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Figure 68 Parabolic antenna feed methods
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To make the dish lighter and to withstand strong wind, a dish made of metal mesh instead of solid metal can be used
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653 Microwave Antenna Selection
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Standard, open-grid (for low wind loading), and high-performance dishes, and single or dual polarized antenna models, are available Many antennas come with galvanized steel mounts based on EIA standards RS195B and RS222 As the antenna beamwidth decreases, antenna alignment (and thus stability) become more critical Furthermore, weight and wind loading are greater with large antennas As a consequence, the antenna mounting structure must be several times more rigid (against twist and sway) for each successive increase in antenna size The selection of antenna size should be based on the results of path analysis and calculations The antenna size must be determined before a frequency coordination study can be performed and before applying for the license to operate the microwave system9 The main parameters of interest when choosing the microwave antenna are as follows: 1 Operating frequency band (MHz or GHz) 2 Radiation pattern 3 Gain (dB) 4 Polarization (single or dual polarized) 5 Half-power beamwidth (degrees) 6 Wind load (mph) 7 Front-to-back ratio (dB) 8 Cross-polarization discrimination (dB) (This is the difference between the peak of the co-polarized main beam and the maximum cross-polarized signal over an angle twice the 3-dB beamwidth of the co-polarized main beam) 9 Isolation (between inputs of single-band, dual polarized antennas) (dB) 10 Additional options for the most microwave antennas that may be ordered from manufacturers are
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Input anges Antenna color variations Radomes (re ector protectors) of various customized colors High-wind survival options Corrosive environment protection
Microwave Network Deployment
n n n
Packing type and quantity options Various re ector types Special-purpose antennas:
High-performance or shrouded antennas Ultra-high performance antennas; the improvement over highperformance antennas is a higher front-to-back ratio and better cross-polarization discrimination
Special accessories such as struts, ice shields, and so on
Grid-type dish antennas are usually employed for the low-frequency microwave bands at 14 GHz and 2 GHz At 400 MHz and 800/900 MHz, low-cost Yagi types can be used, but grids are preferred for their better front-to-back ratio performance over Yagi antennas Most administrations require the use of relatively high-performance antenna for fixed links in these bands to optimize frequency reuse Grid antennas, on the other hand, are cheaper than the solid types and the use of grids reduces tower wind loads Antennas operating at the low-frequency bands have wider beam widths than at higher microwave bands, reducing the requirements for tower stiffness providing a further cost saving
654 Radomes and Shrouds
High-performance antenna versions include an RF shroud, added to improve side lobe performance Shrouds appear like drums increasing the side profile of the parabolic antenna The shroud is lined with an RF absorber material, improving side and back lobe radiation Back lobe radiation can be reduced more than 10 dB by using shrouds A planar radome is used to protect the antenna against harsh weather conditions and to prevent ice or snow accumulation Radomes must be electrically transparent at operating frequency; that means that the thickness, L, of the radome must be chosen to be equal to one-half wavelength in the material of the radome As an example, we can assume that the permittivity er' of the dielectric radome is 28 at 6 GHz The refractive index of radome will be: n = r ' = 28 167 (619)
The wavelength will be reduced n times in the dielectric material of radome:
radome =
0 n
(620)
Six
So, in our example we have:
0 = radome =
c 3 108 m/sec = = 50 mm 6 109 Hz f
0 50 mm 30 mm = 167 n
So, in order for this particular radome to be electrically transparent at 6 GHz frequency, it has to be 15 mm thick: L=
radome 30 mm = = 15 mm 2 2
The amount of loss for a radome may vary from less than 05 dB for a typical unheated radome to more than 20 dB for a typical heated radome in the high-frequency bands During a rainstorm, water sheeting on the face of the antenna causes an additional loss called wet radome loss Preliminary analyses suggest that the effect of water on the antenna radome may be an important mechanism, in addition to rain attenuation along the path10 This is particularly true in tropical regions where the rainfall rate is very high, and microwave links operating in the higher microwave bands will suffer from high attenuations due to rain The losses of a wet radome depend not only on the rainfall rate, but also on wind conditions during the rain In heavy rain situations, values of 15dB or more attenuation have been reported with nylon radomes The loss for a Teflon-coated radome is 1dB, whereas the loss for a fiberglass or ABS plastic radome depends on the rain rate and frequency band but may be much higher than the Teflon radome The Hypalon radome material used on some high-performance dishes can be as bad as fiberglass Because of the lower wet-radome losses, Teflon-coated radomes are recommended for bands that are affected by rain outage Wet radome losses are lower for Teflon because water tends to form droplets instead of sheets on the slick Teflon surface Teflon is also used on most highperformance antennas
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