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October Expert Advice

October 10, 2009
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In this Ask the Expert column we will review some key tips on how to optimize microwave antenna systems for 4 GHz and 5 GHz networks. The correct and optimal selection of antennas can be the most cost effective way to improve network performance at these now commonly used frequencies.

There are four basic styles or types of antennas utilized for the 4 and 5 GHz bands. The sector (hub) antenna is designed to provide segmented coverage over a selected area. They typically provide a wider beamwidth than parabolic antennas and are commonly manufactured in a range of beamwidths. The flat panel antenna is ideal when aesthetics are critical and performance is not. They are light in weight and visually appealing allowing for easy concealment. They are generally available in several sizes and for all broadband wireless bands. The user should be aware that parabolic antennas will have more gain for the same size flat panel due to the inherent higher efficiency of the parabolic antenna design. The standard in microwave antennas is the parabolic or “dish” antenna. The parabolic antenna consists of a parabolic shaped reflector, which focuses energy at the feed point of the antenna. They have a very narrow beamwidth that focuses energy at a specific point, making them ideal for point-to-point communications. There are also high performance versions that utilize a shroud and absorber material to improve side lobe performance and the front-to-back ratio of the antenna. At lower frequencies, below 5 GHz, a parabolic reflector can be simulated by a “grid” of reflective elements. This arrangement reduces wind loading, but does not provide as good pattern performance or gain as a solid reflector. Additionally, grid antennas are limited to a single polarization.

Different system applications each require a different antenna type to ensure optimum network performance. A point-to-point application requires an antenna with a narrow beamwidth in both planes and high gain. Thus allows for longer paths, as well as minimizing interference issues. Thus a parabolic is the best choice. Where interference may be present and for the best possible communications path a high performance (HP) parabolic should be utilized. Due to the crowded nature of spectrum these days, we are seeing more and more users utilize HP dishes on microwave links even in both the 4 and 5 GHz bands. These HP dishes allow more links to co-exist in the same geographic area. Dual polarized antennas may be utilized to offer system capacity enhancement with a radio such as Motorola’s Canopy Backhaul PTP400 and PTP600 series or polarization diversity to enhance the link performance. In the case of the radio produced by Exalt Communications, the polarization can actually be switched remotely with a software controlled rf switch. Either of these radios would ideally be matched with an antenna such as the HPD4-5.2, which is a high-performance, 4’ dual-polarized parabolic dish. By utilizing the combination of one of these radios and a high-performance dual-polarized antenna, network performance is thus greatly enhanced and susceptibility to interference greatly reduced. The more each user deploys HP antennas, the more we conserve spectrum for all operators to deploy more networks in the future.

Users should also always consider the use of radomes to protect their investment for years to come from the elements. A relatively simple technique to minimize interference is to utilize larger diameter antennas. The larger the antenna, the lower the back lobe and side lobes will be. Thus by utilizing a larger antenna, the interfering signals will be at a lower level. Additionally, the larger the antenna the higher the gain provided by the antenna will be. This will lead to a higher received level for the desired signal. When you consider the cost of a microwave link, the “delta” cost to go to larger diameter antenna provides for a relatively low cost method to improve network performance. It’s also important to consider side struts in high wind areas. The cost to add a side strut at initial installation is low compared to paying for a re-alignment after a major wind storm.

A point-to-multipoint hub (base station) application requires an antenna with a wide horizontal beamwidth and high gain to properly illuminate the coverage area and this best provided by a sector antenna. A point-to-multipoint subscriber application requires a small antenna that can be easily installed and is aesthetically pleasing. This can best be accomplished with a small 1’ or 2’ parabolic. When selecting the beamwidth for the hub (base) antenna users should consider 90 degree horizontal beamwidth antennas as the optimum choice with at least 16 dBi of gain or more. While it may seem that since you are covering 360 degrees you would want three 120 degree antennas, this is actually inefficient. If you “overlay” three 120 degree antennas, there is significant overlap in the three beam patterns. By utilizing three 90 degree antennas, the area is fully covered, there is less wasted overlap and the higher gain of the 90 degree antennas helps the system to work over longer distances. Thus 90 degree sectors are the ideal choice for most hub antenna applications in this frequency range. The user also needs to be careful if selecting sector antennas that make use of PC board material for the radiating elements or feed system. Typically, low-cost antennas have poor or unreliable performance characteristics such as high loss and interference as well as inappropriate beam widths. All too common in low cost PCB antennas is the usage of lower quality board material that has higher losses. Thus as the RF signal travels through the board, more energy is converted to heat and less energy passed through the circuit to eventually be radiated as energy from the antenna system. A higher quality board material will lower the losses and have higher antenna efficiency ultimately providing more energy that is radiated out of the antenna system as true gain. There is no industry or government organization that review manufacturers gain specifications. An antenna manufacturer can claim anything they want. We have seen numerous cases where someone has replaced a “cheap” sector antenna with one of our sectors and the coverage improves dramatically due to our antenna having the gain specified, but the “cheap” antennas actually performing well below the manufacturers advertised specifications.

There are some OEMs and others that have recommended broad band antennas that cover part of the 4 GHz band and the 5 GHz band such as 4.9 to 5.85 GHz. Antennas this broad are not an optimal solution for most applications. While many antenna manufacturers will not inform customers, the gain of these antennas drops off on the band edges. In some cases the drop can be significant. While these broadband antennas make it easier to stock the antennas, they do not provide optimal network performance due to the gain drop one or both band edges.

Keep in mind that the antenna is the most cost effective tool for system optimization. Choosing an antenna that focuses energy in the most useful area is key as well as assuring the antenna selected can minimize interference. Higher gain (larger diameter) antennas have narrower beamwidths that help to reduce interference from unwanted sources and maximize desired signal. Choosing an antenna with good efficiency is also key for assuring optimized performance. When selecting antennas, one should also be careful of “paper specs” in a catalog, as there is no agency or industry organization that assures data in a manufacturers catalog is correct. Users should also carefully check the manufacturers warranty and if they don’t offer at least five years, ask them why not. As the most significant performance improvements are achieved by optimizing performance of antenna systems, it is imperative that designers consider the choice of antennas carefully. Radio Waves provide an arsenal of antennas to solve complexities facing designers in optimizing their networks.

Recent Articles by Andy Singer

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