Page 226 - Maxwell House

206 ANTENNA BASICS

Usually, both values and their Zeus Differential Reflectivity or = − are
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measured in dB. The typical values of ZDR are listed in Table 5.2 . Here chaff refers to
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thin strips or wires of aluminum dropped at times near or inside of clouds to track air flows
and sometimes for military purposes to mask the real target. Moreover, additional
information can be similarly gathered by measuring the differential phase shift between the
received signals in HLP and VLP channels.
10. A line-of-sight path as shown in Figure 5.1.7 can be impaired by obstructions such as
foliage or small buildings. Since CP signal carries two LP components simultaneously, the
probability of signal loss diminishes. Note that due to scattering from tree-trunks the VLP
waves typically suffer 5 dB to 15 dB higher propagation loss in foliage than HLP waves
depending on the frequency band, type, and density of foliage, weather conditions, wind
strength, season, etc. [31].

Figure 5.1.7 Line of sight (point-to-point) communication link
11. CP antenna signals are typically more efficiently penetrated through building walls
providing overall better in-building communication links.
12. Handheld devices for mobile communications typically include the simplest LP antenna
(no room for a bulkier CP antenna). Their antenna orientation often randomly changes
depending on how the user is holding the device. To prevent the wireless connection from
dropping and establish a reliable communication link, the antennas on the base sites must
be set up for CP. As a result, a handheld device receives the downlink signal regardless of
its antenna alignment. The same is correct for uplink signals coming back to the CP
antennas of base stations.
13. Aircraft, Low Earth Orbit (LEO) and Medium Earth Orbit (MEO) satellites, as well as
Satellite Communications (SATCOM) systems, use CP transmitters to keep reliable
communication links between constantly moving satellites and the fixed terrestrial stations
as well the customers with LP antennas. Likewise, it is not always possible to preserve
antenna alignments between aircraft and other services because of an aircraft’s
regularly pitches, yaw rolls and banked turns. Geostationary Earth Orbit (GEO) satellites

3 Zeus is the name of the highest Greek god that means “shine” or “sky”. Greek believed that he ruled
over earth and humankind, control over the weather, and his weapon was a thunderbolt.
4 Public Domain Image, source: www.weatherwise.org