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Efficient Design and Analysis of Airborne Radomes
There are a number of applications and uses for the 4.4 to 4.99 GHz band. These include the 4.4 to 4.5 GHz band, which is designated in the US 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 to 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 to 4.94 GHz band. More recently, the FCC allocated 50 MHz in the 4.940 to 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 to 4.990 GHz band must be related to the protection of life, health or property and cannot provide services 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. The new FCC allocation of 4.940 to 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.
Figure 1 4 GHz frequency allocation table.
While there are a number of good radios available from companies for the 4.940 to 4.990 GHz public safety band, antenna selection is a 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. Also, by planning in advance to minimize interference in the future, labor is reduced that might otherwise be required to solve interference issues down the road.
Figure 2 Basic antenna types.
Figure 3 HPX-4.7 series antennas.
Radio Waves has a number of antenna families developed specifically for the 4.7 GHz band (see Figure 2). The most recent addition is the HPX-4.7 series of high performance parabolic dish antennas, an example of which is seen in Figure 3. The standard in point-to-point 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. The high performance series of HPX-4.7 antennas utilize a shroud and absorber material to improve side lobe performance and the front-to-back ratio of the antenna. This family of antennas offers the ultimate in front-to-back and front-to-side ratios, which can be a tremendous advantage in minimizing interference during mission critical communications. As an example, a SP4-4.7 has a front-to-back ratio of 40 dB, while the high performance HP4-4.7 has a significantly better front-to-back ratio of 54 dB. Due to the crowded nature of spectrum these days, there are more and more users who utilize HP dishes on microwave links even in the 4.7 and 5.8 GHz bands. These HP dishes allow more links to co-exist in the same geographic area.
Figure 4 HPD4-4.7 four foot dual-polarized parabolic antenna.
Dual-polarized antennas may also 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 HPD4-4.7, which is a high-performance, four-foot dual-polarized parabolic dish pictured in Figure 4. 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.
Figure 5 Model HPD6-4.7 radiation pattern envelope.
The HPX-4.7 series is available in a range of sizes, including the HP2-4.7, HP3-4.7, HP4-4.7, HP6-4.7 and the HP8-4.7 eight-foot dish for maximum possible gain in the 4.7 GHz band. The dual-polarized versions mentioned above are the HPD2-4.7, HPD3-4.7, HPD4-4.7, HPD6-4.7 and the HPD8-4.7. Figure 5 is a radiation pattern envelope (RPE) of the HP6-4.7 that shows the superior side lobe performance of this high performance series of dishes. The HPX-4.7 series has a conservatively rated wind survival rating of 125 mph. By adding one extra side strut the wind survival rating is increased to 150 mph for challenging environments such as sea coast and mountain top installations. Each antenna in the series includes a molded plastic radome that is shaped for optimum side lobe performance.
The antenna can be 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 the desired signal. Choosing an antenna with good efficiency is also important 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 consistent with what one will realize in actual use. There are numerous antennas that have been measured that do not meet the gain specified by the manufacturer in actual use. It is best to visit a manufacturer’s facility and actually witness the antenna gain being measured. Users should also carefully check the manufacturer’s warranty and obtain warranties of at least five years.
As the most significant performance improvements are achieved by optimizing the 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. In addition to the HPX-4.7 series of high performance parabolic dishes, Radio Waves offers standard performance parabolic dish antennas, grids, sectors, omnidirectionals and the Xcelarator™ series of flat panels for the 4.7 GHz band.
Billerica, MA (978) 459-8800,
RS No. 302
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