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Chapter 5 205

are LP. By definition, CP antennas are more expensive, complicated and bulkier. If so, why
do we need CP antennas at all?

Circular Polarization.
8. The answer is quite straightforward. Extra signal diversity and energy mean better
recognition of valuable information signals against the background of intentional and
unexpected noises. Further, the reliability of the communication link can be improved with
CP antennas. The CP usage also helps detect and overcome some damaging effects due to
multipath, precipitation (such as rain, snow or hail), atmospheric turbulence, and other
effects.
To illustrate, consider a few CP application examples.

9. It is well known that the detection range of radars is significantly reduced in the rain
because raindrops absorb and scatter EM waves, and thereby the energy reaching a target
reduces. Moreover, the scattered and returned to radar signal can easily overpower and
totally mask the target signal that is further weakened by absorption. This scattering effect
was named a rain clutter (i.e. mess). Meanwhile, typically spherical raindrops scatter the
CP waves reversing their polarization, i.e. for example, an RHCP wave emitted by radar
antenna will come back to the same antenna as the LHCP wave. As a result, the RHCP
radar antenna suppresses almost entirely the LHCP signal. Typically not more than 1% of
reflected from rain energy can reach the radar receiver (i.e. about 20 dB protection). The

Table 5.2

situation is quite the opposite in surveillance radars for monitoring severe weather
conditions such as approaching tornado, drizzle, rain, sleet, snow, graupel (supercooled
droplets of water), hail, sandstorm, tropical cyclone, and hurricane. Weather radars can
even detect biological target such as birds, bats and insects, drones and airborne debris
lifted from the ground by high winds, effects of Anomalous Propagation (AP) of EM
waves, etc. Many of old LP radars are now modernized and equipped with a dual-polarized
antenna carrying the separated input and output for signals of each polarization. This
approach lets optimize the signal processing independently in each of channels. Typically,
one of the measured parameters is the differential reflectivity. It means the difference
between the reflectivity factor from HLP and that from VLP waves, respectively.