Applications and Antenna Selection in the 4.9 GHz Band
By Andy Singer, President, Radio Waves

This article will begin with a review of various applications and uses for the 4.9 GHz band (4.4 – 4.99 GHz) and then focus on applications and antenna selection for the “new” 4.940 – 4.990 GHz public safety band allocated by the FCC (Federal Communications Commission).

There are a number of applications and uses for the 4.4 – 4.99 GHz band. These include the 4.4 – 4.5 GHz band, which is designated in the U.S. and NATO countries for military fixed and mobile communications. Typical uses include point-to-point microwave links and telemetry applications such as unmanned aerial vehicles (UAV). There are also peacetime training and test networks deployed in this frequency range. This band is also used widely by NATO countries in Europe for military communications networks. In the 4.635 – 4.685 GHz band, the United States Navy operates the Cooperative Engagement Capability network (CEC), which is a radar information distribution network. There is also a radio astronomy service (RAS) allocation globally on a secondary basis in the 4.8 – 4.94 GHz band. More recently, the FCC allocated 50 MHz in the 4.940 – 4.990 GHz band for public safety applications. Any state or local government agency, including municipal utilities, can utilize this “new” band on a shared basis.
Communication networks deployed in the 4.940 – 4.990 GHz band must be related to the protection of life, health or property and can not provide services that are commercially available to the public. Users include state and local governments, police, fire and search and rescue organizations. Figure 1 is a diagram showing these frequency allocations.

This new FCC allocation of 4.940 – 4.990 GHz permits public safety agencies to implement on-scene wireless networks for video, internet and database access, transfer of data and files such as maps, building layouts, medical files, police records and missing person images. This allocation also allows public safety agencies to establish temporary (up to one year) fixed microwave links to support surveillance operations and emergency communications.

The FCC licensing rules grant a public safety agency authorization to use the total 50 MHz of spectrum within its jurisdiction. Fixed point-to-point operation requires an individual license for each station and can be used for temporary (up to one year) operations on a primary basis, or for permanent operations on a secondary basis. The FCC has concluded that Part 90 will guide this allocation and declined to adopt any standard for broadband technology. There is a sliding scale power limit, depending on signal bandwidth. There is an antenna gain limit of 9 dBi. However, high power devices used for point-to-point or point-to-mulitpoint (fixed or temporary) may use transmit antennas with directional gain up to 26 dBi at maximum transmitter output power. Directional gain may exceed 26 dBi if both power transmitted and spectral density are reduced db-per-db by the amount the directional antenna gain exceeds 26 dBi.

The 4.9 GHz band is experiencing a rapid increase in available radio products that can be deployed. Thus far, most of the deployments in the 4.9 GHz band have been utilizing the 4.940 – 4.990 GHz spectrum for microwave backhaul purposes. Less activity has been seen on the access side, but the backhaul link activity deployment has been strong. These links have been for building-to-building, linking temporary stations to a base station and for linking remote devices, such as video surveillance cameras or SWAT vans, to a headquarters. These networks can also be utilized for temporary monitoring of large events, homeland security and for border control activities. Municipal utilities can utilize these networks for remote monitoring and communications. A diagram showing some of these applications can be seen in Figure 2. The available equipment is best suited to these fixed wireless applications, such as point-to-point and point-to-multipoint. As a licensed band, the greatest advantage to the 4.9 GHz is the minimal interference for public safety users relative to the unlicensed bands such as 5 GHz. These networks are easy and fast to deploy, with a wide selection of equipment available. This new 4.9 GHz band is very attractive to public safety communications users.

While there are a number of good radios available from companies for the 4.940 – 4.990 GHz public safety band, antenna selection is by far the most critical decision relative to network performance. Because the antenna cost is a fraction of the radio cost, the antenna system offers perhaps the best return on investment (ROI) of any network component. Selecting and deploying the optimum antenna is critical to ensuring maximized network performance. In fact, choosing the right mix of antennas can lead to significant cost savings in a network. Designers can maximize the coverage for each antenna and minimize interference, thus minimizing the number of radio points required. For point-to-point links, we will focus on microwave parabolic dishes and for point-to-multipoint networks, we will focus on sector antennas. Figure 3 is a diagram showing the two applications.

There are four basic styles or types of antennas utilized for the 4.9 GHz band. These four can be seen in Figure 4. 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 beamwidths of 40, 60, 90 and 120 degrees. The flat panel antenna is ideal when aesthetics are critical. 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. Due to the narrow beam, they have a relatively high gain compared to other types of antennas. 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. This 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 the 4.9 and 5.2 GHz bands. These HP dishes allow more links to coexist 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.

Users should also always consider the use of radomes to protect their investment from the elements for years to come. 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. At Radio Waves, we have had customers resolve interference problems simply by replacing an existing antenna with a larger diameter antenna at a site. When you consider the cost of a microwave link, the “delta” cost to go to a larger diameter antenna provides a relatively low cost method to improve network performance.

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 is best provided by a sector antenna. A typical sector antenna horizontal pattern can be seen in Figure 5. 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 passes 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.

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 manufacturer’s catalog is correct. There are numerous antennas we have measured that did not meet the gain specified by the manufacturer. It is best if you can visit a manufacturer’s facility and actually witness the antenna gain being measured. Users should also carefully check the manufacturer’s 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 provides an arsenal of antennas to solve complexities facing designers in optimizing their networks.

RADIO WAVES
www.radiowavesinc.com
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