic feet) or less. In a pressurized system, the waveguide is connected to an external pump, pressurized and then disconnected from the pump—much like inflating a tyre. Since the microwave system is not hermetically sealed, frequent recharging is required.
2. Dynamic systems
A dynamic system remains connected to a pressurization pump that automatically provides additional dry gas as needed to maintain a specified pressure level. This gas may be nitrogen—stored in a tank with a regulator—or it may be one of several kinds of dehydrators.
Nitrogen tanks are ideal for small, tight systems where ac power is unavailable. They have no moving parts and Provide a low dew point to stave off moisture. The drawbacks of using nitrogen are their relatively high cost and the frequency at which they must be replaced in locations where leaks bleed them quickly.
Heat regeneration dehydrators are used for smallto medium-sized systems where low pressure and low power. Use are preferred. They operate continuously, using an integrated controller to monitor system pressure and adjust the air pumps appropriately. These perform their own regeneration of the desiccant, so they require no periodic maintenance.
Membrane dehydrators are designed for low to high system volumes and remote site locations. These systems use a membrane filtration system to remove moisture from the air. They operate continuously, using an integrated controller to monitor system pressure and adjust the air pumps appropriately.
All these choices are predicated on knowing the volume of air to be pressurized, but—as microwave backhaul increases its prominence in the fast-growing world of wireless communications—it’s a certainty that the amount of waveguide will expand. Manifolds are one way to keep up with this expansion. As additional waveguide lines are added, a manifold system allows multiple lines to be serviced by a single dehydrator, and makes it a simple matter to add pressure or flow gauges to the system.
Because pressurization systems often operate in remote locations, it’s important to know when something goes wrong or maintenance is required. Heat regeneration, automatic regeneration and some manual regeneration dehydrator systems include basic alarm functions—alerting operators to such problems as low pressure, excess run time and power failure. Some include humidity alarms and the ability to monitor other alarm conditions. Each product features its own published specification regarding which alarm function it supports.
HOW MUCH IS ENOUGH?
The positive pressure inside an elliptical waveguide transmission line should be below the maximum pressure rating of all the components involved, including the waveguide itself, the antenna feed and pressure window. Most components are rated up to 70 kPa (10 PSI), but pressure need not be this high to be effective. Pressurization of only 3.5 to 35 kPa (0.5 to 5 PSI) is generally recommended.
No matter how well the link has been planned—or how much care has been taken in the selection of equipment—poor installation practices will jeopardize the link’s reliability and deliver performance far below planned expectations.
Further complicating things, every installation is different. The location of the site, the direction of the antenna, the presence of nearby equipment, and other prevailing local conditions mean each installation presents unique challenges requiring careful attention. There are no one-size-fits-all solutions or processes for this challenge. While manufacturers provide detailed instructions on how to best assemble and install equipment, it’s the expertise, skill and care of the installation crew that will ultimately determine if the link fulfills its performance and reliability goals.
Summarry:
There’s an entire science underpinning the use of elliptical waveguide in microwave transmission systems. Ideal performance comes only with ideal conditions— and that means a dry air dielectric inside the waveguide at all times. Depending on the needs and budget, there are several ways to accomplish this level of dehydration, but only by examining the specifics of the individual deployment will the best answer present itself.
Elliptical waveguide must be kept dry internally:
• Any moisture infiltration can attenuate signal and increase VSWR
• Pressurized, dehydrated gas—air or nitrogen— keeps moisture out
• Dehydration can be achieved via nitrogen gas or the use of desiccants
