Wireless Systems Design

Microwave Absorbers Help Wi-Fi Perform

Wi-Fi is truly ushering in a communications revolution. This technology is the first of its kind in an age when thousands of people are unplugging their phone jacks. Now, the wireless Internet is beginning to fully encompass the globe without phone lines and buried cable. During the past year, manufacturers of materials that control electromagnetic interference have been providing supporting technology to the network-infrastructure companies that manufacture indoor/outdoor Wi-Fi switches.

A few years ago, 3G was the ring in everyone's ears as cellular companies paid for transmission rights and pushed for multimedia smart phones. Yet Wi-Fi technology had already emerged. It could be found in small areas around the globe. Sure, 3G is still around. But with problems still looming and some wondering about its feasibility and its future, it's no wonder that Wi-Fi finally got the foothold that it needed to go mainstream. So how does Wi-Fi work?

Wi-Fi antennas use the IEEE 802.11a or 802.11b standard to provide secure, reliable, and fast wireless connectivity. Wi-Fi networks operate in the unlicensed 2.4- and 5-GHz radio bands with an 11-Mbps (802.11b) or 54-Mbps (802.11a) data rate. They also work with products that contain both bands (i.e., dual band). Nonetheless, both standards are generally limited to providing wireless-Internet access to only several dozen users within a few hundred feet of the transmitter. But don't throw away that Wi-Fi-compatible laptop just yet.

Thankfully, some military designs are now available to the public. For instance, military-designed phased-array antennas have found their way into the commercial world of telecommunications. Now, those antennas are making it possible to steer numerous radio beams from a single point. Focusing the beams increases their signal strength, while using many of them greatly increases overall capacity. In fact, distances of a few thousand feet within buildings and about three-to-five miles outdoors have been achieved using phased-array antennas. As a result, Wi-Fi is becoming more widespread in corporate and academic campuses nationwide.

These antennas, which are packaged as a single integrated unit, create highly directed, relatively narrow energy beams. Those beams result in very-long-range transmissions, which can be rapidly switched between users. Within this very high-power microwave package, however, electronics like isolators and mismatched circulators radiate EM fields. These fields can drastically interfere with and diminish the overall performance of the system itself. Cavity resonances and backlobe reduction all become very serious concerns. After all, each of them can cause disruptions that are very different from the other. Ultimately, these concerns must be addressed in order to ensure top performance.

Cavity resonances can occur inside the housings that are designed to protect RF components or help shield them from other electronics. Many design engineers try to "channelize" components within the housing. Although this approach does work to some degree, it rarely solves the problem entirely. Yet these enclosures can actually trap the spurious signals radiating from a microwave circuit. Such signals can create a resonant voltage standing wave ratio (VSWR) inside the enclosure.

In a microwave package with highly sensitive components, a VSWR can be very troublesome. It can cause system impedances to become mismatched, which results in performance degradation. When the impedances are improperly matched, RF power will be reflected. The outcome can be loss of signal power, weak transmissions, and poor reception.

Eliminating these resonances can be as simple as inserting a high permeability or permittivity absorber in the cavity above the circuit. This action will reduce the VSWR so that the EM devices are no longer adversely affected (FIG. 1). A magnetically loaded absorber is usually recommended, as the E field goes to zero at the cavity walls while the H field is at a maximum. As a result, a material with high magnetic loss will have the best absorption characteristics. If thickness and conductivity aren't an issue, however, a dielectrically lossy foam also can be used to restore peak performance.

Another common interference problem is the transmission or reception of unwanted signals through an antenna's sidelobes or backlobes. Improper design and poor directivity will cause a drop in an antenna's front-to-back ratio. As a result, interfering signals will tend to seep through these lobes. This problem can sometimes be drastically magnified when installed on a base station. Geography and the proximity to other antennas will surely alter its performance.

Outside of the lab and in the field, this interference can quickly waste a company's time, money, and resources. In the consumer's eyes, such interference results in real day-to-day problems. Examples include decreasing data rates and weak connectivity. These nuisances are enough to make consumers switch providers. The ISP then loses precious market share. Clearly, fixing these leakage problems up front is both beneficial and cost effective in the long run.

Microwave absorbers can be used to easily attenuate this unwanted lobe energy with high-performance, inexpensive, and environmentally friendly materials. If a high-loss dielectric or magnetically lossy material is properly placed within the antenna housing, it will usually sufficiently reduce the backlobe radiation to a point at which it no longer causes a problem. In most cases, this reduction can be achieved with an inexpensive dielectrically loaded, carbon-filled polyurethane foam. When properly placed behind the antenna, a high-loss foam will absorb a transmission wave by attenuating the signal. It will convert the signal to heat and ultimately decrease the overall amplitude of the wave (FIG. 2).

Today's new technologies and materials have opened the door for a range of possibilities when approaching these problems. High-tack pressure-sensitive adhesives allow for the easy attachment of absorbers. They save both time and money in assembly operations while assuring excellent adhesion properties over time. New coatings also have been developed for the weatherproof conditioning of absorbers. Many foams won't function properly when they're wet, causing performance to drop off until the water is removed. Weatherproofing these foams will maintain peak performance at a minimal cost. It also allows a single absorber to be used for both indoor and outdoor applications.

Until now, Internet providers have rejected Wi-Fi because of its high engineering and equipment costs as well as the need to pay for costly installation by end users. In addition, security was and still is a very big concern. When this new antenna technology is coupled with the right absorber and security measures, however, it can be a viable alternative. As these new designs arise and microwave packages continue to get smaller, absorber manufacturers will remain ready to assist design engineers in solving their EMI problems before products go to market.

TAGS: Components
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